Otter Tail 424B5 Prospectus supplement for primary offering

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Filed Pursuant to Rule 424(b)(5)
Registration No. 333-224649

CALCULATION OF REGISTRATION FEE

Title of each class of securities to be registered		Proposed Maximum Aggregate Offering Price			Amount of Registration Fee(1)

Common Shares, par value $5 per share		$	75,000,000		$	9,735

(1): The registration fee was calculated in accordance with Rule 457(o) and 457(r) of the Securities Act of 1933, as amended (the "Securities Act") and is being paid in accordance with Rule 456(b) of the Securities Act.

Table of Contents

Prospectus supplement
(To prospectus dated May 3, 2018)

$75,000,000

Common Shares

We have entered into a distribution agreement with KeyBanc Capital Markets Inc. ("KeyBanc Capital Markets"), relating to our common shares, par value $5 per share.

Under the distribution agreement, we may offer and sell common shares having an aggregate offering price of up to $75,000,000 from time to time through KeyBanc Capital Markets, as our distribution agent. Sales of the shares, if any, will be made by means of ordinary brokers' transactions on the Nasdaq Global Select Market at market prices or as otherwise agreed by us and KeyBanc Capital Markets.

Under the distribution agreement, we may also sell common shares to KeyBanc Capital Markets, as principal for its own account, at a price to be agreed upon at the time of sale. If we sell shares to KeyBanc Capital Markets as principal, we will enter into a separate terms agreement with KeyBanc Capital Markets, and we will describe the agreement in a separate prospectus supplement or pricing supplement.

Our common shares are listed on the Nasdaq Global Select Market under the symbol "OTTR." The last reported sale price of our common shares on the Nasdaq Global Select Market on November 7, 2019 was $49.95 per share.

Investing in our common shares involves risks. See "Risk factors" beginning on page S-3 of this prospectus supplement and the discussion of risk factors contained in our annual, quarterly and current reports filed with the Securities and Exchange Commission under the Securities Exchange Act of 1934, as amended, which are incorporated by reference into this prospectus supplement and the accompanying prospectus, to read about certain factors you should consider before investing in our common shares.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or determined if this prospectus supplement or the accompanying prospectus is truthful or complete. Any representation to the contrary is a criminal offense.

KeyBanc Capital Markets will receive from us a commission of up to 2% of the gross sales price per share for any shares sold through it as our distribution agent under the distribution agreement. Subject to the terms and conditions of the distribution agreement, KeyBanc Capital Markets will use its commercially reasonable efforts to sell on our behalf any shares to be offered by us under the distribution agreement.

KeyBanc Capital Markets

November 8, 2019

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TABLE OF CONTENTS

		Page

Prospectus Supplement
About this prospectus supplement		S-ii
Cautionary statement regarding forward-looking statements		S-ii
Information incorporated by reference		S-iii
Prospectus supplement summary		S-1
The offering		S-2
Risk factors		S-3
Use of proceeds		S-5
Price range of common shares and dividends		S-6
Material U.S. federal tax considerations for non-U.S. holders of common shares		S-7
Plan of distribution		S-11
Conflicts of interest		S-13
Legal matters		S-13
Experts		S-13
Prospectus
ABOUT THIS PROSPECTUS		2
WHERE YOU CAN FIND MORE INFORMATION		2
RISK FACTORS		4
OTTER TAIL CORPORATION		5
USE OF PROCEEDS		6
RATIO OF EARNINGS TO FIXED CHARGES AND TO FIXED CHARGES AND PREFERRED DIVIDEND REQUIREMENTS		6
DESCRIPTION OF COMMON SHARES		6
DESCRIPTION OF CUMULATIVE PREFERRED SHARES		10
DESCRIPTION OF DEPOSITARY SHARES		15
DESCRIPTION OF DEBT SECURITIES		19
DESCRIPTION OF SECURITIES WARRANTS		31
DESCRIPTION OF UNITS		32
PLAN OF DISTRIBUTION		33
VALIDITY OF SECURITIES		34
EXPERTS		34

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About this prospectus supplement

This prospectus supplement and the accompanying prospectus are part of a registration statement that we filed with the Securities and Exchange Commission ("SEC") using a "shelf" registration process. Under the shelf registration process, from time to time, we may sell any combination of the securities described in the accompanying prospectus in one or more offerings, subject in certain cases to the receipt of regulatory approval. This document is in two parts. The first part is this prospectus supplement, which describes the specific terms of the offering of the common shares and also adds to and updates information contained in the accompanying prospectus and the documents incorporated by reference in this prospectus supplement and the accompanying prospectus. The second part is the accompanying prospectus, which gives more general information, some of which does not apply to the common shares. If the description of the offering of the common shares varies between this prospectus supplement and the accompanying prospectus, you should rely on the information in this prospectus supplement.

We have not authorized anyone to provide any information other than that contained or incorporated by reference in this prospectus supplement, the accompanying prospectus or in any free writing prospectus prepared by or on behalf of us or to which we have referred you. We take no responsibility for, and can provide no assurance as to the reliability of, any other information that others may give you. We are not, and KeyBanc Capital Markets is not, making an offer of the common shares in any state where the offer or sale is not permitted. You should assume that the information appearing in this prospectus supplement, the accompanying prospectus and any such free writing prospectus is accurate only as of their respective dates and that the information contained in the documents incorporated by reference is accurate only as of the respective dates of those documents. Our business, financial condition, results of operations and prospects may have changed since those dates.

Unless we state otherwise or the context otherwise requires, references appearing in this prospectus supplement to "Company," "we," "us" and "our" should be read to refer to Otter Tail Corporation and its subsidiaries.

Cautionary statement regarding forward-looking statements

This prospectus supplement, the accompanying prospectus, the documents they incorporate by reference and any related free writing prospectus issued by us may contain forward-looking statements with respect to the financial condition, results of operations, plans, objectives, future performance and business of Otter Tail Corporation and its subsidiaries. Statements preceded by, followed by or that include words such as "may," "will," "expect," "anticipate," "continue," "estimate," "project," "believes" or similar expressions are intended to identify some of the forward-looking statements within the meaning of the Private Securities Litigation Reform Act of 1995 and are included, along with this statement, for purposes of complying with the safe harbor provisions of that Act. These forward-looking statements involve risks and uncertainties. Actual results may differ materially from those contemplated by the forward-looking statements due to, among other factors, the risks and uncertainties described in this prospectus supplement, including under "Risk factors," the factors discussed in any documents incorporated by reference in this prospectus supplement and any other cautionary statements, written or oral, which may be made or referred to in connection with any such forward-looking statements. Any forward-looking statement contained in this prospectus supplement, the accompanying prospectus and the documents incorporated by reference speaks only as of the date on which the statement is made, and we undertake no obligation to update publicly or revise any forward-looking statements for any reason, whether as a result of new information, future events or otherwise. New factors emerge from time to time, and it is not possible for us to predict all of the factors, nor can we assess the effect of each factor on our business or the extent to which any factor, or combination of factors, may cause actual results to differ materially from those contained in any forward-looking statement.

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Information incorporated by reference

The SEC allows us to incorporate by reference the information we file with them. This allows us to disclose important information to you by referencing those filed documents. We have previously filed the following documents with the SEC and are incorporating them by reference into this prospectus supplement (other than any portions of such documents that are not deemed "filed" under the Securities Exchange Act of 1934, as amended (the "Exchange Act"), in accordance with the Exchange Act and applicable SEC rules):

·: Annual Report on Form 10-K for the year ended December 31, 2018;
·: Quarterly Reports on Form 10-Q for the quarters endedMarch 31, 2019,June 30, 2019 and September 30, 2019;
·: Current Reports on Form 8-K filed onFebruary 19, 2019,April 9, 2019,May 7, 2019,June 11, 2019,July 16, 2019,August 6, 2019,September 16, 2019,October 11, 2019,November 5, 2019 and November 8, 2019; and
·: The description of our common shares contained in any registration statement on Form 8-A that we have filed, and any amendment or report filed for the purpose of updating this description.

We are also incorporating by reference any future filings made by us with the SEC under Section 13(a), 13(c), 14 or 15(d) of the Exchange Act after the date of this prospectus supplement until we sell all of the common shares offered by this prospectus supplement (other than portions of such documents that are not deemed "filed" under the Exchange Act in accordance with the Exchange Act and applicable SEC rules). The information contained in any such document will be considered part of this prospectus supplement from the date the document is filed with the SEC.

Any statement made in a document incorporated by reference into this prospectus supplement or the accompanying prospectus is deemed to be modified or superseded for purposes of this prospectus supplement or the accompanying prospectus to the extent that a statement in this prospectus supplement or the accompanying prospectus or in any other subsequently filed document, which is also incorporated by reference, modifies or supersedes such statement. Any statement made in this prospectus supplement or the accompanying prospectus is deemed to be modified or superseded to the extent a statement in any subsequently filed document, which is incorporated by reference into this prospectus supplement or the accompanying prospectus, modifies or supersedes such statement.

You can request or view any of the documents which are incorporated by reference in this prospectus supplement, except for exhibits which are specifically incorporated by reference into those documents, at no cost, by visiting our website atwww.ottertail.com (it being understood that information contained, or accessed through, our website is expressly not incorporated by reference into this prospectus supplement) or by emailing, writing or telephoning us at:

Otter Tail Corporation Shareholder Services Department
215 South Cascade Street, Box 496
Fergus Falls, Minnesota 56538-0496
(800) 664-1259 (toll free)
(218) 739-8479 (locally)

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Prospectus supplement summary

This summary highlights information contained in this prospectus supplement and the accompanying prospectus. Because it is a summary, it does not contain all the information you should consider before investing in our common shares. You should read this entire prospectus supplement and the accompanying prospectus carefully, including the "Risk factors" section and the information incorporated by reference herein and therein, before making an investment decision.

Otter Tail Corporation

Otter Tail Corporation and its subsidiaries conduct business primarily in the United States. We had approximately 2,321 full-time employees at December 31, 2018. Our businesses have been classified into three segments: Electric, Manufacturing and Plastics.

·: Electric includes the production, transmission, distribution and sale of electric energy in Minnesota, North Dakota and South Dakota by Otter Tail Power Company ("OTP"). In addition, OTP is a participant in the Midcontinent Independent System Operator, Inc. markets. OTP's operations have been the Company's primary business since 1907.
·: Manufacturing consists of businesses in the following manufacturing activities: contract machining, metal parts stamping, fabrication and painting, and production of plastic thermoformed horticultural containers, life science and industrial packaging, and material handling components. These businesses have manufacturing facilities in Georgia, Illinois and Minnesota, and sell products primarily in the United States.
·: Plastics consists of businesses producing polyvinyl chloride ("PVC") pipe at plants in North Dakota and Arizona. The PVC pipe is sold primarily in the upper Midwest and Southwest regions of the United States.

OTP is our wholly owned subsidiary. Our manufacturing and plastic pipe businesses are owned by our wholly owned subsidiary, Varistar Corporation.

The chart below indicates the companies included in each of our reporting segments.

Our executive offices are located at 215 South Cascade Street, P.O. Box 496, Fergus Falls, Minnesota 56538-0496 and 4150 19^th Avenue South, Suite 101, P.O. Box 9156, Fargo, North Dakota 58106-9156. Our telephone number is (866) 410-8780.

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The offering


Issuer		Otter Tail Corporation
Common shares offered		Common shares, par value $5 per share, having an aggregate offering price of up to $75,000,000.
Use of proceeds		We intend to use the net proceeds from this offering for general corporate purposes, which may include, among other things, working capital, capital expenditures, debt repayment and the financing of possible acquisitions. See "Use of proceeds."
Conflicts of interest		We may use more than 5% of the net proceeds from the sale of our common shares to repay indebtedness under the Otter Tail Corporation line of credit and the OTP line of credit, under which an affiliate of KeyBanc Capital Markets is a lender. Accordingly, the offering is being made in compliance with the requirements of Rule 5121 of the Financial Industry Regulatory Authority. See "Conflicts of interest."
Risk factors		Investing in our common shares involves a high degree of risk. Please see "Risk factors" beginning on page S-3 of this prospectus supplement and the discussion of risk factors contained in our annual, quarterly and current reports filed with the SEC under the Exchange Act, which are incorporated by reference into this prospectus supplement and the accompanying prospectus, to read about certain factors you should consider before investing in our common shares.
Nasdaq Global Select Market symbol		OTTR
Co-Transfer agent and registrar		Equiniti Shareowner Services

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Risk factors

An investment in our common shares involves risks. You should carefully consider and evaluate all of the risks and uncertainties described below and any information included and incorporated by reference in this prospectus supplement and the accompanying prospectus, including the risk factors incorporated by reference from our most recentannual report on Form 10-K, as updated by our subsequentquarterly reports on Form 10-Q and other filings we make with the SEC. It is possible that our business, financial condition, results of operations or prospects could be materially adversely affected by any of these risks. See also "Cautionary statement regarding forward-looking statements" in this prospectus.

Risks Related to our Common Shares and this Offering

There may be future sales or other dilution of our common shares, which may adversely affect the market price of our common shares.

We may issue common shares having an aggregate offering price of up to $75,000,000 in connection with the distributions contemplated by this prospectus supplement. In addition, we are not restricted from issuing additional common shares, including any securities that are convertible into or exchangeable for, or that represent the right to receive, common shares or any substantially similar securities. The market price of our common shares could decline as a result of sales of a large number of common shares in the market after this offering or the perception that such sales could occur.

Our board of directors has the power to issue series of cumulative preferred shares and cumulative preference shares and to designate the rights and preferences of those series, which could adversely affect the voting, dividend, liquidation and other rights of holders of our common shares.

Under our articles of incorporation, our board of directors has the power to issue series of cumulative preferred shares and cumulative preference shares and to designate the rights and preferences of those series. Therefore, our board of directors may designate a new series of cumulative preferred shares or cumulative preference shares with the rights, preferences and privileges that the board of directors deems appropriate, including special dividend, liquidation and voting rights. The creation and designation of a new series of cumulative preferred shares or cumulative preference shares could adversely affect the voting, dividend, liquidation and other rights of holders of our common shares and, possibly, any other class or series of stock that is then in existence.

The market price of our common shares may be volatile, and this may make it difficult for holders to sell our common shares at the desired time and for the desired price.

The market price of our common shares may fluctuate significantly in response to a number of factors, some of which may be beyond our control. These factors include the perceived prospects and actual operating results of our electric and nonelectric businesses; changes in estimates of our operating results by analysts, investors or our Company; our actual operating results relative to such estimates or expectations; actions or announcements by us or our competitors; litigation and judicial decisions; legislative or regulatory actions; and changes in general economic or market conditions. In addition, the stock market in general has from time to time experienced extreme price and volume fluctuations. These market fluctuations could reduce the market price of our common shares for reasons unrelated to our operating performance.

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Our charter documents and Minnesota law contain provisions that could delay or prevent an acquisition of the Company, which could inhibit your ability to receive a premium on your investment from a possible sale of the Company.

Our charter documents contain provisions that may discourage third parties from seeking to acquire us. These provisions and specific provisions of Minnesota law relating to business combinations with interested shareholders may have the effect of delaying, deterring or preventing a merger or change in control of the Company. Some of these provisions may discourage a future acquisition of the Company even if shareholders would receive an attractive value for their shares or if a significant number of our shareholders believed such a proposed transaction to be in their best interests. As a result, shareholders who desire to participate in such a transaction may not have the opportunity to do so.

The payment of future dividends on our common shares will be subject to the discretion of our board of directors and will depend on the earnings and cash flows of our subsidiaries, as well as other factors.

We have historically paid quarterly dividends on our common shares. Any determination to pay dividends in the future will be at the discretion of our board of directors. In addition, we are a holding company with no significant operations of our own. The primary source of funds for payment of our financial obligations and dividends to our shareholders is cash provided by our subsidiary companies. Our ability to meet our financial obligations and pay dividends on the common shares principally depends on the actual and projected earnings, cash flows, capital requirements and general financial position of our subsidiary companies, as well as regulatory factors, financial covenants, general business conditions and other factors that our board of directors may deem relevant.

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Use of proceeds

We intend to use the net proceeds from the sale of our common shares pursuant to the distribution agreement for general corporate purposes, which may include, among other things, working capital, capital expenditures, debt repayment and the financing of possible acquisitions. Pending application of the net proceeds as described above, we intend to invest the proceeds in highly liquid, investment-grade securities and money market funds.

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Price range of common shares and dividends

The following table shows the high and low intraday sales prices for our common shares as reported on the Nasdaq Global Select Market and the cash dividends per share paid for the periods indicated. Our common shares are traded on the Nasdaq Global Select Market under the symbol "OTTR".

	High		Low		Quarterly Cash Dividends Paid

Fiscal Year 2017
First Quarter	$	40.80	$	35.65	$	.320
Second Quarter		41.95		36.45		.320
Third Quarter		44.50		38.75		.320
Fourth Quarter		48.65		43.30		.320
Fiscal Year 2018
First Quarter	$	44.95	$	39.00	$	.335
Second Quarter		48.75		42.30		.335
Third Quarter		49.75		46.85		.335
Fourth Quarter		51.88		44.22		.335
Fiscal Year 2019
First Quarter	$	51.70	$	45.94	$	.350
Second Quarter		53.345		48.48		.350
Third Quarter		55.10		48.09		.350

On November 7, 2019, the last reported sale price of common shares on the Nasdaq Global Select Market was $49.95 per share. As of November 7, 2019, there were approximately 12,545 registered holders of record of our common shares.

We have paid dividends on our common shares each quarter since 1938 without interruption or reduction. We presently intend to pay quarterly cash dividends, subject to our earnings and financial condition, regulatory requirements and such other factors as our board of directors may deem relevant.

Through our Automatic Dividend Reinvestment and Share Purchase Plan, we offer our common shareholders, retail customers of OTP and other interested investors the opportunity to purchase our common shares without paying any brokerage fees or service charges. Current common shareholders can purchase additional common shares by reinvesting all of their dividends and/or making optional cash investments. Customers of OTP who are not currently our shareholders and other interested investors can purchase common shares and thereafter participate in this plan by enrolling with a minimum initial investment of $100 (for customers of OTP) or $250 (for other interested investors). At November 7, 2019, approximately 82% of eligible common shareholders holding approximately 11% of our eligible common shares participated in the dividend reinvestment portion of this plan.

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Material U.S. federal tax considerations for
non-U.S. holders of common shares

The following is a summary of U.S. federal income tax considerations relating to the purchase, ownership, and disposition of our common shares. This summary is based on the U.S. Internal Revenue Code of 1986 (the "Code") and the Treasury Regulations promulgated thereunder, judicial decisions, and published rulings and administrative pronouncements of the Internal Revenue Service (the "IRS"), all as of the date hereof and all of which are subject to change, possibly with retroactive effect.

Prospective purchasers of our common shares should consult their own tax advisors with respect to the U.S. federal, state, local, and other tax consequences of purchasing, owning, or disposing of our common shares.

This summary is for general information only and is not intended to constitute a complete analysis of all U.S. federal income tax considerations relating to the purchase, ownership, and disposition of our common shares. It does not address alternative minimum tax consequences or the additional tax on net investment income, nor does it address the U.S. federal estate and gift tax, or any state, local, or non-U.S. tax consequences. This summary applies only to Non-U.S. Holders (as defined below) that hold our common shares as "capital assets" (generally, property held for investment).

This discussion does not address all aspects of U.S. federal income taxation that may be relevant to particular holders of our common shares in light of their specific circumstances or the tax considerations applicable to holders that may be subject to special U.S. federal income tax rules, such as: certain accrual-method taxpayers subject to special tax accounting rules under Section 451(b) of the Code; financial institutions; insurance companies; real estate investment trusts; regulated investment companies; brokers, dealers or traders in stocks, securities, or currencies or notional principal contracts; tax-exempt entities; former citizens or long-term residents of the United States; persons that will hold our common shares as part of a straddle, hedge, conversion, constructive sale, or other integrated transaction for U.S. federal income tax purposes; controlled foreign corporations; passive foreign investment companies; and partnerships or other pass-through entities. If a partnership (or any other entity or arrangement treated as a partnership for U.S. federal income tax purposes) holds our common shares, the tax treatment of a partner in that partnership will generally depend on the status of the partner and the activities of the partnership. Partnerships considering an investment in our common shares, and the partners in those partnerships, should consult their own tax advisors.

For purposes of this discussion, a "Non-U.S. Holder" means a beneficial owner of our common shares that is not, for U.S. federal income tax purposes:

·: an individual who is a citizen or resident of the United States;
·: a corporation created or organized in or under the laws of the United States, any state thereof, or the District of Columbia;
·: an estate the income of which is subject to U.S. federal income taxation regardless of its source; or
·: a trust, if (i) a court within the United States is able to exercise primary supervision over the administration of the trust and one or more United States persons have the authority to control

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all substantial decisions of the trust or (ii) the trust has a valid election in effect under applicable Treasury Regulations to be treated as a United States person.

Distributions

Distributions of cash or property with respect to our common shares will constitute dividends for U.S. federal income tax purposes to the extent paid from our current or accumulated earnings and profits, as determined under U.S. federal income tax principles. To the extent a distribution exceeds both our current and our accumulated earnings and profits, it will constitute a return of capital and will be applied against and reduce the holder's adjusted tax basis in our common shares, but not below zero. Any remainder will be treated as gain from the sale or exchange of our common shares and will be subject to U.S. federal income tax in the manner described below under "—Gain on Disposition of Common Shares."

Dividends paid to a Non-U.S. Holder generally will be subject to U.S. withholding tax at a rate of 30% of the gross amount, unless the Non-U.S. Holder is eligible for and properly claims a reduced rate of withholding under an applicable income tax treaty. However, dividends that are effectively connected with the conduct of a trade or business by the Non-U.S. Holder within the United States (and, if required by an applicable income tax treaty, are attributable to a United States permanent establishment of the Non-U.S. Holder) will not be subject to U.S. withholding tax, provided certain certification and disclosure requirements are satisfied. Instead, such dividends are subject to U.S. federal income tax on a net income basis in the same manner as if the Non-U.S. Holder were a United States person, as defined under the Code. Any such effectively connected dividends received by a foreign corporation may be subject to an additional "branch profits tax" equal to 30% of its effectively connected earnings and profits (subject to certain adjustments) or at such lower rate as may be specified by an applicable income tax treaty.

A Non-U.S. Holder eligible for a reduced rate of U.S. withholding tax pursuant to an income tax treaty may obtain a refund of any excess amounts withheld by timely filing an appropriate claim for refund with the IRS.

Gain on Disposition of Common Shares

Subject to the remainder of this discussion, any gain realized by a Non-U.S. Holder on the disposition of our common shares generally will not be subject to U.S. federal income tax unless:

·: the gain is effectively connected with the conduct of a trade or business by the Non-U.S. Holder within the United States (and, if required by an applicable income tax treaty, is attributable to a United States permanent establishment of the Non-U.S. Holder);
·: the Non-U.S. Holder is an individual who is present in the United States for 183 days or more in the taxable year of that disposition and certain other conditions are met; or
·: Otter Tail Corporation is or has been a "United States real property holding corporation" for U.S. federal income tax purposes at any time during the shorter of the five-year period ending on the date of disposition or the period that the Non-U.S. Holder held our common shares.

A Non-U.S. Holder with gain described in the first bullet point immediately above will be subject to tax on the net gain derived from the disposition of our common shares in the same manner as if the Non-U.S. Holder were a United States person, as defined under the Code. If a Non-U.S. Holder that is a foreign corporation falls under the first bullet point immediately above, it may also be subject to

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the "branch profits" tax equal to 30% of its effectively connected earnings and profits (subject to certain adjustments) or at such lower rate as may be specified by an applicable income tax treaty.

Except as otherwise provided by an applicable income tax treaty, an individual Non-U.S. Holder described in the second bullet point immediately above will be subject to a flat 30% tax on the gain derived from the disposition, which may be offset by United States source capital losses recognized in the same taxable year by the individual, even though the individual is not considered a resident of the United States under the Code.

We do not believe that Otter Tail Corporation is or has been, and do not expect Otter Tail Corporation to become, a United States real property holding corporation for U.S. federal income tax purposes.

Information Reporting and Backup Withholding

Annual reporting to the IRS and to each Non-U.S. Holder is required as to the amount of distributions (other than certain distributions treated as gain on a sale of stock) paid to such Non-U.S. Holder and the amount, if any, of tax withheld with respect to such distributions. This information may also be made available to the tax authorities of the country in which the Non-U.S. Holder resides or is established. Dividends generally are not subject to "backup withholding" if the Non-U.S. Holder properly certifies as to its non-U.S. status (usually by completing an IRS Form W-8BEN, Form W-8BEN-E, or Form W-8ECI). Payments of proceeds from the sale or other disposition of our common shares generally will be subject to information reporting if the disposition is effected within the United States or through certain U.S.-related financial intermediaries unless the Non-U.S. Holder provides the applicable withholding agent with a statement certifying, among other things, that the Non-U.S. Holder is not a "United States person" within the meaning of the Code (generally on IRS Form W-8BEN or W-8BEN-E). Payments subject to information reporting may be subject to backup withholding if the Non-U.S. Holder fails to certify the holder's non-U.S. status as described above. Backup withholding is not an additional tax. Any amounts withheld under the backup withholding rules may be allowed as a refund or a credit against the Non-U.S. Holder's U.S. federal income tax liability, if any, provided that the required information is timely furnished to the IRS. Certain holders are exempt from information reporting and backup withholding. Potential holders should consult their own tax advisors regarding qualification for an exemption and the procedures for obtaining such an exemption.

Foreign Account Tax Compliance

Sections 1471 through 1474 of the Code (commonly referred to as "FATCA") impose a separate reporting regime and potentially a 30% withholding tax on certain payments, including payments of dividends on our common shares. Withholding under FATCA generally applies to payments made to or through a foreign entity if such entity fails to satisfy certain disclosure and reporting rules. These rules generally require (i) in the case of a foreign financial institution, that the financial institution agree to identify and provide information in respect of financial accounts held (directly or indirectly) by U.S. persons and U.S.-owned entities, and, in certain instances, to withhold on payments to account holders that fail to provide the required information, and (ii) in the case of a non-financial foreign entity, that the entity either identify and provide information in respect of its substantial U.S. owners or certify that it has no such U.S. owners.

FATCA withholding also potentially applies to payments of gross proceeds from the sale or other disposition of our common shares. Proposed regulations, however, would eliminate FATCA

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withholding on such payments, and the U.S. Treasury Department has indicated that taxpayers may rely on this aspect of the proposed regulations until final regulations are issued.

Non-U.S. Holders typically will be required to furnish certifications (generally on the applicable IRS Form W-8) or other documentation to provide the information required by FATCA or to establish compliance with or an exemption from withholding under FATCA. FATCA withholding may apply where payments are made through a non-U.S. intermediary that is not FATCA compliant, even where the Non-U.S. Holder satisfies the holder's own FATCA obligations.

The United States and a number of other jurisdictions have entered into intergovernmental agreements to facilitate the implementation of FATCA. Any applicable intergovernmental agreement may alter one or more of the FATCA information reporting and withholding requirements. You are encouraged to consult with your own tax advisor regarding the possible implications of FATCA on your investment in our common shares, including the applicability of any intergovernmental agreements.

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Plan of distribution

We have entered into a distribution agreement with KeyBanc Capital Markets under which we may issue and sell from time to time common shares having an aggregate offering price of up to $75,000,000 through KeyBanc Capital Markets as our distribution agent. Sales of our common shares, if any, will be made by means of ordinary brokers' transactions on the Nasdaq Global Select Market at market prices. As agent, KeyBanc Capital Markets will not engage in any transactions that stabilize our common shares.

KeyBanc Capital Markets will offer our common shares subject to the terms and conditions of the distribution agreement on a daily basis or as otherwise agreed upon by us and KeyBanc Capital Markets. We will designate the maximum amount of our common shares to be sold through KeyBanc Capital Markets on a daily basis or otherwise determine such maximum amount together with KeyBanc Capital Markets. Subject to the terms and conditions of the distribution agreement, KeyBanc Capital Markets will use its commercially reasonable efforts as the distribution agent to sell on our behalf all of the designated common shares. We may instruct KeyBanc Capital Markets not to sell our common shares if the sales cannot be effected at or above the price designated by us in any such instruction. KeyBanc Capital Markets will not be obligated to use reasonable efforts to sell shares at any price below the designated price. We or KeyBanc Capital Markets may suspend the offering of our common shares under the distribution agreement upon proper notice and subject to other conditions.

KeyBanc Capital Markets will receive from us a commission equal to up to 2% of the gross sales price per share for any shares sold through it as our distribution agent under the distribution agreement. The remaining sales proceeds, after deducting any expenses payable by us and any transaction fees imposed by any governmental, regulatory or self-regulatory organization in connection with the sales, will equal our net proceeds for the sale of such shares. We have agreed to pay for certain of KeyBanc Capital Markets's expenses in certain circumstances.

KeyBanc Capital Markets will provide written confirmation to us following the close of trading on the Nasdaq Global Select Market each day on which our common shares are sold by it for us under the distribution agreement. Each confirmation will include the number of shares sold on that day, the gross sales price per share and the net proceeds to us.

Settlement for sales of our common shares will occur, unless the parties agree otherwise, on the second business day following the date on which any sales were made in return for payment of the net proceeds to us. There is no arrangement for funds to be received in an escrow, trust or similar arrangement.

Under the terms of the distribution agreement, we also may sell common shares to KeyBanc Capital Markets as principal for its own account, at a price agreed upon at the time of sale. If we sell shares to KeyBanc Capital Markets as principal, we will enter into a separate terms agreement with KeyBanc Capital Markets, and we will describe such agreement in a separate prospectus supplement or pricing supplement.

We will report in a prospectus supplement and/or our filings under the Exchange Act at least quarterly the number of common shares sold through KeyBanc Capital Markets under the distribution agreement, the net proceeds to us and the compensation paid by us to KeyBanc Capital Markets in connection with the sales of our common shares.

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In connection with the sale of common shares on our behalf, KeyBanc Capital Markets may be deemed to be an "underwriter" within the meaning of the Securities Act of 1933, as amended (the "Securities Act"), and the compensation paid to KeyBanc Capital Markets may be deemed to be underwriting commissions or discounts. We have agreed in the distribution agreement to provide indemnification and contribution to KeyBanc Capital Markets against certain civil liabilities, including liabilities under the Securities Act.

KeyBanc Capital Markets and its affiliates have engaged in, and may in the future engage in, investment banking and other commercial dealings in the ordinary course of business with us or our affiliates. They have received or may in the future receive, customary fees and commissions for these transactions. An affiliate of KeyBanc Capital Markets is a lender under the Otter Tail Corporation line of credit and the OTP line of credit. Such affiliate may receive a portion of the proceeds from the sale of our common shares hereunder in the event that such proceeds are used to repay our loans under the Otter Tail Corporation line of credit or the OTP line of credit.

If either KeyBanc Capital Markets or we have reason to believe that the exemptive provisions set forth in Rule 101(c)(1) of Regulation M under the Exchange Act are not satisfied, that party will promptly notify the other and sales of our common shares under the distribution agreement will be suspended until that or other exemptive provisions have been satisfied in the judgment of KeyBanc Capital Markets and us.

The offering of our common shares pursuant to the distribution agreement will terminate upon the earlier of (i) the sale of all common shares subject to the distribution agreement or (ii) the termination of the distribution agreement by us or by KeyBanc Capital Markets.

We estimate that the total expenses of the offering payable by us, excluding discounts and commissions payable to KeyBanc Capital Markets under the distribution agreement, will be approximately $150,000.

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Conflicts of interest

We may use more than 5% of the net proceeds from the sale of our common shares to repay indebtedness under the Otter Tail Corporation line of credit and the OTP line of credit, under which an affiliate of KeyBanc Capital Markets is a lender. Accordingly, the offering is being made in compliance with the requirements of Rule 5121 of the Financial Industry Regulatory Authority's conduct rules. In accordance with Rule 5121, if KeyBanc Capital Markets receives more than 5% of the net proceeds from this offering, it will not sell our common shares to a discretionary account without receiving the written approval from the account holder.

Legal matters

The validity of the common shares offered hereby will be passed upon for us by Dorsey & Whitney LLP. Certain legal matters in connection with this offering will be passed upon for KeyBanc Capital Markets by Davis Polk & Wardwell LLP.

Experts

The consolidated financial statements and the related financial statement schedule, incorporated in this prospectus supplement and the accompanying prospectus by reference from Otter Tail Corporation'sAnnual Report on Form 10-K, and the effectiveness of Otter Tail Corporation's internal control over financial reporting have been audited by Deloitte & Touche LLP, an independent registered public accounting firm, as stated in their report, which is incorporated herein by reference. Such consolidated financial statements and financial statement schedule have been so incorporated in reliance upon the report of such firm given upon their authority as experts in accounting and auditing.

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PROSPECTUS

Common Shares
Cumulative Preferred Shares
Depositary Shares
Debt Securities
Securities Warrants
Units

We may offer for sale, from time to time, either separately or together in any combination, the securities described in this prospectus. This prospectus provides you with a general description of the securities we may offer. Each time we sell any of these securities, we will provide one or more prospectus supplements containing specific information about the terms of that offering. The prospectus supplements may also add, update or change information contained in this prospectus. If information in the prospectus supplement is inconsistent with the information in this prospectus, then the information in the prospectus supplement will apply and will supersede the information in this prospectus. You should carefully read both this prospectus and any prospectus supplement together with additional information described under the heading "Where You Can Find More Information" before you invest.

We may offer and sell these securities directly or to or through underwriters, agents or dealers. The supplements to this prospectus will describe the terms of any particular plan of distribution including names of any underwriters, agents or dealers.

This prospectus may not be used to carry out sales of securities unless accompanied by a prospectus supplement.

Our common shares are traded on the Nasdaq Global Select Market under the symbol "OTTR."

Investing in our securities involves risks. See "Risk Factors" beginning on page 4 of this prospectus and, if applicable, any risk factors described in any applicable prospectus supplement and in our Securities and Exchange Commission filings that are incorporated by reference in this prospectus.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or determined if this prospectus is truthful or complete. Any representation to the contrary is a criminal offense.

The date of this prospectus is May 3, 2018.

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ABOUT THIS PROSPECTUS		2
WHERE YOU CAN FIND MORE INFORMATION		2
RISK FACTORS		4
OTTER TAIL CORPORATION		5
USE OF PROCEEDS		6
RATIO OF EARNINGS TO FIXED CHARGES AND TO FIXED CHARGES AND PREFERRED DIVIDEND REQUIREMENTS		6
DESCRIPTION OF COMMON SHARES		6
DESCRIPTION OF CUMULATIVE PREFERRED SHARES		10
DESCRIPTION OF DEPOSITARY SHARES		15
DESCRIPTION OF DEBT SECURITIES	��	19
DESCRIPTION OF SECURITIES WARRANTS		31
DESCRIPTION OF UNITS		32
PLAN OF DISTRIBUTION		33
VALIDITY OF SECURITIES		34
EXPERTS		34

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ABOUT THIS PROSPECTUS

This prospectus is part of an automatic shelf registration statement on Form S-3 that we filed with the Securities and Exchange Commission ("SEC") as a "well-known seasoned issuer" as defined in Rule 405 under the Securities Act of 1933, as amended ("Securities Act"). By using this shelf registration statement, we may, at any time and from time to time, in one or more offerings, sell any of the securities described in this prospectus. The registration statement that contains this prospectus (including the exhibits to the registration statement) contains additional information about us and the securities we are offering under this prospectus. You can read that registration statement at the SEC web site atwww.sec.gov or at the SEC office mentioned under the heading "Where You Can Find More Information."

This prospectus provides you with a general description of the securities we may offer. Each time we offer any of these securities, we will provide one or more prospectus supplements containing specific information about the terms of that offering. The prospectus supplements may also add, update or change information contained in this prospectus. If information in the prospectus supplement is inconsistent with the information in this prospectus, then the information in the prospectus supplement will apply and will supersede the information in this prospectus. You should carefully read both this prospectus and any prospectus supplement together with additional information described under the heading "Where You Can Find More Information" before you invest.

All references in this prospectus to "Otter Tail," "we," "us," "our," "our company" and "the corporation" are to Otter Tail Corporation including our consolidated subsidiaries, unless otherwise indicated or the context otherwise requires. All references in this prospectus to "$," "U.S. Dollars" and "dollars" are to United States dollars.

You should rely only on the information provided or incorporated by reference in this prospectus, any prospectus supplement or any free writing prospectus prepared by us or on our behalf. We have not authorized anyone else to provide you with different or additional information. If anyone provides you with additional or different information, you should not rely on it. We are not making an offer of these securities in any jurisdiction where the offer is not permitted. You should not assume that the information in this prospectus, any prospectus supplement or any free writing prospectus is accurate as of any date other than the date on the front of those documents. Our business, properties, financial condition, results of operation and prospects may have changed since those dates.

WHERE YOU CAN FIND MORE INFORMATION

We file annual, quarterly and current reports, proxy statements and other information with the SEC. Our SEC filings are available to the public through the Internet at the SEC's web site atwww.sec.gov. You may also read and copy any document we file with the SEC at the SEC's public reference room at 100 F Street N.E., Washington, D.C. 20549. Please call the SEC at 1-800-SEC-0330 for further information about its public reference facilities and their copy charges.

The SEC allows us to incorporate by reference the information we file with them. This allows us to disclose important information to you by referencing those filed documents. We have previously filed the following documents with the SEC and are incorporating them by reference into this prospectus (other than any portions of such documents that are not deemed "filed" under the Securities Exchange Act of 1934, as amended (the "Exchange Act"), in accordance with the Exchange Act and applicable SEC rules):

•: Annual Report onForm 10-K for the year ended December 31, 2017, as amended by Amendment No. 1 thereto onForm 10-K/A filed on February 22, 2018;
•: Current Reports on Form 8-K filed onFebruary 13, 2018,April 10, 2018 andApril 20, 2018; and

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•: the description of our common shares contained in any registration statement on Form 8-A that we have filed, and any amendment or report filed for the purpose of updating this description.

We also are incorporating by reference any future filings made by us with the SEC under Section 13(a), 13(c), 14 or 15(d) of the Exchange Act after the date of this prospectus and prior to the expiration or termination of the registration statement of which this prospectus is a part (other than portions of such documents that are not deemed "filed" under the Exchange Act, in accordance with the Exchange Act and applicable SEC rules). The information contained in any such document will be considered part of this prospectus from the date the document is filed with the SEC.

Any statement made in a document incorporated by reference into this prospectus is deemed to be modified or superseded for purposes of this prospectus to the extent that a statement in this prospectus or in any other subsequently filed document, which is also incorporated by reference, modifies or supersedes such statement. Any statement made in this prospectus is deemed to be modified or superseded to the extent a statement in any subsequently filed document, which is incorporated by reference into this prospectus, modifies or supersedes such statement.

You can obtain a copy of any documents which are incorporated by reference in this prospectus or the accompanying prospectus supplement, except for exhibits which are specifically incorporated by reference into those documents, at no cost, by writing or telephoning us at:

Otter Tail Corporation
Shareholder Services Department
215 South Cascade Street, Box 496
Fergus Falls, Minnesota 56538-0496
(800) 664-1259 (toll free)
(218) 739-8479 (locally)

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RISK FACTORS

Our business is subject to various risks and uncertainties. You should carefully consider and evaluate all of the information included and incorporated by reference in this prospectus, including the risk factors incorporated by reference from our most recent annual report on Form 10-K, as updated by our subsequent quarterly reports on Form 10-Q and other filings we make with the SEC. It is possible that our business, financial condition, liquidity or results of operations could be materially adversely affected by any of these risks. The applicable prospectus supplement for any securities we may offer may contain a discussion of additional risks applicable to an investment in us and the particular type of securities we are offering under that prospectus supplement.

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OTTER TAIL CORPORATION

Otter Tail Corporation and its subsidiaries conduct business primarily in the United States. We had approximately 2,097 full-time employees at December 31, 2017. Our businesses have been classified into three segments: Electric, Manufacturing and Plastics.

•: Electric includes the production, transmission, distribution and sale of electric energy in Minnesota, North Dakota and South Dakota by Otter Tail Power Company ("OTP"). In addition, OTP is a participant in the Midcontinent Independent System Operator, Inc. markets. OTP's operations have been the corporation's primary business since 1907.
•: Manufacturing consists of businesses in the following manufacturing activities: contract machining, metal parts stamping, fabrication and painting, and production of plastic thermoformed horticultural containers, life science and industrial packaging, and material handling components. These businesses have manufacturing facilities in Georgia, Illinois and Minnesota and sell products primarily in the United States.
•: Plastics consists of businesses producing polyvinyl chloride ("PVC") pipe at plants in North Dakota and Arizona. The PVC pipe is sold primarily in the upper Midwest and Southwest regions of the United States.

OTP is a wholly owned subsidiary of Otter Tail Corporation. The corporation's manufacturing and plastic pipe businesses are owned by our wholly owned subsidiary, Varistar Corporation.

Otter Tail Corporation was incorporated in June 2009 under the laws of the State of Minnesota in connection with our holding company reorganization on July 1, 2009. As a result of the reorganization, Otter Tail Power Company, which had previously been operated as a division of Otter Tail Corporation, became a wholly owned subsidiary of the new parent holding company named Otter Tail Corporation. Our executive offices are located at 215 South Cascade Street, P.O. Box 496, Fergus Falls, Minnesota 56538-0496 and 4150 19^th Ave. South, Suite 101, P.O. Box 9156, Fargo, North Dakota 58106-9156. Our telephone number is (866) 410-8780.

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USE OF PROCEEDS

Unless the applicable prospectus supplement states otherwise, we will use the net proceeds we receive from the sale of the securities for general corporate purposes, which may include, among other things, working capital, capital expenditures, debt repayment or refinancing, the financing of possible acquisitions or stock repurchases. The prospectus supplement relating to a particular offering of securities by us will identify the use of proceeds for that offering.

RATIO OF EARNINGS TO FIXED CHARGES AND
TO FIXED CHARGES AND PREFERRED DIVIDEND REQUIREMENTS

Our consolidated ratio of earnings to fixed charges and of earnings to fixed charges and preferred dividend requirements for the periods indicated are as follows:

	Year Ended December 31,

	2013		2014		2015		2016		2017
Ratio of Earnings to Fixed Charges		3.36		3.29		3.38		3.39		4.10
Ratio of Earnings to Fixed Charges and Preferred Dividend Requirements		3.29		3.29		3.38		3.39		4.10

For purposes of computing the ratios, earnings consist of consolidated income from continuing operations before income taxes plus fixed charges. Fixed charges consist of interest on long-term debt, amortization of debt expense, premium and discount, and the portion of interest expense on operating leases we believe to be representative of the interest factor. All outstanding cumulative preferred shares were redeemed on March 1, 2013 for $15.7 million, including $0.2 million in call premiums charged to equity and included as preferred dividends paid and as part of our preferred dividend requirement for the year ended December 31, 2013.

DESCRIPTION OF COMMON SHARES

This section summarizes the general terms of the common shares that we may offer using this prospectus. The following description is only a summary and does not purport to be complete and is qualified by reference to our restated articles of incorporation (the "articles") and restated bylaws (the "bylaws"). Our articles and bylaws have been incorporated by reference as exhibits to the registration statement of which this prospectus is a part. See "Where You Can Find More Information" for information on how to obtain copies.

General

Our articles currently authorize the issuance of three classes of shares:

•: cumulative preferred shares, without par value (1,500,000 shares authorized),
•: cumulative preference shares, without par value (1,000,000 shares authorized), and
•: common shares, par value $5 per share (50,000,000 shares authorized).

As of December 31, 2017, there were outstanding no cumulative preferred shares, no cumulative preference shares and 39,557,491 common shares.

The board of directors is authorized to provide for the issue from time to time of cumulative preferred shares and cumulative preference shares in series and, as to each series, to fix the designation, annual dividend rate, quarterly dividend payment dates, redemption price or prices, voluntary and involuntary liquidation prices, conversion provisions, if any, and sinking fund provisions, if any, applicable to the shares of such series. As a result, our board of directors could, without shareholder approval, authorize the issuance of cumulative preferred shares or cumulative preference

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shares with dividend, redemption or conversion provisions that could have an adverse effect on the availability of earnings for distribution to the holders of common shares, or with voting, conversion or other rights that could proportionately reduce, minimize or otherwise adversely affect the voting power and other rights of holders of common shares. See "Description of Cumulative Preferred Shares."

The common shares are not entitled to any conversion or redemption rights. Holders of common shares do not have any preemptive right to subscribe for additional securities we may issue. Our outstanding common shares are, and any newly issued common shares will be, fully paid and non-assessable. The transfer agent and registrar for the common shares is EQ Shareowner Services.

Dividend Rights

Subject to the prior dividend rights of the holders of the cumulative preferred shares and the cumulative preference shares and the other limitations set forth in the following paragraphs, dividends may be declared by the board of directors and paid from time to time upon the outstanding common shares from any funds legally available therefor.

We and our subsidiaries are parties to agreements pursuant to which we borrow money, and certain covenants in these agreements may limit our ability to pay dividends or other distributions with respect to the common shares or to repurchase common shares. In addition, we and our subsidiaries may become parties to future agreements that contain such restrictions. These covenants will be described in more detail in the prospectus supplement relating to any common shares that we offer using this prospectus.

If, and so long as, any cumulative preferred shares are outstanding, we shall not, without the consent of the holders of a majority of the aggregate voting power of the cumulative preferred shares of all series then outstanding (two-thirds if more than one-fourth vote negatively), declare, pay or set apart for payment any dividend on or purchase, redeem or otherwise acquire any common shares unless, after giving effect thereto, Common Share Equity shall equal at least 25% of Total Capitalization and our earned surplus shall not be less than $831,398.

"Common Share Equity" is the sum of

•: our stated capital applicable to our common shares and to all other shares ranking junior to the cumulative preferred shares with respect to the payment of dividends or the distribution of assets (collectively "Subordinate Shares"), including any shares proposed to be issued substantially contemporaneously,
•: capital surplus to the extent of premium on our common shares and on all other Subordinate Shares, including any premium on any shares proposed to be issued substantially contemporaneously,
•: contributions in aid of construction, and
•: earned surplus,

all determined in accordance with such system of accounts as may be prescribed by governmental authorities having jurisdiction in the premises or, in the absence thereof, in accordance with generally accepted accounting practice.

"Total Capitalization" means the sum of

•: the Common Share Equity,
•: the involuntary liquidation preference of all cumulative preferred shares and all other shares prior to or on a parity with the cumulative preferred shares to be outstanding after the proposed event, and

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•: the principal amount of all interest bearing debt (including debt to which property theretofore acquired or to be acquired substantially contemporaneously is or will be subject) to be outstanding after the proposed event, excluding, however, all indebtedness maturing by its terms within one year from the time of creation thereof unless we, without the consent of the lender, have the right to extend the maturity of such indebtedness for a period or periods which, with the original period of such indebtedness, aggregates one year or more.

Moreover, no dividend shall be declared, paid or set apart for payment on the common shares (other than a dividend or distribution payable solely in common shares) nor shall any common shares be purchased or acquired by us at any time while there is a default or deficiency with respect to a sinking or purchase fund established for the benefit of any series of the cumulative preferred shares or the cumulative preference shares.

Voting Rights

Subject to the rights of the holders of the cumulative preferred shares, as described under "Description of Cumulative Preferred Shares—Voting Rights," and the cumulative preference shares, as described below, only the holders of common shares have voting rights and are entitled to one vote for each share held.

In the event that four full quarterly dividend payments on the cumulative preference shares of any series shall be in default, the holders of the cumulative preference shares of all series at the time outstanding, voting as a class, shall thereafter elect two members of an eleven member board of directors. After any such default shall have been cured, the cumulative preference shares, as the case may be, shall be divested of such voting rights, subject to being revested in the event of subsequent such defaults.

The consent of the holders of at least two-thirds of the aggregate voting power of the cumulative preference shares of all series then outstanding is required to

•: create or authorize any shares of any class (other than the cumulative preferred shares, whether now or hereafter authorized) ranking prior to the cumulative preference shares as to dividends or assets, or
•: amend our articles so as to affect adversely any of the preferences or other rights of the cumulative preference shares, provided that if less than all series of cumulative preference shares are so affected, only the consent of the holders of at least two-thirds of the aggregate voting power of the affected series shall be required.

A majority (two-thirds if more than one-fourth vote negatively) of the aggregate voting power of the cumulative preference shares of all series then outstanding is required to

•: increase the number of authorized cumulative preference shares or create or authorize any shares of any class ranking on a parity with the cumulative preference shares as to dividends or assets, or
•: consolidate or merge into or with any other corporation or corporations or sell, lease or exchange all or substantially all of our property and assets unless specified conditions are met.

Liquidation Rights

Upon any liquidation, dissolution or winding up of the corporation, the holders of common shares shall be entitled to receive pro rata all assets of the corporation distributable to shareholders after the payment of the respective liquidation preferences to the holders of the cumulative preferred shares and the cumulative preference shares.

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Minnesota Anti-Takeover Laws

We are governed by the provisions of Sections 302A.671, 302A.673 and 302A.675 of the Minnesota Business Corporation Act. These provisions may discourage a negotiated acquisition or unsolicited takeover of us and deprive our shareholders of an opportunity to sell their shares at a premium over the market price.

In general, Section 302A.671 provides that a public Minnesota corporation's shares acquired in a control share acquisition have no voting rights unless voting rights are approved in a prescribed manner. A "control share acquisition" is a direct or indirect acquisition of beneficial ownership of shares that would, when added to all other shares beneficially owned by the acquiring person, entitle the acquiring person to have voting power of 20% or more in the election of directors.

In general, Section 302A.673 prohibits a public Minnesota corporation from engaging in a business combination with an interested shareholder for a period of four years after the date of the transaction in which the person became an interested shareholder, unless the business combination is approved in a prescribed manner. The term "business combination" includes mergers, asset sales and other transactions resulting in a financial benefit to the interested shareholder. An "interested shareholder" is a person who is the beneficial owner, directly or indirectly, of 10% or more of a corporation's voting stock, or who is an affiliate or associate of the corporation, and who, at any time within four years before the date in question, was the beneficial owner, directly or indirectly, of 10% or more of the corporation's voting stock. Section 302A.673 does not apply if a committee of our board of directors consisting of one or more of our disinterested directors (excluding directors who are our current and former officers and employees) approves the proposed transaction or the interested shareholder's acquisition of shares before the share acquisition date, or on the share acquisition date but before the interested shareholder becomes an interested shareholder.

If a takeover offer is made for our shares, Section 302A.675 of the Minnesota Business Corporation Act precludes the offeror from acquiring additional shares of stock (including in acquisitions pursuant to mergers, consolidations or statutory share exchanges) within two years following the completion of the takeover offer, unless shareholders selling their shares in the later acquisition are given the opportunity to sell their shares on terms that are substantially the same as those contained in the earlier takeover offer. A "takeover offer" is a tender offer which results in an offeror who owned 10% or less of a class of our shares acquiring more than 10% of that class, or which results in the offeror increasing its beneficial ownership of a class of our shares by more than 10% of the class, if the offeror owned 10% or more of the class before the takeover offer. Section 302A.675 does not apply if a committee of our board of directors approves the proposed acquisition before any shares are acquired pursuant to the earlier tender offer. The committee must consist solely of directors who were directors or nominees for our board of directors at the time of the first public announcement of the takeover offer, and who are not our current or former officers and employees, offerors, affiliates or associates of the offeror or nominees for our board of directors by the offeror or an affiliate or associate of the offeror.

Certain Provisions of Articles and Bylaws

Except at such times when holders of cumulative preferred shares and/or cumulative preference shares have special voting rights for the election of directors as described in this prospectus, our directors are elected for three-year, staggered terms by the holders of the common shares. Cumulative voting of the common shares in the election of directors is prohibited. In addition, our bylaws provide that a vote of 75% of the common shares is required to remove directors who have been elected by the holders of common shares. The affirmative vote of 75% of the common shares is required to amend provisions of our articles and bylaws relating to the staggered terms and the removal of directors, unless approved by all of the continuing directors as specified therein.

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Our articles contain "fair price" provisions which require the affirmative vote of 75% of the voting power of the common shares to approve business combinations, including mergers, consolidations and sales of a substantial part of our assets, with an interested shareholder or its affiliates or associates, unless specified price criteria and procedural requirements are met or unless the transaction is approved by the majority of the continuing directors. Our articles also contain "anti-greenmail" provisions which preclude us from making certain purchases of common shares at a price per share in excess of the fair market price from a substantial shareholder unless approved by the affirmative vote of 66²/3% of the voting power of the common shares held by the disinterested shareholders. The "fair price" and "anti-greenmail" provisions of our articles may not be amended without the affirmative vote of the holders of at least 75% of the voting power of the common shares, unless approved by all of the continuing directors as specified therein.

The overall effect of the foregoing provisions of our articles and bylaws, together with the ability of the board of directors to issue additional common shares, cumulative preferred shares and cumulative preference shares, may be to delay or prevent attempts by other persons or entities to acquire control of the corporation without negotiations with our board of directors.

DESCRIPTION OF CUMULATIVE PREFERRED SHARES

This section summarizes the general terms and provisions of the cumulative preferred shares that we may offer using this prospectus. This section is only a summary and does not purport to be complete. You must look at our articles and the relevant certificate of designation for a full understanding of all the rights and preferences of any series of cumulative preferred shares. Our articles and the certificates of designation have been or will be filed or incorporated by reference as exhibits to the registration statement of which this prospectus is a part. See "Where You Can Find More Information" for information on how to obtain copies.

A prospectus supplement will describe the specific terms of any particular series of cumulative preferred shares offered under that prospectus supplement, including any of the terms in this section that will not apply to that series of cumulative preferred shares, and any special considerations, including tax considerations, applicable to investing in that series of cumulative preferred shares.

General

As discussed above, our articles currently authorize the issuance of three classes of shares:

•: cumulative preferred shares, without par value (1,500,000 shares authorized),
•: cumulative preference shares, without par value (1,000,000 shares authorized), and
•: common shares, par value $5 per share (50,000,000 shares authorized).

As of December 31, 2017, there were outstanding no cumulative preferred shares, no cumulative preference shares and 39,557,491 common shares.

The board of directors is authorized to provide for the issue from time to time of cumulative preferred shares and cumulative preference shares in series and, as to each series, to fix the designation, annual dividend rate, quarterly dividend payment dates, redemption price or prices, voluntary and involuntary liquidation prices, conversion provisions, if any, and sinking fund provisions, if any, applicable to the shares of such series. The cumulative preferred shares are senior to the cumulative preference shares and the common shares as to dividend and liquidation rights.

Any cumulative preferred shares will, when issued, be fully paid and nonassessable. Holders of cumulative preferred shares do not have any preemptive right to subscribe for additional securities we may issue. The transfer agent and registrar for any series of cumulative preferred shares will be specified in the applicable prospectus supplement.

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The prospectus supplement relating to any particular series of cumulative preferred shares that we offer using this prospectus will describe the following terms of that series, if applicable:

•: the number of shares, their stated value and their designation or title;
•: the initial public offering price of the series;
•: that series' rights as to dividends;
•: the rights of holders of shares of that series upon the dissolution or distribution of our assets;
•: whether and upon what terms the shares of that series will be redeemable;
•: whether and upon what terms a sinking fund will be used to purchase or redeem the shares of that series;
•: whether and upon what terms the shares of that series may be converted and the securities into which that series of cumulative preferred shares may be converted;
•: the voting rights, if any, that will apply to that series; and
•: any additional rights and preferences of the series.

We may elect to offer depositary shares evidenced by depositary receipts, each representing a fractional interest in a share of the particular series of cumulative preferred shares issued and deposited with a depositary. See "Description of Depositary Shares."

Dividend Rights

The holders of cumulative preferred shares of each series are entitled to receive, when and as declared by the board of directors, on a parity with the other outstanding series of cumulative preferred shares, cumulative dividends at the annual rate (which may be fixed or variable or both) for such series, payable quarterly on the dividend payment dates fixed for such series. Each series of cumulative preferred shares that we offer using this prospectus will be entitled to dividends at the annual rate set forth in the applicable prospectus supplement, cumulative from the date of original issue of such share, and payable quarterly on the dates set forth in the applicable prospectus supplement.

We and our subsidiaries are parties to agreements pursuant to which we borrow money, and certain covenants in these agreements may limit our ability to pay dividends or other distributions with respect to the cumulative preferred shares or to redeem or repurchase these shares. In addition, we and our subsidiaries may become parties to future agreements that contain such restrictions. These covenants will be described in more detail in the prospectus supplement relating to any particular series of cumulative preferred shares that we offer using this prospectus.

If, and so long as, any cumulative preferred shares are outstanding, no dividends or other distributions may be made on the cumulative preference shares, the common shares or any other shares ranking junior to the cumulative preferred shares with respect to the payment of dividends or the distribution of assets (collectively "Subordinate Shares"), nor may any Subordinate Shares be purchased, redeemed or otherwise acquired (including through the operation of any sinking fund), if dividends on the cumulative preferred shares are accumulated and unpaid for any period and a sum sufficient for the payment thereof has not been set apart or we shall in any respect be in default under any sinking fund for the benefit of cumulative preferred shares. Moreover, if, and so long as, any cumulative preferred shares are outstanding, we shall not, without the consent of the holders of a majority of the aggregate voting power of the cumulative preferred shares of all series then outstanding (two-thirds if more than one-fourth vote negatively), declare, pay or set apart for payment any dividend on or purchase, redeem or otherwise acquire (including through the operation of any sinking fund) any

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Subordinate Shares unless, after giving effect thereto, Common Share Equity shall equal at least 25% of Total Capitalization and our earned surplus shall be not less than $831,398.

"Common Share Equity" is the sum of

•: our stated capital applicable to our common shares and to all other Subordinate Shares, including any shares proposed to be issued substantially contemporaneously,
•: capital surplus to the extent of premium on our common shares and on all other Subordinate Shares, including any premium on any shares proposed to be issued substantially contemporaneously,
•: contributions in aid of construction, and
•: earned surplus,

"Total Capitalization" means the sum of

•: the Common Share Equity,
•: the involuntary liquidation preference of all cumulative preferred shares and all other shares prior to or on a parity with the cumulative preferred shares to be outstanding after the proposed event, and
•: the principal amount of all interest bearing debt (including debt to which property theretofore acquired or to be acquired substantially contemporaneously is or will be subject) to be outstanding after the proposed event, excluding, however, all indebtedness maturing by its terms within one year from the time of creation thereof unless we, without the consent of the lender, have the right to extend the maturity of such indebtedness for a period or periods which, with the original period of such indebtedness, aggregates one year or more.

If we shall be in default in the payment of any dividend on the cumulative preferred shares of any series, we shall not purchase, redeem or otherwise acquire (including through the operation of any sinking fund) any cumulative preferred shares unless all of the cumulative preferred shares are redeemed.

Redemption and Repurchase

A series of cumulative preferred shares that we may offer using this prospectus may be redeemable, in whole or in part, at our option, and may be subject to mandatory redemption pursuant to a sinking fund or otherwise, or may be subject to repurchase at the option of the holders, as described in the applicable prospectus supplement, subject to the restriction described in the last paragraph under the caption "—Dividend Rights." If a series of cumulative preferred shares is subject to mandatory redemption, the applicable prospectus supplement will specify the terms of redemption, the procedure used for redemption, the number of shares that we will redeem each year and the redemption price. The applicable prospectus supplement will also specify whether the redemption price will be payable in cash or other property.

Provision may be made whereby, subject to certain conditions, all rights (other than the right to receive redemption moneys) of the holders of cumulative preferred shares called for redemption, whether at our option or through a sinking fund, will terminate before the redemption date upon the deposit with a bank or trust company of the funds necessary for redemption.

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Cumulative preferred shares acquired by us upon redemption or conversion thereof, through operation of any sinking fund therefor or otherwise, may be reissued in the same manner as authorized but unissued cumulative preferred shares.

Conversion or Exchange

If any series of cumulative preferred shares that we may offer using this prospectus may be converted or exchanged into common shares, another series of cumulative preferred shares or debt securities, the applicable prospectus supplement will state the terms on which shares of that series may be converted or exchanged.

Voting Rights

Unless otherwise provided in the prospectus supplement relating to any series of cumulative preferred shares that we offer using this prospectus, the holders of the cumulative preferred shares are not entitled to vote at any meetings of our shareholders, except as required by law or as described below.

In the event that four full quarterly dividend payments on the cumulative preferred shares of any series shall be in default, the holders of the cumulative preferred shares of all series at the time outstanding, voting as a class, shall thereafter elect three members of an eleven member board of directors; and, if such default shall increase to twelve full quarterly dividend payments, such holders shall thereafter elect six members of an eleven member board of directors. After any such default shall have been cured, the cumulative preferred shares shall be divested of such voting rights, subject to being revested in the event of subsequent such defaults.

The consent of the holders of at least two-thirds of the aggregate voting power of the cumulative preferred shares of all series then outstanding is required to

•: create, authorize or issue any shares of any class ranking prior to (or any securities of any kind or class convertible into shares of any class ranking prior to) the cumulative preferred shares as to dividends or assets, or
•: amend our articles so as to affect adversely any of the preferences or other rights of the holders of the cumulative preferred shares, provided that if less than all series of cumulative preferred shares are so affected, only the consent of the holders of at least two-thirds of the aggregate voting power of the affected series shall be required.

A majority (two-thirds if more than one-fourth vote negatively) of the aggregate voting power of the cumulative preferred shares of all series then outstanding is required to

•

increase the number of authorized cumulative preferred shares or create, authorize or issue shares of any class ranking on a parity with the cumulative preferred shares as to dividends or assets, or any securities of any kind or class convertible into cumulative preferred shares or shares of any class on a parity with the cumulative preferred shares;

•

issue any cumulative preferred shares of any series if as a result thereof more than 60,000 cumulative preferred shares of all series will then be outstanding, unless, after giving effect thereto

•: Adjusted Income Available for Interest shall equal at least 1.5 times Adjusted Interest and Preferred Charges,
•: Adjusted Income Available for Preferred Dividends shall equal at least 2.5 times Adjusted Preferred Charges, and
•: Common Share Equity shall equal at least 25% of Total Capitalization;

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•

declare, pay or set apart for payment any dividend on any Subordinate Shares, or purchase, redeem or otherwise acquire for value any Subordinate Shares, or pay or set aside or make available any moneys for a purchase fund or sinking fund for the purchase or redemption of any such Subordinate Shares, unless after giving effect to the payment of such dividend or such purchase, redemption or other acquisition of such payment or setting aside of moneys in a purchase fund or sinking fund,

•: The Common Share Equity shall equal at least one-fourth of the Total Capitalization, and
•: The earned surplus of the corporation shall be not less than $831,398;

•

consolidate or merge into or with any other corporation or corporations unless, after giving effect thereto

•: the cumulative preferred shares outstanding immediately prior to such transaction shall remain outstanding or be constituted as shares of the resulting corporation in the same number and with the same relative rights and preferences as the cumulative preferred shares, with no increase in the authorized number and no outstanding or authorized shares ranking prior to or on a parity with the cumulative preferred shares (except our shares outstanding or authorized immediately prior to such transaction), and the outstanding indebtedness of the resulting corporation shall not exceed our outstanding indebtedness immediately preceding such transaction, or
•: each of the following must be satisfied with respect to the resulting corporation: the Adjusted Income Available for Interest shall equal at least 1.5 times Adjusted Interest and Preferred Charges, the Adjusted Income Available for Preferred Dividends shall equal at least 2.5 times Adjusted Preferred Charges, and the Common Share Equity of the resulting corporation shall equal at least 25% of Total Capitalization; and

•

sell, lease or exchange all or substantially all of our property and assets unless, after giving effect thereto, the fair value of our assets shall at least equal the preference on voluntary liquidation of all outstanding cumulative preferred shares and of all other outstanding shares ranking on a parity with the cumulative preferred shares, after deducting an amount equal to our outstanding indebtedness plus an amount equal to the preference on voluntary liquidation of all shares ranking prior to the cumulative preferred shares.

"Adjusted Income Available for Interest" is based upon gross income of the corporation or of the resulting corporation, as the case may be, for a then current 12-month period available for the payment of interest, after deducting all taxes (including income taxes).

"Adjusted Income Available for Preferred Dividends" equals Adjusted Income Available for Interest minus interest charges for one year and the dividend requirement for one year on any shares ranking prior to the cumulative preferred shares.

"Adjusted Interest and Preferred Charges" means the sum of

•: the interest charges for one year on all our interest bearing indebtedness outstanding at the time of issuance of such cumulative preferred shares or of the proposed consolidation or merger (including that, if any, proposed to be issued or assumed substantially contemporaneously, or to which property theretofore acquired or to be acquired substantially contemporaneously is or will be subject (adjusted for all amortization of debt discount and expense, or of premium on debt, as the case may be)), and
•: the dividend requirements for one year on all outstanding cumulative preferred shares, and on all other shares of a class ranking prior to or on a parity with the cumulative preferred shares as to dividends or assets, outstanding at the time of issuance of such additional cumulative

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preferred shares, or of such consolidation or merger, including all such shares proposed to be issued, or all such shares of the resulting corporation, as the case may be.

"Adjusted Preferred Charges" is the Adjusted Interest and Preferred Charges for one year determined at the time of issuance of such cumulative preferred shares or of the proposed consolidation or merger, less the interest charges for one year and the dividend requirements for one year on any shares ranking prior to the cumulative preferred shares, included in determining the Adjusted Interest and Preferred Charges.

Holders of cumulative preferred shares entitled to vote as described above shall have voting power in proportion to the involuntary liquidation preference of the cumulative preferred shares so held and shall be entitled to cumulate votes in the election of directors.

Liquidation Rights

In the event of dissolution, liquidation or winding up of the corporation, the holders of cumulative preferred shares of each series outstanding shall be entitled to receive out of our assets, before any payment shall be made to the holders of Subordinate Shares, such amount per share as shall have been fixed by the board of directors as the voluntary liquidation price or the involuntary liquidation price, as the case may be, for the shares of such series, together with a sum, in the case of each share, computed at the annual dividend rate for the series of which the particular share is a part, from the date on which dividends on such shares became cumulative to and including the date fixed for such distribution or payment, less the aggregate amount of all dividends which have theretofore been paid or which have been declared on the share and for which moneys have been set apart and remain available for payment. If upon any such dissolution, liquidation or winding up, our assets available for payment to shareholders are not sufficient to make payment in full to the holders of cumulative preferred shares as above provided, payment shall be made to such holders ratably in accordance with the respective distributive amounts to which such holders shall be entitled. A consolidation or merger of the corporation shall not be construed as a dissolution, liquidation or winding up of the corporation within the meaning of the foregoing provisions.

The voluntary and involuntary liquidation prices for any series of cumulative preferred shares that we offer using this prospectus will be set forth in the applicable prospectus supplement. The involuntary liquidation price for each series of cumulative preferred shares issued after April 1, 1977 must be equal to the gross consideration received by us upon the issuance thereof (without regard to any premium received, underwriting discount or commission, private placement fee or other expense of issuance).

Certain Provisions of Articles and Bylaws

For a description of some additional provisions of our articles and bylaws, see "Description of Common Shares—Certain Provisions of Articles and Bylaws."

DESCRIPTION OF DEPOSITARY SHARES

This section summarizes the general terms and provisions of the depositary shares represented by depositary receipts that we may offer using this prospectus. This section is only a summary and does not purport to be complete. You must look at the applicable forms of depositary receipt and deposit agreement for a full understanding of the specific terms of any depositary shares and depositary receipts. The forms of the depositary receipts and the deposit agreement will be filed or incorporated by reference as exhibits to the registration statement to which this prospectus is a part. See "Where You Can Find More Information" for information on how to obtain copies.

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A prospectus supplement will describe the specific terms of the depositary shares and the depositary receipts offered under that prospectus supplement, including any of the terms in this section that will not apply to those depositary shares and depositary receipts, and any special considerations, including tax considerations, applicable to investing in those depositary shares.

General

We may offer fractional interests in cumulative preferred shares, rather than full shares of cumulative preferred shares. If we do so, we will provide for the issuance to the public by a depositary of depositary receipts evidencing depositary shares. Each depositary share will represent a fractional interest in a share of a particular series of cumulative preferred shares.

The shares of any series of cumulative preferred shares underlying the depositary shares will be deposited under a separate deposit agreement between us and a bank or trust company having its principal office in the United States and having a combined capital and surplus of at least $50 million. The applicable prospectus supplement will state the name and address of the depositary. Subject to the terms of the deposit agreement, each owner of a depositary share will have a fractional interest in all the rights and preferences of the cumulative preferred shares underlying the depositary share. Those rights include any dividend, voting, redemption, conversion and liquidation rights.

While the final depositary receipts are being prepared, we may order the depositary to issue temporary depositary receipts substantially identical to the final depositary receipts, although not in final form. The holders of temporary depositary receipts will be entitled to the same rights as if they held the depositary receipts in final form. Holders of temporary depositary receipts can exchange them for final depositary receipts at our expense.

Withdrawal of Cumulative Preferred Shares

If you surrender depositary receipts at the principal office of the depositary you will be entitled to receive at that office the number of shares of cumulative preferred shares and any money or other property then represented by the depositary shares, unless the depositary shares have been called for redemption. We will not, however, issue any fractional shares of cumulative preferred shares. Accordingly, if you deliver depositary receipts for a number of depositary shares that, when added together, represents more than a whole number of shares of cumulative preferred shares, the depositary will issue to you a new depositary receipt evidencing the excess number of depositary shares at the same time as you receive your share of cumulative preferred shares. You will no longer be entitled to deposit the shares of cumulative preferred shares you have withdrawn under the deposit agreement or to receive depositary shares in exchange for those shares of cumulative preferred shares. There may be no market for any withdrawn shares of cumulative preferred shares.

Dividends and Other Distributions

The depositary will distribute all cash dividends or other cash distributions received with respect to the deposited cumulative preferred shares, less any taxes required to be withheld, to the record holders of the depositary receipts in proportion to the number of the depositary shares owned by each record holder on the relevant date. The depositary will distribute only the amount that can be distributed without attributing to any holder a fraction of one cent. Any balance will be added to the next sum to be distributed to holders of depositary receipts.

If there is a distribution other than in cash, the depositary will distribute property to the holders of depositary receipts, unless the depositary determines that it is not practical to make the distribution. If this occurs, the depositary may, with our approval, sell the property and distribute the net proceeds from the sale to the holders.

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The deposit agreement will contain provisions relating to how any subscription or similar rights offered by us to holders of the cumulative preferred shares will be made available to the holders of depositary receipts.

Redemption and Repurchase of Deposited Cumulative Preferred Shares

If any series of cumulative preferred shares underlying the depositary shares is subject to redemption, the depositary shares will be redeemed from the redemption proceeds, in whole or in part, of the series of cumulative preferred shares held by the depositary. The depositary will mail a notice of redemption between 30 and 60 days prior to the date fixed for redemption to the record holders of the depositary receipts to be redeemed at their addresses appearing in the depositary's records. The redemption price per depositary share will bear the same relationship to the redemption price per share of cumulative preferred shares that the depositary share bears to the underlying cumulative preferred shares. Whenever we redeem cumulative preferred shares held by the depositary, the depositary will redeem, as of the same redemption date, the number of depositary shares representing the cumulative preferred shares redeemed. If less than all of the depositary shares are to be redeemed, the depositary shares to be redeemed will be selected by the depositary by lot or pro rata or other equitable method, as we determine.

After the date fixed for redemption, the depositary shares called for redemption will no longer be outstanding. If depositary shares are no longer outstanding, the holders will have no rights with regard to those depositary shares other than the right to receive money or other property that they were entitled to receive upon redemption. The payments will be made when the holder surrenders its depositary receipts to the depositary.

Depositary shares are not subject to repurchase at the option of the holders. However, if shares of cumulative preferred shares underlying the depositary shares become subject to repurchase at the option of the holders, the holders may surrender their depositary receipts to the depositary and direct the depositary to instruct us to repurchase the deposited cumulative preferred shares at the price specified in the applicable prospectus supplement. If we have sufficient funds available, we will, upon receipt of the instructions, repurchase the requisite whole number of shares of cumulative preferred shares from the depositary, which will, in turn, repurchase the depositary receipts. However, holders of depositary receipts will only be entitled to request the repurchase of a number of depositary shares that represents in total one or more whole shares of the underlying cumulative preferred shares. The repurchase price per depositary share will equal the repurchase price per share of the underlying cumulative preferred shares multiplied by the fraction of that share represented by one depositary share. If the depositary shares evidenced by any depositary receipt are repurchased in part only, the depositary will issue one or more new depositary receipts representing the depositary shares not repurchased.

Voting of Deposited Cumulative Preferred Shares

Upon receipt of notice of any meeting at which the holders of the series of cumulative preferred shares underlying the depositary shares are entitled to vote, the depositary will mail information about the meeting to the record holders of the related depositary receipts. Each record holder of depositary receipts on the record date (which will be the same date as the record date for the holders of the related cumulative preferred shares) will be entitled to instruct the depositary as to how to vote the cumulative preferred shares underlying the holder's depositary shares. The depositary will try, if practicable, to vote the number of shares of cumulative preferred shares underlying the depositary shares according to the instructions it receives. We will agree to take all action requested and considered necessary by the depositary to enable it to vote the cumulative preferred shares in that manner. The depositary will not vote any shares of cumulative preferred shares for which it does not receive specific instructions from the holders of the depositary receipts.

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Conversion and Exchange of Deposited Cumulative Preferred Shares

If we provide for the exchange of the cumulative preferred shares underlying the depositary shares, the depositary will exchange, as of the same exchange date, that number of depositary shares representing the cumulative preferred shares to be exchanged, so long as we have issued and deposited with the depositary the securities for which the cumulative preferred shares are to be exchanged. The exchange rate per depositary share will equal the exchange rate per share of the underlying cumulative preferred shares multiplied by the fraction of that share represented by one depositary share. If less than all of the depositary shares are exchanged, the depositary shares to be exchanged will be selected by the depositary by lot or pro rata or other equitable method, as we determine. If the depositary shares evidenced by a depositary receipt are exchanged in part only, the depositary will issue one or more new depositary receipts representing the depositary shares not exchanged.

Depositary shares may not be converted or exchanged for other securities or property at the option of the holders. However, if shares of cumulative preferred shares underlying the depositary shares are converted into or exchanged for other securities at the option of the holders, the holders may surrender their depositary receipts to the depositary and direct the depositary to instruct us to convert or exchange the deposited cumulative preferred shares into the whole number or principal amount of securities specified in the applicable prospectus supplement. Upon receipt of instructions, we will cause the conversion or exchange and deliver to the holders the whole number or principal amount of our securities and cash in lieu of any fractional security. The exchange or conversion rate per depositary share will equal the exchange or conversion rate per share of the underlying cumulative preferred shares multiplied by the fraction of the cumulative preferred share represented by one depositary share. If the depositary shares evidenced by a depositary receipt are converted or exchanged in part only, the depositary will issue a new depositary receipt evidencing any depositary shares not converted or exchanged.

Amendment and Termination of the Deposit Agreement

The form of depositary receipt evidencing the depositary shares and any provision of the deposit agreement may be amended by agreement between us and the depositary. However, any amendment that materially and adversely alters the rights of the existing holders of depositary receipts will not be effective unless the amendment has been approved by the record holders of at least a majority of the depositary receipts. A deposit agreement may be terminated only if all related outstanding depositary shares have been redeemed or there has been a final distribution on the underlying cumulative preferred shares in connection with our liquidation, dissolution or winding up, and the distribution has been distributed to the holders of the related depositary receipts.

Charges of Depositary

We will pay all transfer and other taxes and governmental charges arising solely from the existence of the depositary arrangements. We will pay charges of the depositary for the initial deposit of the cumulative preferred shares and any redemption of the cumulative preferred shares. Holders of depositary receipts will pay transfer and other taxes and governmental charges and any other charges that are stated in the deposit agreement to be their responsibility.

Miscellaneous

The depositary will forward to the holders of depositary receipts all reports and communications from us that are delivered to the depositary and that we are required to furnish to the holders of the underlying cumulative preferred shares.

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Neither we nor the depositary will be liable if the depositary is prevented or delayed by law or any circumstance beyond its control in performing its obligations under the deposit agreement. Our obligations and the depositary's obligations under the deposit agreement will be limited to the performance in good faith of our respective duties under the deposit agreement. Neither we nor the depositary will be obligated to prosecute or defend any legal proceeding connected with any depositary shares or cumulative preferred shares unless satisfactory indemnity is furnished. We and the depositary may rely upon written advice of counsel or accountants or upon information provided by persons presenting cumulative preferred shares for deposit, holders of depositary receipts or other persons believed to be competent and on documents believed to be genuine.

Resignation and Removal of Depositary

The depositary may resign at any time by delivering notice to us. We also may at any time remove the depositary. Resignations or removals will take effect upon the appointment of a successor depositary and its acceptance of the appointment. The successor depositary must be appointed within 60 days after delivery to us of notice of resignation or removal and must be a bank or trust company having its principal office in the United States and having a combined capital and surplus of at least $50 million.

DESCRIPTION OF DEBT SECURITIES

This section describes the general terms and provisions of the debt securities that we may offer using this prospectus and the related indenture. This section is only a summary and does not purport to be complete. You must look to the relevant form of debt security and the related indenture for a full understanding of all terms of any series of debt securities. The form of debt security and the related indenture have been or will be filed or incorporated by reference as exhibits to the registration statement of which this prospectus is a part. See "Where You Can Find More Information" for information on how to obtain copies.

A prospectus supplement will describe the specific terms of any particular series of debt securities offered under that prospectus supplement, including any of the terms in this section that will not apply to that series, and any special considerations, including tax considerations, applicable to investing in those debt securities. In some instances, certain of the precise terms of debt securities you are offered may be described in a further prospectus supplement, known as a pricing supplement.

General

We will issue the debt securities in one or more series under the indenture dated as of November 1, 1997, as amended by the First Supplemental Indenture dated as of July 1, 2009, between us and U.S. Bank National Association (formerly First Trust National Association), as trustee. The indenture does not limit the amount of debt securities that we may issue under it at any time. We may issue additional debt securities under the indenture in one or more series from time to time with terms different from those of other debt securities already issued under the indenture. In this section, we include references in parentheses to specific sections of the indenture.

Ranking

The debt securities will be our unsecured and unsubordinated obligations and will rank equally and ratably with our other current and future unsecured and unsubordinated debt. As of December 31, 2017, we and our subsidiaries had approximately $605 million of debt, none of which was outstanding under the indenture. The debt securities will be subordinated to all of our secured debt (as to the collateral pledged to secure this debt). As of December 31, 2017, we had no secured debt. In addition, except to the extent we have a priority or equal claim against our subsidiaries as a creditor, the debt

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securities will be effectively subordinated to debt and other obligations at the subsidiary level because, as the common shareholder of our direct and indirect subsidiaries, we will be subject to the prior claims of creditors of our subsidiaries. As of December 31, 2017, our subsidiaries had approximately $524 million of aggregate outstanding debt. Otter Tail Corporation's obligations under its $130 million revolving credit facility and $80 million of outstanding 3.55% senior notes due 2026 are guaranteed by its wholly owned subsidiary, Varistar Corporation, and certain of its wholly owned subsidiaries. The indenture does not restrict the amount of secured or unsecured debt that we or our subsidiaries may incur.

Terms

The prospectus supplement, including any separate pricing supplement, relating to a series of debt securities that we offer using this prospectus will describe the following terms of that series, if applicable:

•

the title of the offered debt securities;

•

any limit on the aggregate principal amount of the offered debt securities;

•

the person or persons to whom interest on the offered debt securities will be payable if other than the persons in whose names the offered debt securities are registered;

•

the date or dates on which the principal of the offered debt securities will be payable;

•

the rate or rates, which may be fixed or variable, and/or the method of determination of the rate or rates at which the offered debt securities will bear interest, if any;

•

the date or dates from which interest, if any, will accrue, the interest payment dates on which interest will be payable and the regular record date for any interest payable on any interest payment date;

•

the place or places where

•: the principal of or any premium or interest on the offered debt securities will be payable;
•: registration of transfer may be effected;
•: exchanges may be effected; and
•: notices and demands to or upon us may be served;

•

the security registrar for the offered debt securities and, if such is the case, that the principal of the offered debt securities will be payable without presentment or surrender thereof;

•

the period or periods within which, or the date or dates on which, the price or prices at which and the terms and conditions upon which any of the offered debt securities may be redeemed, in whole or in part, at our option;

•

our obligation or obligations, if any, to redeem or purchase any of the offered debt securities pursuant to any sinking fund or other mandatory redemption provisions or at the option of the holder, and the period or periods within which, or the date or dates on which, the price or prices at which and the terms and conditions upon which any of the offered debt securities will be redeemed or purchased, in whole or in part, pursuant to that obligation, and applicable exceptions to the requirements of a notice of redemption in the case of mandatory redemption or redemption at the option of the holder;

•

the denominations in which the offered debt securities will be issuable, if other than denominations of $1,000 and any integral multiple thereof;

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•: if other than the currency of the United States, the currency or currencies, including composite currencies, in which payment of the principal of and any premium and interest on the offered debt securities will be payable;
•: if the principal of or any premium or interest on any of the offered debt securities will be payable, at the election of us or the holder, in a coin or currency other than in which the offered debt securities are stated to be payable, the period or periods within which and the terms and conditions upon which, the election will be made;
•: if the principal of or any premium or interest on the offered debt securities will be payable, or will be payable at the election of us or a holder, in securities or other property, the type and amount of securities or other property, or the formula or other method or other means by which the amount will be determined, and the period or periods within which, and the terms and conditions upon which, any such election may be made;
•: if the amount of payment of principal of or any premium or interest on the offered debt securities may be determined with reference to an index or other fact or event ascertainable outside the indenture, the manner in which the amounts will be determined;
•: if other than the principal amount of the offered debt securities, the portion of the principal amount of the offered debt securities which will be payable upon declaration of acceleration of the maturity;
•: any addition to the events of default applicable to the offered debt securities and any addition to our covenants for the benefit of the holders of the offered debt securities;
•: the terms, if any, pursuant to which the offered debt securities may be converted into or exchanged for shares of our capital stock or other securities or any other person;
•: the obligations or instruments, if any, which will be considered to be eligible obligations for the offered debt securities denominated in a currency other than U.S. dollars or in a composite currency, and any additional or alternative provisions for the reinstatement of our indebtedness in respect of the debt securities after the satisfaction and discharge thereof;
•: if the offered debt securities will be issued in global form, any limitations on the rights of the holder to transfer or exchange the same or obtain the registration of transfer and to obtain certificates in definitive form in lieu of temporary form, and any and all other matters incidental to such debt securities;
•: if the offered debt securities will be issuable as bearer securities;
•: any limitations on the rights of the holders of the offered debt securities to transfer or exchange the debt securities or to obtain the registration of transfer, and if a service charge will be made for the registration of transfer or exchange of the offered debt securities, the amount or terms thereof;
•: any exceptions to the provisions governing payments due on legal holidays or any variations in the definition of business day with respect to the offered debt securities; and
•: any other terms of the offered debt securities, or any tranche thereof, not inconsistent with the provisions of the indenture. (Section 301)

Although debt securities offered by this prospectus may be issued under the indenture, there is no requirement that we issue future debt securities under the indenture. Accordingly, we may use other indentures or documentation containing different provisions in connection with future issuances of our debt.

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We may issue the debt securities as original issue discount securities, which will be offered and sold at a substantial discount below their stated principal amount. The prospectus supplement relating to those debt securities will describe the federal income tax consequences and other special considerations applicable to them. In addition, if we issue any debt securities denominated in foreign currencies or currency units, the prospectus supplement relating to those debt securities will also describe any federal income tax consequences and other special considerations applicable to them.

The indenture does not contain covenants or other provisions designed to afford holders of debt securities protection in the event of a highly-leveraged transaction or change of control involving us. If this protection is provided for the offered debt securities, we will describe the applicable provisions in the prospectus supplement relating to those debt securities.

Form, Exchange and Transfer

Unless the applicable prospectus supplement specifies otherwise, we will issue the debt securities only in fully registered form without interest coupons and in denominations of $1,000 and integral multiples of $1,000. (Sections 201 and 302)

At the option of the holder, subject to the terms of the indenture and the limitations applicable to global securities, debt securities of any series will be exchangeable for other debt securities of the same series, of any authorized denomination and of like tenor and aggregate principal amount. (Section 305)

Subject to the terms of the indenture and the limitations applicable to global securities, holders may present debt securities for exchange as provided above and for registration of transfer at the office of the security registrar or at the office of any transfer agent designated by us for that purpose. Unless the applicable prospectus supplement indicates otherwise, no service charge will be made for any registration of transfer or exchange of debt securities, but we may require payment of a sum sufficient to cover any tax or other governmental charge associated with the transfer or exchange. Debt securities presented or surrendered for registration of transfer or exchange must (if so required by us, the trustee or the security registrar) be duly endorsed or accompanied by an executed written instrument of transfer in form satisfactory to us, the trustee or the security registrar. (Section 305) Any transfer agent (in addition to the security registrar) initially designated by us for the offered debt securities will be named in the applicable prospectus supplement. We may at any time designate additional transfer agents or rescind the designation of any transfer agent or approve a change in the office through which any transfer agent acts. We are required to maintain a transfer agent in each place of payment for the debt securities of a particular series. We may maintain an office that performs the functions of the transfer agent. (Section 602) Unless the applicable prospectus supplement specifies otherwise, the trustee will act as security registrar and transfer agent with respect to each series of debt securities offered by this prospectus.

We will not be required to execute or register the transfer or exchange of debt securities, or any tranche thereof, during a period of 15 days preceding the notice to be given identifying the debt securities called for redemption, or any debt securities so selected for redemption, in whole or in part, except the unredeemed portion of any debt securities being redeemed in part. (Section 305)

If a debt security is issued as a global security, only the depositary or its nominee as the sole holder of the debt security will be entitled to transfer and exchange the debt security as described in this prospectus under "—Global Securities."

Payment and Paying Agent

Unless the applicable prospectus supplement indicates otherwise, we will pay interest on the offered debt securities on any interest payment date to the person in whose name the debt security is registered at the close of business on the regular record date. (Section 307)

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Unless the applicable prospectus supplement indicates otherwise, we will pay the principal of and any premium and interest on the offered debt securities at the office of the paying agent or paying agents as we may designate for that purpose from time to time. Unless the applicable prospectus supplement indicates otherwise, the corporate trust office of the trustee in New York, New York will be our sole paying agent for payment for each series of debt securities. Any other paying agents initially designated by us for the debt securities of a particular series will be named in the applicable prospectus supplement. We may at any time designate additional paying agents or rescind the designation of any paying agent or approve a change in the office through which any paying agent acts. We are required to maintain a paying agent in each place of payment for the debt securities of a particular series. (Section 602)

Any moneys deposited by us with the trustee or any paying agent for the payment of the principal of or any premium or interest on any offered debt securities which remain unclaimed at the end of two years after the applicable payment has become due and payable will be paid to us. The holder of that debt security, as an unsecured general creditor and not as a holder, thereafter may look only to us for the payment. (Section 603)

Redemption

Any terms for the optional or mandatory redemption of the offered debt securities will be set forth in the applicable prospectus supplement. Except as otherwise provided in the applicable prospectus supplement with respect to debt securities that are redeemable at the option of the holder, the offered debt securities will be redeemable only upon notice by mail not less than 30 days nor more than 60 days prior to the redemption date. If less than all the debt securities of a series, or any tranche thereof, are to be redeemed, the particular debt securities to be redeemed will be selected by the securities registrar by the method as provided for the particular series, or in the absence of any such provision, by such method of random selection as the security registrar deems fair and appropriate. (Sections 403 and 404)

Any notice of redemption at our option may state that the redemption will be conditional upon receipt by the paying agent or agents, on or prior to the redemption date, of money sufficient to pay the principal of and any premium and interest on the offered debt securities. If sufficient money has not been so received, the notice will be of no force and effect and we will not be required to redeem the debt securities. (Section 404)

Consolidation, Merger, Conveyance or Other Transfer

Under the terms of the indenture, we may not consolidate with or merge into any other corporation or convey, transfer or lease our properties and assets substantially as an entirety to any person, unless:

•: the corporation formed by the consolidation or into which we are merged or the person which acquires by conveyance or transfer, or which leases, our properties and assets substantially as an entirety is organized and existing under the laws of the United States, any state thereof or the District of Columbia and assumes our obligations on the debt securities and under the indenture;
•: immediately after giving effect to the transaction, no Event of Default shall have occurred and be continuing; and
•: we have delivered to the trustee an officer's certificate and an opinion of counsel as provided in the indenture. (Section 1101)

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Events of Default

Each of the following will constitute an "Event of Default" under the indenture with respect to any series of debt securities:

•: failure to pay any interest on any debt securities of that series within 60 days after the same becomes due and payable;
•: failure to pay principal of or premium, if any, on any debt securities of that series within three business days after the same becomes due and payable;
•: failure to perform or breach of any of our other covenants or warranties in the indenture (other than a covenant or warranty in the indenture solely for the benefit of a series of debt securities other than that series) for 60 days after written notice to us by the trustee, or to us and the trustee by the holders of at least 33% in principal amount of the outstanding debt securities of that series as provided in the indenture;
•: the occurrence of events of bankruptcy, insolvency or reorganization relating to us; and
•: any other Event of Default specified in the applicable prospectus supplement with respect to debt securities of a particular series. (Section 801)

An Event of Default with respect to a series of debt securities may not necessarily constitute an Event of Default with respect to debt securities of any other series issued under the indenture.

If an Event of Default with respect to any series of debt securities occurs and is continuing, then either the trustee or the holders of not less than 33% in principal amount of the outstanding debt securities of that series may declare the principal amount (or if the debt securities of that series are original issue discount securities, such portion of the principal amount thereof as may be specified in the applicable prospectus supplement) of all of the debt securities of that series to be due and payable immediately. However, if an Event of Default occurs and is continuing with respect to more than one series of debt securities, the trustee or the holders of not less than 33% in aggregate principal amount of the outstanding securities of all such series, considered as one class, may make the declaration of acceleration and not the holders of the debt securities of any one of such series. (Section 802) There is no automatic acceleration, even in the event of our bankruptcy or insolvency.

Subject to the provisions of the indenture relating to the duties of the trustee in case an Event of Default shall occur and be continuing, the trustee will be under no obligation to exercise any of its rights or powers under the indenture at the request or direction of any holder, unless the holder has offered to the trustee reasonable security or indemnity. (Section 903) Subject to the provisions of the indemnification of the trustee, the holders of a majority in principal amount of the outstanding debt securities of any series will have the right to direct the time, method and place of conducting any proceeding for any remedy available to the trustee, or exercising any trust or power conferred on the trustee, with respect to the debt securities of that series; provided, however, that if an Event of Default occurs and is continuing with respect to more than one series of debt securities, the holders of a majority in aggregate principal amount of the outstanding debt securities of all those series, considered as one class, will have this right, and not the holders of any one series of debt securities. (Section 812)

No holder of debt securities of any series will have any right to institute any proceeding related to the indenture, or for the appointment of a receiver or a trustee, or for any other remedy thereunder, unless:

•: the holder has previously given written notice to the trustee of a continuing Event of Default with respect to the debt securities of that series;

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•: the holders of not less than a majority in aggregate principal amount of the outstanding debt securities of that series have made written request to the trustee, and offered reasonable indemnity to the trustee, to institute the proceeding as trustee; and
•: the trustee has failed to institute the proceeding, and has not received from the holders of a majority in aggregate principal amount of the outstanding debt securities of that series a direction inconsistent with such request, within 60 days after the notice, request and offer. (Section 807)

Notwithstanding the provisions described in the immediately preceding paragraph or any other provision of the indenture, the holder of any debt security will have the right, which is absolute and unconditional, to receive payment of the principal, premium, if any, and interest on that debt security and to institute suit for enforcement of any payment, and that right will not be impaired without consent of that holder. (Section 808)

We will be required to furnish to the trustee annually, not later than October in each year, a statement by an appropriate officer as to the officer's knowledge of our compliance with all conditions and covenants under the indenture, such compliance to be determined without regard to any period of grace or requirement of notice under the indenture. (Section 606)

Right to Cure

At any time after the declaration of acceleration with respect to a series of debt securities has been made but before a judgment or decree for payment of the money due has been obtained, the Event or Events of Default giving rise to the declaration of acceleration will, without further act, be deemed to have been waived, and the declaration and its consequences will, without further act, be deemed to have been rescinded and annulled, if:

•

we have paid or deposited with the trustee a sum sufficient to pay:

•: all overdue interest, if any, on all debt securities of that series;
•: the principal of and premium, if any, on any debt securities of that series which have become due otherwise than by that declaration of acceleration and interest thereon at the rate or rates prescribed in the debt securities;
•: interest upon overdue interest, if any, at the rate or rates prescribed in the debt securities, to the extent payment of that interest is lawful; and
•: all amounts due to the trustee under the indenture; and

•

any other Event of Default with respect to the debt securities of that series, other than the non-payment of the principal of the debt securities of that series which has become due solely by the declaration of acceleration, have been cured or waived as provided in the indenture. (Section 802)

Modification and Waiver

Without the consent of any holder of debt securities, we and the trustee may enter into one or more supplemental indentures to the indenture for any of the following purposes:

•: to evidence the assumption by any permitted successor to us of our covenants under the indenture and the debt securities;
•: to add to our covenants or other provisions for the benefit of the holders of all or any series of outstanding debt securities or to surrender any right or power conferred upon us by the indenture;

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•

to add any additional Events of Default with respect to all or any series of outstanding debt securities;

•

to change or eliminate any provision of the indenture or to add any new provision to the indenture, provided that if the change, elimination or addition will adversely affect the interests of the holders of any series of debt securities in any material respect, that change, elimination or addition will become effective with respect to that series only when the consent of the holders of that series so affected has been obtained or when there is no outstanding debt security of that series under the indenture;

•

to provide collateral security for the debt securities;

•

to establish the form or terms of any series of debt securities as permitted by the indenture;

•

to provide for the authentication and delivery of bearer securities and coupons appertaining thereto representing interest, if any, thereon and for the procedures for the registration, exchange and replacement thereof and for giving of notice to, and the solicitation of the vote or consent of, the holders thereof and for any and all other matters incidental thereto;

•

to evidence and provide for the acceptance of appointment of a separate or successor trustee under the indenture with respect to debt securities of one or more series and to add or to change any of the provisions of the indenture as will be necessary to provide for or to facilitate the administration of the trusts under the indenture by more than one trustee;

•

to provide for the procedures required to permit the utilization of a noncertificated system of registration for any series of debt securities;

•

to change any place where

•: the principal of and any premium and interest on any debt securities will be payable;
•: any debt securities may be surrendered for registration of transfer or exchange; or
•: notices and demands to or upon us in respect of the debt securities and indenture may be served; or

•

to cure any ambiguity, to correct or supplement any defective or inconsistent provision or to make or change any other provisions with respect to matters and questions arising under the indenture, provided that action does not adversely affect the interests of the holders of debt securities of any series in any material respect. (Section 1201)

The holders of not less than a majority in aggregate principal amount of the outstanding debt securities of any series may waive our compliance with some restrictive provisions of the indenture. (Section 607) The holders of not less than a majority in principal amount of the outstanding debt securities of any series may waive any past default under the indenture with respect to that series, except a default

•: in the payment of principal, premium or interest; and
•: related to certain covenants and provisions of the indenture that cannot be modified or be amended without the consent of the holder of each outstanding debt security of the series affected. (Section 813)

Without limiting the generality of the foregoing, if the Trust Indenture Act is amended after the date of the indenture in such a way as to require changes to the indenture or the incorporation of additional provisions or so as to permit changes to, or the elimination of provisions which, at the date of the indenture or at any time thereafter, were required by the Trust Indenture Act to be contained in the indenture, the indenture will be deemed to have been amended so as to conform to such

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amendment or to effect such changes or elimination. We and the trustee may, without the consent of any holders, enter into one or more supplemental indentures to evidence or effect such amendment. (Section 1201)

Except as provided above, the consent of the holders of not less than a majority in aggregate principal amount of the debt securities of all series then outstanding, considered as one class, is required for the purpose of adding any provisions to, or changing in any manner, or eliminating any of the provisions of the indenture pursuant to one or more supplemental indentures. However, if less than all of the series of outstanding debt securities are directly affected by a proposed supplemental indenture, then the consent only of the holders of a majority in aggregate principal amount of outstanding debt securities of all series so directly affected, considered as one class, will be required. Further, if the debt securities of any series have been issued in more than one tranche and if the proposed supplemental indenture directly affects the rights of the holders of one or more, but less than all, tranches, then the consent only of the holders of a majority in aggregate principal amount of the outstanding debt securities of all tranches so directly affected, considered as one class, will be required.

Without the consent of each holder of debt securities affected by the modification, no supplemental indenture may:

•: change the stated maturity of the principal of, or any installment of principal of or interest on, any debt security;
•: reduce the principal amount of the debt security;
•: reduce the rate of interest on the debt security (or the amount of any installment of interest thereon) or change the method of calculating the rate;
•: reduce any premium payable upon redemption of the debt security;
•: reduce the amount of the principal of any original issue discount security that would be due and payable upon a declaration of acceleration of maturity;
•: change the coin or currency (or other property) in which any debt security or any premium or the interest thereon is payable;
•: impair the right to institute suit for the enforcement of any payment on or after the stated maturity of any debt security (or, in the case of redemption, on or after the redemption date);
•: reduce the percentage in principal amount of the outstanding debt securities of any series, or any tranche thereof, the consent of the holders of which is required for any such supplemental indenture, or the consent of the holders of which is required for any waiver of compliance with any provision of the indenture or any default thereunder and its consequences, or reduce the requirements for quorum or voting; or
•: modify certain of the provisions of the indenture relating to supplemental indentures, waivers of certain covenants and waivers of past defaults with respect to the debt securities of any series, or any tranche thereof.

A supplemental indenture which changes or eliminates any covenant or other provision of the indenture which has expressly been included solely for the benefit of one or more particular series of debt securities or one or more tranches thereof, or modifies the rights of the holders of debt securities of that series or tranche with respect to such covenant or other provision, will be deemed not to affect the rights under the indenture of the holders of the debt securities of any other series or tranche. (Section 1202)

The indenture provides that in determining whether the holders of the requisite principal amount of the outstanding debt securities have given any request, demand, authorization, direction, notice,

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consent or waiver under the indenture as of any date, or whether or not a quorum is present at a meeting of holders:

•: debt securities owned by us or any other obligor upon the debt securities or any affiliate of ours or of such other obligor (unless we, the affiliate or the obligor own all securities outstanding under the indenture, or all outstanding debt securities of each such series and each such tranche, as the case may be, determined without regard to this clause) will be disregarded and deemed not to be outstanding;
•: the principal amount of an original issue discount security that will be deemed to be outstanding for such purposes will be the amount of the principal thereof that would be due and payable as of the date of such determination upon a declaration of acceleration of the maturity thereof as provided in the indenture; and
•: the principal amount of a debt security denominated in one or more foreign currencies or a composite currency that will be deemed to be outstanding will be the U.S. dollar equivalent, determined as of such date in the manner prescribed for such debt security, of the principal amount of the debt security (or, in the case of a debt security described in the second bullet above, of the amount described in that clause). (Section 101)

If we solicit from holders any request, demand, authorization, direction, notice, consent, election, waiver or other act, we may, at our option, by board resolution, fix in advance a record date for the determination of holders entitled to give such request, demand, authorization, direction, notice, consent, election, waiver or other act. If a record date is fixed, such request, demand, authorization, direction, notice, consent, election, waiver or other act may be given before or after that record date, but only the holders of record at the close of business on the record date will be deemed to be holders for the purposes of determining whether holders of the requisite proportion of the outstanding debt securities have authorized or agreed or consented to such request, demand, authorization, direction, notice, consent, election, waiver or other act, and for that purpose the outstanding debt securities will be computed as of the record date. Any request, demand, authorization, direction, notice, consent, election, waiver or other act of a holder will bind every future holder of the same debt security and the holder of every debt security issued upon the registration of transfer thereof or in exchange therefor or in lieu thereof in respect of anything done, omitted or suffered to be done by the trustee or us in reliance thereon, whether or not notation of that action is made upon the debt security. (Section 104)

Defeasance

Unless the applicable prospectus supplement otherwise indicates, any debt securities, or any portion of the principal amount thereof, will be deemed to have been paid for purposes of the indenture, and, at our election, our entire indebtedness in respect of the debt securities will be deemed to have been satisfied and discharged, if there has been irrevocably deposited with the trustee or any paying agent (other than us), in trust: (a) money in an amount which will be sufficient, or (b) eligible obligations (as described below), which do not contain provisions permitting the redemption or other prepaying at the option of the issuer thereof, the principal of and the interest on which when due, without any regard to reinvestment thereof, will provide monies which, together with money, if any, deposited with or held by the trustee or the paying agent, will be sufficient, or (c) a combination of (a) and (b) which will be sufficient, to pay when due the principal of and any premium and interest due and to become due on the debt securities or portions thereof. (Section 701)

For this purpose, unless the applicable prospectus supplement otherwise indicates, eligible obligations include direct obligations of, or obligations unconditionally guaranteed by, the United States, entitled to the benefit of the full faith and credit thereof, and certificates, depositary receipts or other instruments which evidence a direct ownership interest in such obligations or in any specific interest or principal payments due in respect thereof. (Section 101)

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Resignation of Trustee

The trustee may resign at any time by giving written notice to us or may be removed at any time by act of the holders of a majority in principal amount of the outstanding debt securities of a series. No resignation or removal of the trustee and no appointment of a successor trustee will become effective until the acceptance of appointment by a successor trustee in accordance with the requirements of the indenture. So long as no Event of Default or event which, after notice or lapse of time, or both, would become an Event of Default has occurred and is continuing and except with respect to a trustee appointed by act of the holders of a majority in principal amount of the outstanding debt securities, if we have delivered to the trustee a board resolution appointing a successor trustee and the successor has accepted the appointment in accordance with the terms of the indenture, the trustee will be deemed to have resigned and the successor will be deemed to have been appointed as trustee in accordance with the indenture. (Section 910)

Notices

Notices to holders of debt securities will be given by mail to the addresses of the holders as they appear in the security register. (Section 106)

Title

We, the trustee and any agent of ours or the trustee may treat the person in whose name a debt security is registered as the absolute owner (whether or not the debt security may be overdue) for the purpose of making payment and for all other purposes. (Section 308)

Governing Law

The indenture and the debt securities will be governed by, and construed in accordance with, the laws of the State of New York, except to the extent the law of any other jurisdiction is mandatorily applicable. (Section 112)

Limitation on Suits

The indenture limits a holder's right to institute any proceeding with respect to the indenture, the appointment of a receiver or trustee, or for any other remedy under the indenture. (Section 807)

Maintenance of Properties

A provision in the indenture provides that we will cause (or, with respect to property owned in common with others, make reasonable effort to cause) all our properties used or useful in the conduct of our business to be maintained and kept in good condition, repair and working order and will cause (or, with respect to property owned in common with others, make reasonable effort to cause) to be made all necessary repairs, renewals, replacements, betterments and improvements, all as, in our judgment, may be necessary so that the business carried on in connection therewith may be properly conducted. However, nothing in this provision will prevent us from discontinuing, or causing the discontinuance of the operation and maintenance of any of our properties if the discontinuance is, in our judgment, desirable in the conduct of our business. (Section 605)

Concerning the Trustee

In the ordinary course of business, U.S. Bank National Association and its affiliates have engaged, and may in the future engage, in commercial or investment banking transactions with us and our affiliates. U.S. Bank National Association is a lender under the Otter Tail Corporation and OTP credit agreements and also provides cash management services to us.

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Global Securities

We may issue a series of debt securities offered by this prospectus, in whole or in part, in the form of one or more global securities, which will have an aggregate principal amount equal to that of the debt securities represented thereby.

Unless it is exchanged in whole or in part for the individual debt securities it represents, a global security may be transferred only as a whole

•: by the applicable depositary to a nominee of the depositary;
•: by any nominee to the depositary itself or another nominee; or
•: by the depositary or any nominee to a successor depositary or any nominee of the successor.

We will describe the specific terms of the depositary arrangement related to a series of debt securities in the applicable prospectus supplement. We anticipate that the following provisions will generally apply to depositary arrangements.

Each global security will be registered in the name of a depositary or its nominee identified in the applicable prospectus supplement and will be deposited with the depositary or its nominee or a custodian. The global security will bear a legend regarding the restrictions on exchanges and registration of transfer referred to below and any other matters as may be provided in the indenture.

As long as the depositary, or its nominee, is the registered holder of the global security, the depositary or nominee, as the case may be, will be considered the sole owner and holder of the debt securities represented by the global security for all purposes under the indenture. Except in limited circumstances, owners of beneficial interests in a global security:

•: will not be entitled to have the global security or any of the underlying debt securities registered in their names;
•: will not receive or be entitled to receive physical delivery of any of the underlying debt securities in definitive form; and
•: will not be considered to be the owners or holders under the indenture relating to those debt securities.

All payments of principal of and any premium and interest on a global security will be made to the depositary or its nominee, as the case may be, as the registered owner of the global security representing these debt securities. The laws of some states require that some purchasers of securities take physical delivery of securities in definitive form. These limits and laws may impair the ability to transfer beneficial interests in a global security.

Ownership of beneficial interests in a global security will be limited to institutions that have accounts with the depositary or its nominee, which institutions we refer to as the participants, and to persons that may hold beneficial interests through participants. In connection with the issuance of any global security, the depositary will credit, on its book-entry registration and transfer system, the respective principal amounts of debt securities represented by the global security to the accounts of its participants. Ownership of beneficial interests in a global security will be shown only on, and the transfer of those ownership interests will be effective only through, records maintained by the depositary and its participants. Payments, transfers, exchanges and other matters relating to beneficial interests in a global security may be subject to various policies and procedures adopted by the depositary from time to time. Neither we, the trustee nor any of our or the trustee's agents will have any responsibility or liability for any aspect of the depositary's or any participant's records relating to, or for payments made on account of, beneficial interests in a global security, or for maintaining, supervising or reviewing any records relating to beneficial interests.

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DESCRIPTION OF SECURITIES WARRANTS

This section summarizes the general terms and provisions of the securities warrants represented by warrant agreements and warrant certificates that we may offer using this prospectus. The securities warrants may be issued for the purchase of common shares, cumulative preferred shares or debt securities. This section is only a summary and does not purport to be complete. You must look at the applicable forms of warrant agreement and warrant certificate for a full understanding of the specific terms of any securities warrant. The forms of the warrant agreement and the warrant certificate will be filed or incorporated by reference as exhibits to the registration statement to which this prospectus is a part. See "Where You Can Find More Information" for information on how to obtain copies.

A prospectus supplement will describe the specific terms of the securities warrants offered under that prospectus supplement, including any of the terms in this section that will not apply to those securities warrants, and any special considerations, including tax considerations, applicable to investing in those securities warrants.

General

We may issue securities warrants alone or together with other securities offered by the applicable prospectus supplement. Securities warrants may be attached to or separate from those securities. Each series of securities warrants will be issued under a separate warrant agreement between us and a bank or trust company, as warrant agent, as described in the applicable prospectus supplement. The warrant agent will act solely as our agent in connection with the securities warrants and will not act as an agent or trustee for any holders or beneficial owners of the securities warrants.

The prospectus supplement relating to any securities warrants that we offer using this prospectus will describe the following terms of those securities warrants, if applicable:

•: the offering price;
•: the currencies in which the securities warrants will be offered;
•: the designation, total principal amount, currencies, denominations and terms of the series of debt securities that may be purchased upon exercise of the securities warrants;
•: the principal amount of the series of debt securities that may be purchased if a holder exercises the securities warrants and the price at which and currencies in which the principal amount may be purchased upon exercise;
•: the total number of shares that may be purchased if all of the holders exercise the securities warrants and, in the case of securities warrants for the purchase of cumulative preferred shares, the designation, total number and terms of the series of cumulative preferred shares that can be purchased upon exercise of the securities warrants;
•: the number of common shares or cumulative preferred shares that may be purchased if a holder exercises any one securities warrant and the price at which and currencies in which the common shares or cumulative preferred shares may be purchased upon exercise;
•: the designation and terms of any series of securities with which the securities warrants are being offered, and the number of securities warrants offered with each security;
•: the date on and after which the holder of the securities warrants can transfer them separately from the related series of securities;
•: the dates on which the right to exercise the securities warrants begins and expires;
•: the triggering event and the terms upon which the exercise price and the number of underlying securities that the securities warrants are exercisable into may be adjusted;

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•: whether the securities warrants will be issued in registered or bearer form;
•: the identity of any warrant agent with respect to the securities warrants and the terms of the warrant agency agreement with that warrant agent;
•: a discussion of material U.S. federal income tax consequences; and
•: any other terms of the securities warrants.

A holder of securities warrants may

•: exchange them for new securities warrants of different denominations;
•: present them for registration of transfer, if they are in registered form; and
•: exercise them at the corporate trust office of the warrant agent or any other office indicated in the applicable prospectus supplement.

Until the securities warrants are exercised, holders of the securities warrants will not have any of the rights of holders of the underlying securities.

Exercise of Securities Warrants

Each holder of a securities warrant is entitled to purchase the number of common shares or cumulative preferred shares or the principal amount of debt securities, as the case may be, at the exercise price described in the applicable prospectus supplement. After the close of business on the day when the right to exercise terminates (or a later date if we extend the time for exercise), unexercised securities warrants will become void.

Holders of securities warrants may exercise them by

•: delivering to the warrant agent the payment required to purchase the underlying securities, as stated in the applicable prospectus supplement;
•: properly completing and signing the reverse side of their warrant certificate(s), if any, or other exercise documentation; and
•: delivering their warrant certificate(s), if any, or other exercise documentation to the warrant agent within the time specified by the applicable prospectus supplement.

If you comply with the procedures described above, your securities warrants will be considered to have been exercised when the warrant agent receives payment of the exercise price. As soon as practicable after you have completed these procedures, we will issue and deliver to you the common shares, cumulative preferred shares or debt securities, as the case may be, that you purchased upon exercise. If you exercise fewer than all of the securities warrants represented by a warrant certificate, we will issue to you a new warrant certificate for the unexercised amount of securities warrants.

Amendments and Supplements to Warrant Agreements

We may amend or supplement a warrant agreement or warrant certificates without the consent of the holders of the securities warrants if the changes are not inconsistent with the provisions of the securities warrants and do not adversely affect the interests of the holders.

DESCRIPTION OF UNITS

We may, from time to time, issue units comprised of one or more of the other securities described in this prospectus in any combination. A prospectus supplement will describe the specific terms of the units offered under that prospectus supplement, and any special considerations, including tax considerations, applicable to investing in those units. You must look at the applicable prospectus

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supplement and any applicable unit agreement for a full understanding of the specific terms of any units. The form of unit agreement will be filed or incorporated by reference as an exhibit to the registration statement to which this prospectus is a part. See "Where You Can Find More Information" for information on how to obtain copies.

PLAN OF DISTRIBUTION

We may offer and sell the securities offered by this prospectus in any of three ways:

•: through agents;
•: through underwriters or dealers; or
•: directly to one or more purchasers.

The securities may be distributed from time to time in one or more transactions at negotiated prices, at a fixed price (that is subject to change), at market prices prevailing at the time of sale, at various prices determined at the time of sale or at prices related to the prevailing market prices.

The applicable prospectus supplement will set forth the specific terms of the offering of securities, including:

•: the securities offered;
•: the price of the securities;
•: the proceeds to us from the sale of the securities;
•: the names of the securities exchanges, if any, on which the securities are listed;
•: the name of the underwriters or agents, if any;
•: any underwriting discounts, agency fees or other compensation to underwriters or agents; and
•: any discounts or concessions allowed or paid to dealers.

We may authorize underwriters, dealers and agents to solicit offers from specified institutions to purchase the securities from us at the public offering price listed in the applicable prospectus supplement. These sales may be made under "delayed delivery contracts" that provide for payment and delivery on a specified future date. Any contracts like this will be subject to the conditions listed in the applicable prospectus supplement. The applicable prospectus supplement also will state the commission to be paid to underwriters, dealers and agents who solicit these contracts.

We may make sales of our common shares to or through one or more underwriters or agents in at-the-market offerings. We will do so pursuant to the terms of a distribution agreement between us and the underwriters or agents. If we engage in at-the-market sales pursuant to a distribution agreement, we will issue and sell the common shares to or through one or more underwriters or agents, which may act on an agency basis or on a principal basis. During the term of any such distribution agreement, we may sell shares on a daily basis in exchange transactions or otherwise as we agree with the underwriters or agents. The distribution agreement may provide that any common shares sold will be sold at prices related to the then prevailing market prices for our securities. Therefore, exact figures regarding net proceeds to us or commissions to be paid are impossible to determine and will be described in a prospectus supplement. Pursuant to the terms of the distribution agreement, we also may agree to sell, and the relevant underwriters or agents may agree to solicit offers to purchase, blocks of our common shares. The terms of each such distribution agreement will be set forth in more detail in a prospectus supplement to this prospectus. To the extent that any named underwriter or agent acts as principal pursuant to the terms of a distribution agreement, or if we offer to sell our common shares through another broker dealer acting as underwriter, then such named underwriter or agent or broker

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dealer may engage in certain transactions that stabilize, maintain or otherwise affect the price of our common shares. We will describe any such activities in the prospectus supplement relating to the transaction. To the extent that any named broker dealer or agent acts as agent on a best efforts basis pursuant to the terms of a distribution agreement, such broker dealer or agent will not engage in any such stabilization transactions.

Any underwriter, dealer or agent who participates in the distribution of an offering of securities may be considered by the SEC to be an underwriter under the Securities Act. Any discounts or commissions received by an underwriter, dealer or agent on the sale or resale of securities may be considered by the SEC to be underwriting discounts and commissions under the Securities Act. We may agree to indemnify any underwriters, dealers and agents against or contribute to any payments the underwriters, dealers or agents may be required to make with respect to, civil liabilities, including liabilities under the Securities Act. Underwriters and agents and their affiliates are permitted to be customers of, engage in transactions with, or perform services for us and our affiliates in the ordinary course of business.

Unless otherwise indicated in the applicable prospectus supplement, the obligations of the underwriters to purchase any offered securities will be subject to conditions precedent and the underwriters will be obligated to purchase all of the offered securities if any are purchased.

Unless otherwise indicated in the applicable prospectus supplement and other than our common shares, all securities we offer using this prospectus will be new issues of securities with no established trading market. Any underwriters to whom we sell securities for public offering and sale may make a market in the securities, but the underwriters will not be obligated to do so and may discontinue any market-making at any time without notice. We cannot assure you that a secondary trading market for any of the securities will ever develop or, if one develops, that it will be maintained or provide any significant liquidity.

VALIDITY OF SECURITIES

The validity of the securities offered by this prospectus will be passed upon for us by Dorsey & Whitney LLP.

EXPERTS

The consolidated financial statements, and the related financial statement schedule, incorporated in this prospectus by reference from Otter Tail Corporation'sAnnual Report on Form 10-K, and the effectiveness of Otter Tail Corporation's internal control over financial reporting have been audited by Deloitte & Touche LLP, an independent registered public accounting firm, as stated in their report, which is incorporated herein by reference. Such consolidated financial statements and financial statement schedule have been so incorporated in reliance upon the report of such firm given upon their authority as experts in accounting and auditing.

Table of Contents

$75,000,000

Common Shares

Prospectus supplement

KeyBanc Capital Markets

November 8, 2019

Otter Tail (OTTR) 424B5Prospectus supplement for primary offering