Alliant Energy (LNT) 424B5 Prospectus supplement for primary offering

Filed Pursuant to Rule 424(b)(5)
Registration Statement No. 333-222076

CALCULATION OF REGISTRATION FEE

Title of each class of securities to be registered	Amount to be registered/ Proposed maximum offering price per unit	Amount of registration fee (1)(2)
Common Stock, $.01 par value	$175,000,000	$21,788

(1) Calculated in accordance with Rule 457(o), based on the proposed maximum offering price, and Rule 457(r) under the Securities Act of 1933, as amended (the “Securities Act”).

(2) This “Calculation of Registration Fee” table shall be deemed to update the “Calculation of Registration Fee” table in the registrant’s Registration Statement on FormS-3 (FileNo. 333-222076) in accordance with Rules 456(b) and 457(r) under the Securities Act.

PROSPECTUS SUPPLEMENT

(To Prospectus dated December 15, 2017)

Up to $175,000,000 Aggregate Offering Price of Common Stock

Alliant Energy Corporation

We have entered into a distribution agreement with Barclays Capital Inc., BNY Mellon Capital Markets, LLC and J.P. Morgan Securities LLC, each of which we refer to as an agent and together as the agents, relating to the shares of common stock, par value $.01 per share, offered by this prospectus supplement and the accompanying prospectus. In accordance with the terms of the distribution agreement, we may offer and sell common stock having an aggregate offering price of up to $175,000,000 from time to time through any agent, as our sales agent, or directly to any agent acting as principal.

Sales of the common stock, if any, made by the agents, as our sales agents, as contemplated by this prospectus supplement and the accompanying prospectus, may be made by means of ordinary brokers’ transactions on the New York Stock Exchange at market prices, in privately negotiated transactions (including block transactions), or as otherwise agreed between us and the agents. We will pay each agent a commission of 1.00% of the gross sales price per share of common stock sold through such agent, as our sales agent, under the distribution agreement.

The agents are not required to sell any specific number or dollar amount of our common stock but will use their commercially reasonable efforts, consistent with their normal sales and trading practice, as our sales agents, and on the terms and subject to the conditions of the distribution agreement, to sell the common stock offered on terms agreed by the agents and us.

Under the terms of the distribution agreement, we also may sell common stock to the agents as principals at a price to be agreed upon at the time of sale. If we sell common stock to an agent as principal, we will enter into a separate terms agreement with such agent, and we will describe the public offering price, underwriting discount (which may exceed 1.00% of the public offering price) and other terms of the offering of those shares of common stock in a separate prospectus supplement or pricing supplement. See “Plan of Distribution” for a description of compensation to be paid to the agents.

Our common stock is listed on the New York Stock Exchange under the symbol “LNT.” The last reported sale price of our common stock on the New York Stock Exchange on May 17, 2018 was $40.64 per share.

Investing in our common stock involves risk. See “Risk Factors” on pageS-2 of this prospectus supplement, page 4 of the accompanying prospectus and in the documents incorporated by reference into this prospectus supplement and the accompanying prospectus for a discussion of certain risks that prospective investors should consider before investing in our common stock.

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

Barclays      BNY Mellon Capital Markets, LLC      J.P. Morgan

May 18, 2018

TABLE OF CONTENTS

Prospectus Supplement

	Page
About This Prospectus Supplement		S-ii
Forward-Looking Statements		S-iii
Summary		S-1
Risk Factors		S-2
Use of Proceeds		S-4
Plan of Distribution		S-5
Where You Can Find More Information		S-7
Legal Matters		S-8
Experts		S-8

Prospectus

	Page
About This Prospectus		1
Forward-Looking Statements		2
Alliant Energy Corporation		3
Risk Factors		4
Ratio of Earnings To Fixed Charges		5
Use of Proceeds		5
Description of Common Stock		6
Description of Debt Securities		8
Description of Warrants		18
Description of Stock Purchase Contracts and Stock Purchase Units		19
Selling Shareowners		19
Plan of Distribution		20
Where You Can Find More Information		22
Legal Matters		23
Experts		23

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ABOUT THIS PROSPECTUS SUPPLEMENT

This document is in two parts. The first part is this prospectus supplement, which describes the specific terms of this offering of common stock. The second part, the accompanying prospectus, gives more general information, some of which may not apply to this offering. You should read the entire prospectus supplement, as well as the accompanying prospectus and the documents incorporated by reference that are described under “Where You Can Find More Information” in this prospectus supplement. In the event that the description of the offering varies between this prospectus supplement and the accompanying prospectus, you should rely on the information contained in this prospectus supplement. Generally, when we refer to the “prospectus,” we are referring to this prospectus supplement and the accompanying prospectus combined.

Any statement made in this prospectus or in a document incorporated or deemed to be incorporated by reference into this prospectus will be deemed to be modified or superseded for purposes of this prospectus to the extent that a statement contained in this prospectus or any subsequently filed document that is also incorporated by reference into this prospectus modifies or supersedes that statement. Any statement so modified or superseded will not be deemed, except as so modified or superseded, to constitute a part of this prospectus.

You should rely only on the information contained or incorporated by reference in this prospectus or any “free writing prospectus” we may authorize to be delivered to you. “Incorporated by reference” means that we can disclose important information to you by referring you to another document filed separately with the Securities and Exchange Commission, or the SEC. We have not authorized any other person to provide you with different information. If anyone provides you with different or inconsistent information, you should not rely on it. We are not making an offer to sell nor soliciting an offer to buy these securities in any jurisdiction where the offer or sale is not permitted. You should assume that the information appearing in this prospectus and the documents incorporated by reference is accurate only as of the respective dates of those documents in which the information is contained. Our business, financial condition, results of operations and prospects may have changed since any such date.

Unless we otherwise indicate or unless the context requires otherwise, all references in this prospectus supplement to “Alliant Energy,” “we,” “our,” “us” or similar references mean Alliant Energy Corporation.

Our principal executive offices are located at 4902 North Biltmore Lane, Madison, Wisconsin 53718, and our telephone number is(608) 458-3311.

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FORWARD-LOOKING STATEMENTS

This prospectus supplement, the accompanying prospectus and the information incorporated by reference herein or therein contain forward-looking statements intended to qualify for the safe harbor from liability established by the Private Securities Litigation Reform Act of 1995. All statements, other than statements of historical fact, included in this prospectus supplement or the accompanying prospectus or incorporated by reference herein or therein, including statements regarding anticipated financial performance, business strategy and management’s plans and objectives for future operations, are forward-looking statements. These forward-looking statements can be identified as such because the statements include words such as “may,” “believe,” “expect,” “anticipate,” “plan,” “project,” “will,” “projections,” “estimate,” or other words of similar import. These forward-looking statements are subject to certain risks and uncertainties that could cause actual results to differ materially from those expressed in, or implied by, such statements. Readers are cautioned not to place undue reliance on these forward-looking statements. All forward-looking statements included in this prospectus supplement, the accompanying prospectus or in a document incorporated by reference herein or therein speak only as of the date of this prospectus supplement, the accompanying prospectus or the document incorporated by reference, as the case may be. Additional information concerning factors that could cause actual results to differ materially from those in the forward-looking statements is contained under “Risk Factors” on pageS-2 of this prospectus supplement, page 4 of the accompanying prospectus and other documents that we file from time to time with the SEC that are incorporated by reference into this prospectus supplement and the accompanying prospectus, including, but not limited to, the risk factor disclosure beginning on page 14 of our Annual Report on Form10-K for the fiscal year ended December 31, 2017. Some, but not all, of the risks and uncertainties that could materially affect actual results include:

• the ability of our direct subsidiaries, Interstate Power and Light Company, or IPL, and Wisconsin Power and Light Company, or WPL, to obtain adequate and timely rate relief to allow for, among other things, earning a return on rate base additions and the recovery of costs, including fuel costs, operating costs, transmission costs, environmental compliance and remediation costs, deferred expenditures, deferred tax assets, capital expenditures, and remaining costs related to electric generating units, or EGUs, that may be permanently closed, earning their authorized rates of return, and the payments to their parent of expected levels of dividends;

• federal and state regulatory or governmental actions, including the impact of energy, tax, financial and health care legislation, and regulatory agency orders;

• the impact of customer- and third party-owned generation, including alternative electric suppliers, in IPL’s and WPL’s service territories on system reliability, operating expenses and customers’ demand for electricity;

• the impact of energy efficiency, franchise retention and customer disconnects on sales volumes and margins;

• the impact that price changes may have on IPL’s and WPL’s customers’ demand for electric, gas and steam services and their ability to pay their bills;

• the ability to utilize tax credits and net operating losses generated to date, and those that may be generated in the future, before they expire;

• the direct or indirect effects resulting from terrorist incidents, including physical attacks and cyber attacks, or responses to such incidents;

• the impact of penalties or third-party claims related to, or in connection with, a failure to maintain the security of personally identifiable information, including associated costs to notify affected persons and to mitigate their information security concerns;

• employee workforce factors, including changes in key executives, ability to hire and retain employees with specialized skills, ability to create desired corporate culture, collective bargaining agreements and negotiations, work stoppages or restructurings;

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• weather effects on results of utility operations;

• issues associated with environmental remediation and environmental compliance, including compliance with the Consent Decree between WPL, the U.S. Environmental Protection Agency, or EPA, and the Sierra Club, the Consent Decree between IPL, the EPA, the Sierra Club, the State of Iowa and Linn County in Iowa, the Coal Combustion Residuals Rule, the Clean Power Plan, future changes in environmental laws and regulations, including the EPA’s regulations for carbon dioxide emissions reductions from new and existing fossil-fueled EGUs, and litigation associated with environmental requirements;

• the ability to defend against environmental claims brought by state and federal agencies, such as the EPA, state natural resources agencies or third parties, such as the Sierra Club, and the impact on operating expenses of defending and resolving such claims;

• continued access to the capital markets on competitive terms and rates, and the actions of credit rating agencies;

• inflation and interest rates;

• the impact of the economy in IPL’s and WPL’s service territories and the resulting impacts on sales volumes, margins and the ability to collect unpaid bills;

• changes in the price of delivered natural gas, purchased electricity and coal due to shifts in supply and demand caused by market conditions and regulations;

• disruptions in the supply and delivery of natural gas, purchased electricity and coal;

• changes in the price of transmission services and the ability to recover the cost of transmission services in a timely manner;

• developments that adversely impact the ability to implement the strategic plan;

• ability to obtain regulatory approval for wind projects with acceptable conditions, to acquire sufficient transmission-ready wind sites, to complete construction within the cost caps set by regulators and to meet all requirements to qualify for the full level of production tax credits;

• the direct or indirect effects resulting from breakdown or failure of equipment in the operation of electric and gas distribution systems, such as mechanical problems and explosions or fires, and compliance with electric and gas transmission and distribution safety regulations;

• issues related to the availability and operations of EGUs, includingstart-up risks, breakdown or failure of equipment, performance below expected or contracted levels of output or efficiency, operator error, employee safety, transmission constraints, compliance with mandatory reliability standards and risks related to recovery of resulting incremental costs through rates;

• impacts that storms or natural disasters in IPL’s and WPL’s service territories may have on their operations and recovery of costs associated with restoration activities;

• any material post-closing adjustments related to any past asset divestitures, including the sales of IPL’s Minnesota electric and natural gas assets, and Whiting Petroleum Corporation, which could result from, among other things, indemnification agreements, warranties, parental guarantees or litigation;

• our ability to sustain our dividend payout ratio goal;

• changes to costs of providing benefits and related funding requirements of pension and other postretirement benefit plans due to the market value of the assets that fund the plans, economic conditions, financial market performance, interest rates, life expectancies and demographics;

• material changes in employee-related benefit and compensation costs;

• risks associated with operation and ownership ofnon-utility holdings;

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• changes in technology that alter the channels through which customers buy or utilize our products and services;

• impacts on equity income from unconsolidated investments due to further potential changes to American Transmission Company LLC’s authorized return on equity;

• impacts of our future tax benefits from Iowa rate-making practices, including deductions for repairs expenditures, allocation of mixed service costs and state depreciation, and recoverability of the associated regulatory assets from customers, when the differences reverse in future periods;

• the impacts of adjustments made to deferred tax assets and liabilities from changes in the tax laws;

• changes to the creditworthiness of counterparties with which we have contractual arrangements, including participants in the energy markets and fuel suppliers and transporters;

• current or future litigation, regulatory investigations, proceedings or inquiries;

• reputational damage from negative publicity, protests, fines, penalties and other negative consequences resulting in regulatory and/or legal actions;

• the effect of accounting standards issued periodically by standard-setting bodies;

• the ability to successfully complete tax audits and changes in tax accounting methods with no material impact on earnings and cash flows; and

• other factors listed in Management’s Discussion and Analysis of Financial Condition and Results of Operations in the combined Quarterly Report on Form10-Q and Annual Report on Form10-K filed by IPL, WPL and us for the quarter ended March 31, 2018 and the year ended December 31, 2017, respectively.

We assume no obligation, and disclaim any duty, to update or revise publicly any forward-looking statements, whether as a result of new information, future events or otherwise, except as required by law.

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SUMMARY

This summary highlights information contained elsewhere in this prospectus supplement and the accompanying prospectus. This summary may not contain all of the information that may be important to you. You should read this entire prospectus supplement and the accompanying prospectus carefully before making a decision to invest in our common stock. References in this prospectus supplement to “common stock” refer to our common stock, $.01 par value per share, and the attached common share purchase rights.

Alliant Energy Corporation

Alliant Energy Corporation is an energy-services provider that operates as a regulated investor-owned public utility holding company. Our primary focus is to provide regulated electric and natural gas service to customers in the Midwest through our two public utility subsidiaries, Interstate Power and Light Company, and Wisconsin Power and Light Company. We also havenon-utility operations. Our utility business is engaged principally in:

• the generation and distribution of electricity to retail customers in select markets in Iowa and Wisconsin;

• the distribution and transportation of natural gas to retail customers in select markets in Iowa and Wisconsin;

• the sale of electricity to wholesale customers in Minnesota, Illinois, Iowa and Wisconsin; and

• the generation and distribution of steam for two customers in Cedar Rapids, Iowa and various other energy-related products and services.

We are a “holding company” under the Public Utility Holding Company Act of 2005 and are subject to regulation by the Federal Energy Regulatory Commission under that Act.

Our principal executive offices are located at 4902 North Biltmore Lane, Madison, Wisconsin 53718, and our telephone number is(608) 458-3311.

The Offering

Common Stock Offered Common stock, par value $.01 per share, having an aggregate offering price of up to $175,000,000.

Use of Proceeds We intend to use the net proceeds from this offering, after deducting the agents’ commissions and our offering expenses, for general corporate purposes, which may include repayment or refinancing of debt, working capital, construction and acquisition expenditures, investments and repurchases and redemptions of securities. Please read “Use of Proceeds.”

Risk Factors You should read “Risk Factors” on pageS-2 of this prospectus supplement, on page 4 of the accompanying prospectus and in the documents incorporated by reference herein for a discussion of certain risks that prospective investors should consider before investing in our common stock.

Transfer Agent and Registrar EQ Shareowner Services

New York Stock Exchange Symbol LNT

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RISK FACTORS

Investing in our common stock involves risk. You should carefully consider the following risk factors together with all the other information contained in this prospectus supplement or incorporated by reference in this prospectus. You should also consider the risks and uncertainties discussed under the caption “Risk Factors” included in our Annual Report on Form10-K for the year ended December 31, 2017, which is incorporated by reference in this prospectus supplement, and which may be amended, supplemented or superseded from time to time by other reports we file with the SEC in the future.

Sales of a substantial number of shares of our common stock, or the perception that such sales might occur, could adversely affect the trading price of our common stock.

As of March 31, 2018, we had 231,481,828 shares of our common stock outstanding. Also, we had, as of March 31, 2018, a total of 11.8 million shares of common stock in the aggregate reserved for issuance pursuant to our Amended and Restated 2010 Omnibus Incentive Plan, Shareowner Direct Plan and 401(k) Savings Plan. If this offering is completed, the number of shares of common stock that we have outstanding will increase. Sales of a substantial number of shares of our common stock, or the perception that such sales might occur, could adversely affect the trading price of our common stock. Further, issuance of shares pursuant to our Amended and Restated 2010 Omnibus Incentive Plan, our Shareowner Direct Plan or our 401(k) Savings Plan, or the perception in the market that the holders of a large number of shares intend to sell shares, could reduce the market price of our common stock.

You may experience dilution.

Giving effect to the issuance of common stock in this offering, the receipt of the expected net proceeds and the use of those proceeds, this offering may have a dilutive effect on our expected earnings per weighted average common share. The actual amount of dilution cannot be determined at this time and will be based on a number of factors. Also, we are not restricted from issuing additional securities in the future, including shares of common stock, securities that are convertible into or exchangeable for, or that represent the right to receive, common stock or substantially similar securities. The price per share at which we sell additional shares of our common stock, or securities convertible or exchangeable into common stock, in future transactions may be higher or lower than the price per share paid by investors in this offering and investors purchasing shares or other securities in the future could have rights superior to existing shareowners. The issuance of these securities may cause further dilution to our shareowners.

The market price of our common stock may fluctuate significantly.

The market price of our common stock could be subject to significant fluctuations in response to factors such as the following, some of which are beyond our control:

• our financial condition, performance, liquidity and prospects;

• the market for similar securities;

• our issuance of common stock or other securities;

• changes in earnings estimates by analysts and our ability to meet analysts’ earnings estimates;

• announcements by third parties of significant claims or proceedings against us;

• government action or regulation;

• our dividend policy;

• changes in the ratings of our securities; and

• general economic, capital markets and real estate market conditions.

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In addition, the stock markets in general, including the New York Stock Exchange, are subject to significant price and trading fluctuations. These fluctuations have resulted in volatility in the market prices of securities that often has been unrelated or disproportionate to changes in operating performance and may affect adversely the market price of our common stock.

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USE OF PROCEEDS

We intend to use the net proceeds from this offering of common stock for general corporate purposes, which may include repayment or refinancing of debt, working capital, construction and acquisition expenditures, investments and repurchases and redemptions of securities. Net proceeds may be placed in short term instruments prior to use.

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PLAN OF DISTRIBUTION

We have entered into a distribution agreement with Barclays Capital Inc., BNY Mellon Capital Markets, LLC and J.P. Morgan Securities LLC, each of which we refer to as an agent and together as the agents, under which we may offer and sell common stock having an aggregate offering price of up to $175,000,000 from time to time through any agent, as our sales agent, or directly to any agent acting as principal. We have filed the distribution agreement as an exhibit to a Current Report on Form8-K, which is incorporated by reference in this prospectus supplement.

Sales, if any, of shares of our common stock by any agent, acting as our sales agent, will be made in “at the market” offerings, as defined in Rule 415 under the Securities Act of 1933, as amended, including sales made directly on the New York Stock Exchange, the existing trading market for our common stock, or sales made to or through a market maker or through an electronic communications network. In addition, shares of our common stock may be offered and sold by such other methods, including privately negotiated transactions (including block transactions), as we and any sales agent agree to in writing.

The agents are not required to sell any specific number or dollar amount of our common stock but have agreed to use their commercially reasonable efforts, consistent with their normal trading and sales practices, as our sales agents and on the terms and subject to the conditions of the distribution agreement, to sell the common stock offered on terms agreed upon by the agents and us. If we elect to offer common stock, we will instruct an agent as to the number of shares of common stock to be sold by it as our sales agent and the date or dates on which such common stock is to be sold. We may instruct such agent not to sell our common stock as our sales agent if the sales cannot be effected at or above a price designated by us. Such agent may decline to accept any such instructions that we may provide to it from time to time. We or any agent may suspend the offering of common stock by an agent, as our sales agent, upon notice to the other parties.

If common stock is sold by any agent, as our sales agent, in an at the market offering, the agents have agreed to confirm to us in writing the number of shares of common stock sold on the applicable trading day and the related gross sales price and net sales price of those shares of common stock on the immediately following trading day. We will report at least quarterly the number of shares of common stock sold through the agents, as our sales agents, under the distribution agreement and information concerning the proceeds from those sales.

The agents will not engage in any transactions that stabilize or maintain the market price of our common stock in connection with any offers or sales of our common stock as our sales agents pursuant to the distribution agreement.

We will pay the agents a commission of 1.00% of the gross sales price per share of common stock sold through the agents, as our sales agents, under the distribution agreement. The remaining sales proceeds, after deducting any transaction fees, transfer taxes or similar taxes or fees imposed by any governmental, regulatory or self-regulatory organization in respect of the sale of the common stock, will constitute our net proceeds (before the expenses referred to in the next paragraph) from the sale of the common stock in the offering.

The expenses of the offering are estimated at approximately $400,000 and are payable by us. We have agreed to reimburse the agents for certain of their expenses.

Under the terms of the distribution agreement, we may, if agreed to by any agent, also sell common stock to such agent, as principal, at a price per share to be agreed upon at the time of sale. If we sell common stock to an agent as principal, we will enter into a separate terms agreement with such agent and we will describe the public offering price, underwriting discount (which may exceed 1.00% of the public offering price) and other terms of the offering of those shares of common stock in a separate prospectus supplement or pricing supplement. However, the agents have no obligation to agree to purchase common stock as principal or to enter into a terms agreement.

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Settlement for sales of our common stock will occur on the second trading day (or on such other date as may be agreed upon by us and an agent) following the respective dates on which any such sales are 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.

The offering of common stock pursuant to the distribution agreement will terminate upon the earliest of (1) the sale of common stock having an aggregate sales price of $175,000,000, (2) the termination of the distribution agreement by us or the agents and (3) May 18, 2019. The distribution agreement may be terminated either by us or the agents at any time and in the sole discretion of us or the agents, as the case may be.

In connection with the sale of our common stock on our behalf, the agents may be deemed to be “underwriters” within the meaning of the Securities Act, and the compensation paid to the agents may be deemed to be underwriting commissions or discounts. We have agreed to provide indemnification and contribution to the agents against certain liabilities, including liabilities under the Securities Act.

We have determined that our common stock is an “actively-traded security’’ excepted from the requirements of Rule 101 of Regulation M under the Securities Exchange Act of 1934, as amended, or the Exchange Act, by Rule 101(c)(1) of Regulation M. If any agent 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 has agreed to promptly notify the other parties and sales of common stock under the distribution agreement will be suspended until that or other exemptive provisions have been satisfied in the judgment of the agents and us.

Selling restrictions

No action has been taken in any jurisdiction (except in the United States) that would permit a public offering of the common stock, or the possession, circulation or distribution of this prospectus supplement, the accompanying prospectus, any free writing prospectus or any other offering material relating to us or our common stock where action for that purpose is required. Accordingly, our common stock may not be offered or sold, directly or indirectly, and neither this prospectus supplement, the accompanying prospectus, any free writing prospectus nor any other offering material or advertisements in connection with our common stock may be distributed or published in or from any country or jurisdiction, except in compliance with any applicable rules and regulations of any such country or jurisdiction.

The agents may arrange to sell the common stock offered by this prospectus supplement and the accompanying prospectus in certain jurisdictions outside the United States, either directly or through affiliates, where they are permitted to do so.

Other relationships

The agents and their respective affiliates are full service financial institutions engaged in various activities, which may include sales and trading, commercial and investment banking, advisory, investment management, investment research, principal investment, hedging, market making, brokerage and other financial andnon-financial activities and services. The agents and their respective affiliates have provided, and may in the future provide, a variety of these services to us and to persons and entities with relationships with us, for which they received or will receive customary fees and expenses. In particular, among other relationships, an affiliate of each of Barclays Capital Inc., BNY Mellon Capital Markets, LLC and J.P. Morgan Securities LLC is a lender under our credit facility.

In the ordinary course of their various business activities, the agents and their respective affiliates, officers, directors and employees may purchase, sell or hold a broad array of investments and actively trade securities, derivatives, loans, commodities, currencies, credit default swaps and other financial instruments for their own account and for the accounts of their customers, and such investment and trading activities may involve or relate

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to our assets, securities and/or instruments (directly, as collateral securing other obligations or otherwise) and/or persons and entities with relationships with us. The agents and their respective affiliates may also communicate independent investment recommendations, market color or trading ideas and/or publish or express independent research views in respect of such assets, securities or instruments and may at any time hold, or recommend to clients that they should acquire, long and/or short positions in such assets, securities and instruments.

WHERE YOU CAN FIND MORE INFORMATION

We file annual, quarterly and current reports, proxy statements and other information with the SEC (FileNo. 1-9894). We also filed a registration statement on FormS-3, including exhibits, under the Securities Act, with respect to the securities offered by this prospectus supplement and the accompanying prospectus. This prospectus supplement does not contain all of the information included in the registration statement or the exhibits. You may read and copy the registration statement and any other document that we file with the SEC at the SEC’s Public Reference Room at 100 F Street, N.E., Washington, D.C. 20549. You can call the SEC at1-800-SEC-0330 for further information on the operation of the Public Reference Room. You can also find our public filings with the SEC on the internet at a web site maintained by the SEC located at www.sec.gov.

We are “incorporating by reference” specified documents that we file with the SEC, which means:

• incorporated documents are considered part of this prospectus supplement;

• we are disclosing important information to you by referring you to those documents; and

• information we file with the SEC will automatically update and supersede information contained in this prospectus supplement.

We incorporate by reference the documents listed below and any future filings we make with the SEC under Sections 13(a), 13(c), 14 or 15(d) of the Exchange Act, after the date of this prospectus supplement and before the end of the offering of the securities pursuant to this prospectus supplement:

• our Annual Report on Form10-K for the year ended December 31, 2017;

• our Quarterly Report on Form10-Q for the quarter ended March 31, 2018;

• our Current Reports on Form8-K filed January 18, 2018, March 15, 2018 and April 30, 2018; and

• the description of our common stock contained in our Registration Statement on Form8-B, dated April 1, 1988, and any amendment or report updating that description.

Notwithstanding the foregoing, information furnished under Items 2.02 and 7.01 of any Current Report on Form8-K, including the related exhibits under Item 9.01, is not incorporated by reference in this prospectus.

You may obtain copies of documents incorporated by reference in this prospectus, at no cost, by request directed to us at the following address or telephone number:

Alliant Energy Corporation

4902 North Biltmore Lane

Madison, Wisconsin 53718

Attention: James H. Gallegos

Senior Vice President,

General Counsel & Corporate Secretary

(608)458-3311

You can also find these filings on our website at www.alliantenergy.com. However, we are not incorporating the information on our website other than these filings into this prospectus.

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Any statement contained herein or in a document incorporated or deemed to be incorporated by reference herein will be deemed to be modified or superseded for purposes of this prospectus to the extent that a statement contained herein, in any other subsequently filed document which also is or is deemed to be incorporated by reference herein or in any prospectus supplement, modifies or supersedes such statement. Any such statement so modified or superseded will not be deemed, except as so modified and superseded, to constitute a part of this prospectus.

LEGAL MATTERS

The validity of the common stock offered in this prospectus supplement and certain legal matters will be passed upon for us by Perkins Coie LLP and for the agents by Gibson, Dunn & Crutcher LLP.

EXPERTS

The consolidated financial statements, and the related financial statement schedules, incorporated in this prospectus by reference from Alliant Energy Corporation’s Annual Report on Form10-K, and the effectiveness of Alliant Energy Corporation and subsidiaries’ internal control over financial reporting have been audited by Deloitte & Touche LLP, an independent registered public accounting firm, as stated in their reports, which are incorporated herein by reference. Such financial statements and financial statement schedules have been so incorporated in reliance upon the reports of such firm given upon their authority as experts in accounting and auditing.

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PROSPECTUS

Alliant Energy Corporation

Common Stock

Debt Securities

Warrants

Stock Purchase Contracts

and

Stock Purchase Units

We may offer and sell from time to time securities in one or more offerings in amounts, at prices and on terms determined at the time of the offering. This prospectus provides you with a general description of the securities we may offer.

We may offer and sell the following securities:

• common stock;

• debt securities, which may be convertible into our common stock;

• warrants to purchase debt securities, common stock or other securities; and

• stock purchase contracts and stock purchase units.

Each time securities are sold using this prospectus, we will provide one or more prospectus supplements containing specific information about the offering and the terms of the securities being sold, including the offering price. Each prospectus supplement may also add, update or change information contained in this prospectus. You should carefully read this prospectus and the prospectus supplement relating to the specific issue of securities, together with the documents we incorporate by reference, before you invest.

We may offer and sell these securities to or through underwriters, dealers or agents, or directly to investors, on a continuous or a delayed basis. The applicable prospectus supplement will provide the specific terms of the plan of distribution.

In addition, selling shareowners to be named in a prospectus supplement may offer and sell from time to time shares of our common stock in such amounts as set forth in a prospectus supplement. Unless otherwise set forth in a prospectus supplement, we will not receive any proceeds from the sale of shares of our common stock by any selling shareowners.

Our common stock is listed on the New York Stock Exchange under the symbol “LNT.”

Investing in our securities involves risk. See “Risk Factors” on page 4 of this prospectus and in any prospectus supplement or such other document we refer you to in any prospectus supplement for a discussion of certain risks that prospective investors should consider before investing in our securities.

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

This prospectus is dated December 15, 2017.

TABLE OF CONTENTS

	Page
About This Prospectus		1
Forward-Looking Statements		2
Alliant Energy Corporation		3
Risk Factors		4
Ratio of Earnings To Fixed Charges		5
Use of Proceeds		5
Description of Common Stock		6
Description of Debt Securities		8
Description of Warrants		18
Description of Stock Purchase Contracts and Stock Purchase Units		19
Selling Shareowners		19
Plan of Distribution		20
Where You Can Find More Information		22
Legal Matters		23
Experts		23

ABOUT THIS PROSPECTUS

References in this prospectus to “we,” “us” and “our” refer to Alliant Energy Corporation, unless the context otherwise requires. References to “common stock” refer to our common stock, $0.01 par value per share, and the attached common share purchase rights.

This prospectus is part of a registration statement that we filed with the Securities and Exchange Commission, or the SEC, utilizing a “shelf” registration process. Under this shelf registration process, we may, from time to time, sell the securities or combinations of the securities described in this prospectus, and one or more of our shareowners may sell our common stock, in one or more offerings. This prospectus provides you with a general description of those securities. Each time we, or, under certain circumstances, our shareowners, offer securities, we will provide a prospectus supplement that will contain specific information about the terms of that offering. The prospectus supplement may also add, update or change information contained in this prospectus and may include other special considerations applicable to such offering of securities. If there is any inconsistency between the information in this prospectus and any prospectus supplement, you should rely on the information in the prospectus supplement. You should read this prospectus and any prospectus supplement together with the additional information described under the heading “Where You Can Find More Information.”

You should rely only on the information contained or incorporated by reference in this prospectus and in any prospectus supplement. “Incorporated by reference” means that we can disclose important information to you by referring you to another document filed separately with the SEC. We have not authorized any other person to provide you with different information. If anyone provides you with different or inconsistent information, you should not rely on it.

This prospectus and any prospectus supplement or information incorporated by reference herein or therein contains summaries of certain agreements that we have filed as exhibits to various SEC filings, as well as certain agreements that we will enter into in connection with the offering of securities covered by any prospectus supplement. The descriptions of these agreements contained in this prospectus and any prospectus supplement or information incorporated by reference herein or therein do not purport to be complete and are subject to, or qualified in their entirety by reference to, the definitive agreements. Copies of the definitive agreements will be made available without charge to you by making a written or oral request to us. See “Where You Can Find More Information.”

We are not making offers to sell nor soliciting offers to buy, nor will we make an offer to sell nor solicit an offer to buy, securities in any jurisdiction where the offer or sale is not permitted.

You should assume that the information appearing in this prospectus and any supplement to this prospectus, or the information we file or previously filed with the SEC that we incorporate by reference in this prospectus or any prospectus supplement, is accurate only as of the respective dates of those documents in which the information is contained. Our business, financial condition, results of operations and prospects may have changed since those dates.

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FORWARD-LOOKING STATEMENTS

This prospectus, any prospectus supplement and the information incorporated by reference herein or therein contain forward-looking statements intended to qualify for the safe harbor from liability established by the Private Securities Litigation Reform Act of 1995. All statements, other than statements of historical fact, included in this prospectus or any prospectus supplement or incorporated by reference herein or therein, including statements regarding anticipated financial performance, business strategy and management’s plans and objectives for future operations, are forward-looking statements. These forward-looking statements can be identified as such because the statements generally include words such as “may,” “believe,” “expect,” “anticipate,” “plan,” “project,” “will,” “projections,” “estimate,” or other words of similar import. These forward-looking statements are subject to certain risks and uncertainties that could cause actual results to differ materially from those expressed in, or implied by, these statements. Readers are cautioned not to place undue reliance on these forward-looking statements. All forward-looking statements included in this prospectus, any prospectus supplement or in a document incorporated by reference herein or therein speak only as of the date of this prospectus, the applicable prospectus supplement or the document incorporated by reference, as the case may be. Additional information concerning factors that could cause actual results to differ materially from those in the forward-looking statements is contained under “Risk Factors” on page 4 of this prospectus and other documents that we file from time to time with the SEC that are incorporated by reference into this prospectus and any prospectus supplement. Numerous important factors described in this prospectus, any prospectus supplement and/or any document incorporated by reference in this prospectus and/or any prospectus supplement could affect these statements and could cause actual results to differ materially from our expectations. We assume no obligation, and disclaim any duty, to update or revise publicly any forward-looking statements, whether as a result of new information, future events or otherwise, except as required by law.

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ALLIANT ENERGY CORPORATION

Alliant Energy Corporation is an energy-services provider that operates as a regulated investor-owned public utility holding company. Our primary focus is to provide regulated electricity and natural gas service to customers in the Midwest through our two public utility subsidiaries, Interstate Power and Light Company, or IPL, and Wisconsin Power and Light Company, or WPL. We also havenon-regulated operations. Our utility business is engaged principally in:

• the generation and distribution of electricity to retail customers in select markets in Iowa and Wisconsin;

• the distribution and transportation of natural gas to retail customers in select markets in Iowa and Wisconsin;

• the sale of electricity to wholesale customers in Minnesota, Illinois, Iowa and Wisconsin; and

• the generation and distribution of steam for two customers in Cedar Rapids, Iowa and various other energy-related products and services.

We are a “holding company” under the Public Utility Holding Company Act of 2005 and are subject to regulation by the Federal Energy Regulatory Commission, or the FERC, under that Act.

Our principal executive offices are located at 4902 North Biltmore Lane, Madison, Wisconsin 53718, and our telephone number is(608) 458-3311.

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RISK FACTORS

Investing in our securities involves risk. You should carefully consider the specific risks discussed or incorporated by reference in the applicable prospectus supplement, together with all the other information contained in the prospectus supplement or incorporated by reference in this prospectus. You should also consider the risks and uncertainties discussed under the caption “Risk Factors” included in our Annual Report onForm 10-K for the year ended December 31, 2016, which is incorporated by reference in this prospectus, and which may be amended, supplemented or superseded from time to time by other reports we file with the SEC in the future.

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RATIO OF EARNINGS TO FIXED CHARGES

The following table shows our ratio of earnings to fixed charges for the periods presented:

Year Ended December 31,									Nine Months Ended September 30,
2012	2013		2014		2015		2016		2017
2.83		3.10		3.11		3.15		2.95		3.44

USE OF PROCEEDS

Unless we inform you otherwise in the applicable prospectus supplement, we intend to use the net proceeds from the sale of the securities for general corporate purposes, which may include repayment or refinancing of debt, acquisitions, working capital, capital expenditures, investments and repurchases and redemptions of securities. Net proceeds may be temporarily invested prior to use.

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DESCRIPTION OF COMMON STOCK

The following description of our common stock summarizes material terms and provisions that apply to the common stock. The summary may not contain all of the information that is important to you and is subject to and qualified in its entirety by reference to our articles of incorporation, our bylaws and our rights agreement, which are filed as exhibits to the registration statement of which this prospectus is a part. See “Where You Can Find More Information.”

General

Our restated articles of incorporation, as amended, provide that we have authority to issue 480,000,000 shares of common stock, $0.01 par value per share. We do not have the authority under our restated articles of incorporation, as amended, to issue shares of preferred stock.

Common Stock

Holders of our common stock are entitled to such dividends as our board of directors may declare from time to time in accordance with applicable law. Our ability to pay dividends is dependent upon a number of factors, including the ability of our subsidiaries to pay dividends. Our utility subsidiaries each have restrictions on the payment of dividends on their common stock based on the regulatory restrictions applicable to them, and IPL has restrictions on the payment of dividends on its common stock based on the terms of its outstanding preferred stock.

Holders of our common stock will be entitled to vote for the election of members to our board of directors and on all other matters. Holders of our common stock are entitled to one vote per share of common stock held by them on all matters properly submitted to a vote of shareowners, subject to Section 180.1150 of the Wisconsin Business Corporation Law. See “—Statutory and Bylaw Provisions.” Shareowners have no cumulative voting rights, which means that the holders of shares entitled to exercise more than 50% of the voting power are able to elect all of the directors to be elected. Our board of directors is divided into three classes, with staggered terms of three years each.

All shares of common stock are entitled to participate equally in distributions in liquidation. Holders of common stock have no preemptive rights to subscribe for or purchase our shares. There are no conversion rights, sinking fund or redemption provisions applicable to our common stock.

Common Share Purchase Rights

We have entered into a rights agreement pursuant to which each outstanding share of our common stock has attached a right to purchaseone-quarter of one share of our common stock. A right will also attach to each share of common stock that we subsequently issue prior to the expiration of the rights agreement. Under circumstances described below, the rights will entitle the holder of the rights to purchase additional shares of common stock. In this prospectus and any accompanying prospectus supplement, unless the context requires otherwise, all references to our common stock include the accompanying rights.

Currently, the rights are not exercisable and trade with the common stock. If the rights become exercisable, each full right, unless held by a person or group that beneficially owns more than 15% of our outstanding common stock, will initially entitle the holder to purchaseone-quarter of one share of our common stock at a purchase price of $55 per full share, subject to adjustment. The rights will become exercisable only if a person or group has acquired, or announced an intention to acquire, 15% or more of our outstanding common stock. Under some circumstances, including the existence of a 15% acquiring party, each holder of a right, other than the acquiring party, will be entitled to purchase at the right’s then-current exercise price, shares of our common stock having a market value of two times the exercise price. If another corporation acquires us after a party acquires

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15% or more of our common stock, each holder of a right will be entitled to receive the acquiring corporation’s common shares having a market value of two times the exercise price. The rights may be redeemed at a price of $0.001 per right until a party acquires 15% or more of our common stock and, after that time, may be exchanged for one share of our common stock per right until a party acquires 50% or more of our common stock. The rights expire on December 11, 2018, subject to extension. Under the rights agreement, our board of directors may reduce the thresholds applicable to the rights from 15% to not less than 10%. The rights do not have voting or dividend rights and, until they become exercisable, have no dilutive effect on our earnings.

The rights have certain anti-takeover effects, in that they could have the effect of delaying, deferring or preventing a change of control of our company by causing substantial dilution to a person or group that attempts to acquire a significant interest in our company on terms not approved by our board of directors.

Statutory and Bylaw Provisions

Provisions of Wisconsin law and our amended and restated bylaws might also discourage some types of transactions that involve an actual or threatened change of control of our company.

Section 196.795(3) of the Wisconsin Statutes provides that no person may take, hold or acquire, directly or indirectly, more than 10% of the outstanding voting securities of a public utility holding company, such as us, without the approval of the Public Service Commission of Wisconsin, or the PSCW.

Section 180.1150 of the Wisconsin Business Corporation Law provides that the voting power of shares of public Wisconsin corporations, such as us, held by any person or persons acting as a group in excess of 20% of the corporation’s voting power is limited to 10% of the full voting power of those shares, unless full voting power of those shares has been restored pursuant to a vote of shareowners. Sections 180.1140 to 180.1144 of the Wisconsin Business Corporation Law contain some limitations and special voting provisions applicable to specified business combinations involving Wisconsin corporations, such as us, and a significant shareowner, unless the board of directors of the corporation approves either the business combination or the purchase of shares that causes the shareowner to become a significant shareowner before such purchase occurs.

Similarly, Sections 180.1130 to 180.1133 of the Wisconsin Business Corporation Law contain special voting provisions applicable to some business combinations, unless specified minimum price and procedural requirements are met. Following commencement of a takeover offer, Section 180.1134 of the Wisconsin Business Corporation Law imposes special voting requirements on share repurchases effected at a premium to the market and on asset sales by the corporation, unless, as it relates to the potential sale of assets, the corporation has at least three independent directors and a majority of the independent directors vote not to have the provision apply to the corporation.

In addition, our amended and restated bylaws establish a procedure that shareowners seeking to call a special meeting of shareowners must satisfy. This procedure involves notice to us, the receipt by us of written demands for a special meeting from holders of 10% or more of the issued and outstanding shares of common stock, a review of the validity of such demands by an independent inspector appointed by us and the fixing of the record and meeting dates by our board of directors. In addition, shareowners demanding such a special meeting must deliver to us a written agreement to pay the costs incurred by us in holding a special meeting, including the costs of preparing and mailing the proxy materials for our solicitation of proxies for use at such meeting, in the event such shareowners are unsuccessful in their proxy solicitation.

Our amended and restated bylaws also provide our board of directors with discretion in postponing shareowner meetings, including, within certain limits, special meetings of shareowners. Additionally, our chairman or board of directors (acting by resolution) may adjourn a shareowner meeting at any time prior to the transaction of business at such meeting, within certain limits. Our amended and restated bylaws also contain strict time deadlines and procedures applicable to shareowners seeking to nominate a person for election as a director or to otherwise bring business before a meeting.

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DESCRIPTION OF DEBT SECURITIES

The following description of the debt securities, or notes, sets forth certain material terms and provisions of the notes to which any prospectus supplement may relate. The following description does not purport to be complete and is subject to, and is qualified in its entirety by reference to, the indenture between us and Wells Fargo Bank, National Association, as trustee, dated September 30, 2009, as it may be supplemented and amended from time to time. The indenture is an exhibit to the registration statement of which this prospectus is a part. The terms of the notes will include those stated in the indenture and those made a part of the indenture by reference to the Trust Indenture Act of 1939, as amended, which we refer to as the 1939 Act.

The particular terms of the notes offered by any prospectus supplement and the extent, if any, to which the provisions described in this prospectus may apply to the offered notes will be described in the prospectus supplement relating to the offered notes. Accordingly, for a description of the terms of a particular issue of notes, reference must be made to both the prospectus supplement relating thereto and to the following description.

The notes will be issued as unsecured debt securities under the indenture and will rank equally with all of our other unsecured and unsubordinated debt. The notes will be effectively subordinated to all of our secured debt. The indenture does not limit the aggregate principal amount of notes that may be issued under the indenture and provides that notes may be issued from time to time in one or more series pursuant to the terms of one or more supplemental indentures, board resolutions or officer’s certificates pursuant to a supplemental indenture or a board resolution. The indenture gives us the ability to reopen a previous issue of notes and issue additional notes of such series, unless otherwise provided.

Provisions of a Particular Series

The prospectus supplement applicable to each series of notes will specify, among other things:

• the title of such notes;

• any limit on the aggregate principal amount of such notes which may be authenticated and delivered under the indenture;

• the person to whom interest on a note of the series shall be payable if other than the person in whose name that note is registered at the close of business on the regular record date for such interest;

• the date or dates on which the principal of such notes is payable or the method or means by which those dates will be determined, and our right, if any, to extend those dates and the duration of any such extension;

• the rate or rates at which such notes shall bear interest, if any, or any method by which such rate or rates will be determined, the date or dates from which such interest will accrue, the interest payment dates on which such interest shall be payable, the regular record date for the interest payable on any interest payment date, and our right, if any, to extend the interest payment periods and the duration of any such extension;

• the place or places where the principal of, premium, if any, and interest, if any, on such notes shall be payable;

• the methods by which registration of transfer of such notes and exchanges of such notes may be effected, and by which notices and demands to or upon us in respect of such notes and the indenture may be made, given, furnished, filed or served;

• the note registrar and paying agent or agents for such notes and, if such is the case, a statement that the principal of such notes will be payable without presentment or surrender of the notes;

• if the time for giving redemption notices for such notes is different from that provided in the indenture, such different time, and the period or periods within which, or date or dates on which, the price or

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prices at which and the terms and conditions upon which such notes may be redeemed, in whole or in part, at our option, as well as any restrictions on such redemptions;

• our obligation(s), if any, to redeem, purchase or repay such notes pursuant to any sinking fund or analogous provision or at the option of a holder and the terms and conditions upon which such notes will be so redeemed, purchased or repaid, as well as any applicable exceptions to the indenture provisions relating to notices of redemption in the case of mandatory redemption or redemption or repayment at the option of a holder;

• the denominations in which such notes shall be issuable;

• the currency or currencies, including composite currencies, in which the principal, premium, if any, and interest, if any, on such notes will be payable if other than U.S. dollars and the method for determining the equivalent amount in U.S. dollars;

• if the principal, premium, if any, or interest, if any, on such notes are to be payable, at our election or at the election of a holder, in a coin or currency other than that in which such notes are stated to be payable: (i) the coin or currency in which any such payment will be payable and (ii) the period(s) within which, and the terms upon which, such election may be made;

• if the principal, premium, if any, or interest, if any, on such notes are to be payable, or are to be payable at our election or at the election of a holder, in securities or other property, the type and amount of such securities or other property, or the method by which such amount shall be determined, and the period(s) within which, and terms upon which, such election may be made;

• if the amount payable in respect of principal of or any premium or interest on any notes may be determined with reference to an index or formula, the manner in which such amount will be determined;

• any deletions from, modifications of or additions to the events of default or covenants as provided in the indenture pertaining to such notes, and any change in the rights of the trustee or holders pursuant to a supplemental indenture;

• any additions to the definitions set forth in the indenture with respect to such notes;

• whether such notes shall be issued in whole or in part in the form of a global security or securities; the terms and conditions, if any, upon which such global security or securities may be exchanged for certificated notes of such series and of like tenor of any authorized denomination and the circumstances under which such exchange may occur, if other than as provided in the indenture; the depositary for such global security or securities if other than The Depository Trust Company; and the form of any legend(s) to be borne by any such global security or securities in addition to or in lieu of the legend provided in the indenture;

• any limitations on the rights of the holders of such notes to transfer or exchange such notes or to obtain registration of transfer of such notes; and if a service charge will be made for the registration of transfer or exchange of such notes, the amount or terms of the service charge;

• any restriction or condition on the transferability of such notes;

• if other than the entire principal amount, the portion of the principal amount of such notes payable upon declaration of acceleration of maturity;

• the terms, if any, pursuant to which such notes may be converted into or exchanged for our or another person’s shares of capital stock or other securities;

• if such notes are denominated in a currency other than U.S. dollars or in a composite currency, the obligations or instruments, if any, which may be deposited in connection with a satisfaction and discharge, and any additional or alternative provisions for the reinstatement of our indebtedness in respect of such notes after satisfaction and discharge as provided in the indenture;

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• any exception to the indenture provisions relating to legal holidays or variation in the definition of “business day” with respect to such notes;

• anynon-applicability of the limitation on liens contained in the indenture to the notes of such series or any exceptions or modifications to the limitation on liens with respect to such notes; and

• any other terms of such notes.

The indenture does not contain provisions that afford holders of notes protection in the event of a highly leveraged transaction involving us.

Some of the notes may be issued as original issue discount notes. Such notes are sold at a discount below their stated principal amount. If we issue these securities, the prospectus supplement will describe any special tax, accounting or other information which we think is important. We encourage you to consult with your own tax and financial advisors on these important matters.

Registration and Transfer

We will not be required to (i) issue, register the transfer of or exchange notes of any series during a period of 15 days immediately preceding the date notice is given identifying the notes of such series called for redemption, or (ii) issue, register the transfer of or exchange any notes so selected for redemption, in whole or in part, except the unredeemed portion of any note being redeemed in part.

Payment and Paying Agent

Unless otherwise indicated in an applicable prospectus supplement, payment of principal of any notes will be made only against surrender to the paying agent of such notes. Principal of and interest on notes will be payable, subject to any applicable laws and regulations, at the office of such paying agent or paying agents as we may designate from time to time. Payment of interest on notes on any interest payment date will be made to the person in whose name the notes (or predecessor security) are registered at the close of business on the record date for such interest payment.

Unless otherwise indicated in an applicable prospectus supplement, the trustee will act as paying agent with respect to the notes. We may at any time designate additional paying agents or rescind the designation of any paying agents or approve a change in the office through which any paying agent acts.

All moneys paid by us to a paying agent for the payment of the principal of (and premium, if any) or interest on the notes of any series which remain unclaimed at the end of two years after such principal or interest shall have become due and payable will be repaid to us, upon request and subject to applicable abandoned property laws, and the holder of such notes will from that time forward look only to us for payment of such principal, premium and interest.

Limitation on Liens

Unless otherwise specified in a prospectus supplement for notes of a particular series, the following covenant will apply to the notes of that series.

So long as any notes remain outstanding, we will not secure any indebtedness with a lien on any shares of the capital stock of any of our significant subsidiaries, which shares of capital stock we directly or indirectly own from the date of the indenture or thereafter, unless we equally and ratably secure all notes. However, this restriction does not apply to or prevent:

(1) (i) liens upon capital stock later acquired, directly or indirectly, by us to secure (A) the purchase price of such capital stock or (B) indebtedness incurred solely for the purpose of financing the acquisition of

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any such capital stock, (ii) liens existing on any such capital stock at the time of acquisition, and (iii) extensions, renewals or replacements of any of the foregoing, provided that in connection with clause (iii), the principal amount of indebtedness so secured shall be for the same or a lesser principal amount of the indebtedness secured by the lien and no such lien shall extend to or cover any capital stock other than the capital stock being acquired or to more than the same proportion of all shares of capital stock as was covered by the lien that was extended, renewed or replaced; or

(2) attachment, judgment or other similar liens arising in connection with court proceedings, provided that the execution or other enforcement of such liens is effectively stayed and (i) the claims secured by the lien are being actively contested in good faith by appropriate proceedings or (ii) payment of the claims is covered in full (subject to customary deductible amounts) by insurance maintained with responsible insurance companies.

Liens on any shares of the capital stock of any of our significant subsidiaries to secure any indebtedness, other than liens described in (1) and (2) above, are referred to in this prospectus as “restricted liens.” The foregoing limitations do not apply to the extent that we create any restricted liens to secure indebtedness that, together with all of our other indebtedness secured by restricted liens, does not at the time exceed 10% of our consolidated net tangible assets, as determined by us as of a month end not more than 90 days prior to the closing or consummation of the proposed transaction.

For this purpose, “indebtedness” applied to any person means (i) any liability of such person (A) for borrowed money, or under any reimbursement obligation relating to a letter of credit (other than trade letters of credit) issued to support indebtedness or obligations of such person or others of the kinds referred to in this definition, or (B) evidenced by a bond, note, debenture or similar instrument, or (C) for payment obligations arising under any conditional sale or other title retention arrangement (including a purchase money obligation) given in connection with the acquisition of any businesses, properties or assets of any kind, or (D) for the payment of money relating to a capitalized lease obligation; (ii) any liability of others described in the preceding clause (i) that such person has guaranteed or that is otherwise its legal liability; and (iii) any amendment, supplement, modification, deferral, renewal, extension or refunding of any liability of the types referred to in clauses (i) and (ii) above.

For this purpose, “consolidated net tangible assets” means the total amount of our assets determined on a consolidated basis in accordance with generally accepted accounting principles, or GAAP, less (i) the sum of our consolidated current liabilities determined in accordance with GAAP and (ii) the amount of our consolidated assets classified as intangible assets determined in accordance with GAAP, including but not limited to, such items as goodwill, trademarks, trade names, patents, and unamortized debt discount and expense and regulatory assets carried as an asset on our consolidated balance sheet.

For this purpose, “significant subsidiary” means any direct, majority owned subsidiary of us that is a “significant subsidiary” as defined in RegulationS-X promulgated by the SEC.

Regulations prohibit or restrict the encumbrance or pledge of public utility assets for the benefit of an associated company. Any pledge of WPL’s common stock to secure the notes could require approval of the FERC or the PSCW. Any pledge of IPL’s common stock to secure the notes could require approval of the FERC. Even with a valid pledge of IPL’s or WPL’s common stock, foreclosure under the indenture may be subject to applicable regulatory requirements, including approval by each of the FERC or the PSCW, if foreclosure or the sale of the pledged IPL or WPL common stock may constitute a transfer of control of IPL or WPL. Applicable law gives each of the FERC and the PSCW broad discretion to define “control” for these purposes, and any such determination would depend upon the facts and circumstances existing at the time. Accordingly, the ability to foreclose on and dispose of IPL or WPL common stock may be restricted or delayed by applicable regulatory requirements.

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Consolidation, Merger and Sale

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 such consolidation or into which we are merged or the person that acquires by conveyance or transfer, or that leases, our properties and assets substantially as an entirety, is a corporation organized and existing under the laws of the United States, any State in the United States or the District of Columbia and such other corporation or person expressly assumes, by supplemental indenture executed and delivered to the trustee, the payment of the principal of (and premium, if any) and interest on all the notes and the performance of every covenant of the indenture on our part to be performed or observed;

• immediately after giving effect to such transactions, no event of default, and no event that, after notice or lapse of time or both, would become an event of default, shall have happened and be continuing; and

• we have delivered to the trustee an officer’s certificate and an opinion of counsel, each stating that such transaction complies with the provisions of the indenture governing consolidation, merger, conveyance, transfer or lease and that all conditions precedent to the transaction have been complied with.

Notwithstanding the foregoing, we may merge or consolidate with or transfer all or substantially all of our assets to an affiliate that has no significant assets or liabilities and was formed solely for the purpose of changing our jurisdiction of organization or our form of organization; provided that the amount of our indebtedness is not increased; and provided, further that the successor assumes all of our obligations under the indenture.

Modification

The indenture contains provisions permitting us and the trustee, with the consent of the holders of not less than a majority in aggregate principal amount of the outstanding notes of each series that is affected, to modify the indenture or the rights of the holders of the notes of such series; provided that no such modification may, without the consent of the holder of each outstanding note that is affected:

• change the stated maturity of the principal of, or any installment of principal of or interest on, any note, or reduce the principal amount of any note or the rate of interest on any note or any premium payable upon the redemption of any note, or change the method of calculating the rate of interest of any note, or impair the right to institute suit for the enforcement of any such payment on or after the stated maturity of any note (or, in the case of redemption, on or after the redemption date); or

• reduce the percentage of principal amount of the outstanding notes of any series, the consent of whose holders is required for any such supplemental indenture, or the consent of whose holders is required for any waiver (of compliance with certain provisions of the indenture or certain defaults under the indenture and their consequences) provided for in the indenture; or

• modify any of the provisions of the indenture relating to supplemental indentures requiring the consent of holders, waiver of past defaults, or waiver of certain covenants, except to increase any percentage vote required for such an amendment or supplemental indenture or to provide that certain other provisions of the indenture cannot be modified or waived without the consent of each affected holder of an outstanding note.

In addition, we and the trustee may execute, without the consent of any holders of notes, any supplemental indenture for certain other customary purposes, including the creation of any new series of notes.

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Events of Default

The indenture provides that any one or more of the following described events with respect to the notes of any series, which has occurred and is continuing, constitutes an “event of default” with respect to the notes of such series:

• failure for 30 days to pay interest on the notes of such series, when due on an interest payment date other than at maturity or upon earlier redemption; or

• failure to pay principal or premium, if any, or interest on the notes of such series when due at maturity or upon earlier redemption; or

• failure for 30 business days to deposit any sinking fund payment when due by the terms of a note of such series; or

• failure to observe or perform any other covenant or warranty of ours in the indenture (other than a covenant or warranty which has expressly been included in the indenture solely for the benefit of one or more series of notes other than such series) for 90 days after written notice to us from the trustee or the holders of at least 33% in principal amount of the outstanding notes of such series; or

• certain events of bankruptcy, insolvency or reorganization of us; or

• any other event of default specified by us pursuant to the indenture with respect to notes of that series.

The holders of not less than a majority in aggregate outstanding principal amount of the notes of any series have the right to direct the time, method and place of conducting any proceeding for any remedy available to the trustee with respect to the notes of such series. If an event of default occurs and is continuing with respect to the notes of any series, then the trustee or the holders of not less than 33% in aggregate outstanding principal amount of the notes of such series may declare the principal amount of the notes due and payable immediately by notice in writing to us (and to the trustee if given by the holders), and upon any such declaration such principal amount shall become immediately due and payable. At any time after such a declaration of acceleration with respect to the notes of any series has been made and before a judgment or decree for payment of the money due has been obtained as provided in the indenture, the holders of not less than a majority in aggregate outstanding principal amount of the notes of such series may rescind and annul such declaration and its consequences if the default has been cured or waived and we have paid or deposited with the trustee a sum sufficient to pay all matured installments of interest and principal (and premium, if any) due otherwise than by acceleration and all sums paid or advanced by the trustee, including reasonable compensation and expenses of the trustee.

The holders of not less than a majority in aggregate outstanding principal amount of the notes of any series, on behalf of the holders of all the notes of such series, may waive any past default with respect to such series, except (i) a default in the payment of principal (or premium, if any) or interest or (ii) a default in respect of a covenant or provision which as provided under the indenture cannot be modified or amended without the consent of the holder of each outstanding note of such series affected.

Satisfaction and Discharge

We will be discharged from our obligations on the notes of any series, or any portion of the principal amount of the notes of any series, if we irrevocably deposit with the trustee or any paying agent (other than us) sufficient cash or eligible obligations (or a combination of both) to pay the principal, or portion of principal, premium, if any, and interest, if any, due and to become due, and deliver to the trustee or such paying agent:

• if such deposit shall have been made prior to the maturity of the notes of such series, a company order stating that the money and eligible obligations shall be held in trust and deposited in accordance with the indenture; and

• if eligible obligations have been deposited, an opinion of counsel that the obligations so deposited constitute eligible obligations and do not contain provisions permitting the redemption or other

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prepayment at the option of the issuer thereof and an opinion of an independent public accountant to the effect that the eligible obligations will be sufficient in accordance with the indenture; and

• if such deposit shall have been made prior to the maturity of the notes of the series, (i) an officer’s certificate stating our intention that, upon delivery of the officer’s certificate, our indebtedness in respect of those notes, or the portions thereof, will have been satisfied and discharged as contemplated in the indenture, and (ii) if such deposit shall have been made prior to the maturity of the notes of the series, an opinion of counsel to the effect that, as a result of a change in law or a ruling of the U.S. Internal Revenue Service issued after the date of issuance of such notes, the holders of the notes of the series, or portions thereof, will not recognize income, gain or loss for U.S. federal income tax purposes as a result of the satisfaction and discharge of our indebtedness and will be subject to U.S. federal income tax on the same amounts, at the same times and in the same manner as if we had not so satisfied and discharged our indebtedness.

For this purpose, “eligible obligations” for U.S. dollar-denominated notes, means securities that are direct obligations of, or obligations the full and timely payment of which are unconditionally guaranteed by, the United States, entitled to the benefit of the full faith and credit thereof, or depository receipts issued by a bank as custodian with respect to these obligations or any specific interest, principal or other payments due in respect thereof held by the custodian in accordance with the indenture for the account of the holder of a depository receipt.

In the event that all of the conditions set forth above have been satisfied for any series of notes, or portions thereof, except that, for any reason, we have not delivered the officer’s certificate and opinion of counsel described in the second and third bulleted items above, the holders of those notes will no longer be entitled to the benefits of certain of our covenants under the indenture, including the covenant described above in “—Limitation on Liens.” Our indebtedness under those notes, however, will not be deemed to have been satisfied and discharged prior to maturity, and the holders of those notes may continue to look to us for payment of the indebtedness represented by those notes.

The indenture will be deemed satisfied and discharged when no notes remain outstanding and we have paid all other sums payable by us under the indenture and delivered an officer’s certificate and opinion of counsel to the trustee stating that the conditions to satisfaction and discharge contained in the indenture have been met. All moneys we pay to the trustee or any paying agent on notes (or hold in trust for payment thereof) which remain unclaimed at the end of two years after payments have become due will be paid to us (or, discharged if so held in trust) upon request, subject to certain notice provisions and applicable abandoned property laws. Thereafter, the holder of those notes may look only to us for payment and not the trustee or any paying agent.

Information Concerning the Trustee

The trustee, prior to an event of default with respect to notes of any series, undertakes to perform, with respect to notes of such series, only such duties as are specifically set forth in the indenture and, in case an event of default with respect to notes of any series has occurred and is continuing, shall exercise, with respect to notes of such series, such of the rights and powers vested in it by the indenture, and use the same degree of care and skill as a prudent individual would exercise in the conduct of his or her own affairs. Subject to such provision, the trustee is under no obligation to exercise any of the powers vested in it by the indenture at the request of any holder of notes of any series, unless offered reasonable security and indemnity by such holder against the costs, expenses and liabilities which might be incurred by the trustee. The trustee is not required to expend or risk its own funds or otherwise incur any financial liability in the performance of its duties if the trustee reasonably believes that repayment or adequate indemnity is not reasonably assured to it. The trustee may decline to follow any such direction from a holder if, among other reasons, the trustee determines in good faith that the actions or proceedings as directed may not lawfully be taken, would involve the trustee in personal liability or would be unduly prejudicial to the holders of the notes of such series not joining in such direction.

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Pursuant to the 1939 Act, if a default occurs with respect to notes of any series, the trustee may be required to resign as trustee under the indenture if it has a conflicting interest (as defined in the 1939 Act), unless the default is cured, duly waived or otherwise eliminated within 90 days.

We and certain of our subsidiaries may maintain deposit accounts and banking relationships with the trustee. The trustee and certain of its affiliates may also serve as trustee under other indentures pursuant to which our securities and certain of our subsidiaries are outstanding.

Governing Law

The indenture and the notes will be governed by, and construed in accordance with, the internal laws of the State of New York.

Global Securities

We will issue the notes in whole or in part in the form of one or more global certificates, which we refer to as global securities. We will deposit the global securities with or on behalf of The Depository Trust Company, which we refer to as DTC, and registered in the name of Cede & Co., as nominee of DTC. Beneficial interests in the global securities may be held through the Euroclear System (“Euroclear”) and Clearstream Banking, S.A. (“Clearstream”) (as indirect participants in DTC).

We have provided the following descriptions of the operations and procedures of DTC, Euroclear and Clearstream solely as a matter of convenience. These operations and procedures are solely within the control of DTC, Euroclear and Clearstream and are subject to change by them from time to time. Neither we, any underwriter nor the trustee take any responsibility for these operations or procedures, and you are urged to contact DTC, Euroclear or Clearstream directly to discuss these matters.

DTC has advised us that:

• DTC is a limited-purpose trust company organized under the New York Banking Law, a “banking organization” within the meaning of the New York Banking Law, a member of the Federal Reserve System, a “clearing corporation” within the meaning of the New York Uniform Commercial Code and a “clearing agency” registered under Section 17A of the Securities Exchange Act of 1934, as amended, or the Exchange Act;

• DTC holds securities that its direct participants deposit with DTC and facilitates the settlement among direct participants of securities transactions, such as transfers and pledges, in deposited securities through electronic computerized book-entry changes in direct participants’ accounts, thereby eliminating the need for physical movement of securities certificates;

• direct participants include securities brokers and dealers, trust companies, clearing corporations and other organizations;

• DTC is a wholly-owned subsidiary of The Depository Trust & Clearing Corporation, which is owned by the users of its regulated subsidiaries;

• access to the DTC system is also available to indirect participants such as securities brokers and dealers, banks and trust companies that clear through or maintain a custodial relationship with a direct participant, either directly or indirectly; and

• the rules applicable to DTC and its direct and indirect participants are on file with the SEC.

We expect that under procedures established by DTC:

• upon deposit of the global securities with DTC or its custodian, DTC will credit on its internal system the accounts of direct participants designated by the underwriters with portions of the principal amounts of the global securities; and

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• ownership of the notes will be shown on, and the transfer of ownership of the notes will be effected only through, records maintained by DTC or its nominee, with respect to interests of direct participants, and the records of direct and indirect participants, with respect to interests of persons other than participants.

Investors in the global securities who are participants in DTC’s system may hold their interests therein directly through DTC. Investors in the global securities who are not participants may hold their interests therein indirectly through organizations (including Euroclear and Clearstream) which are participants in such system. Euroclear and Clearstream may hold interests in the global securities on behalf of their participants through customers’ securities accounts in their respective names on the books of their respective depositories, which are Euroclear Bank S.A./N.V., as operator of Euroclear, and Citibank, N.A., as depository of Clearstream. All interests in the global securities, including those held through Euroclear or Clearstream, may be subject to the procedures and requirements of DTC. Those interests held through Euroclear or Clearstream may also be subject to the procedures and requirements of such systems.

The laws of some jurisdictions require that purchasers of securities take physical delivery of those securities in the form of a certificate. For that reason, it may not be possible to transfer interests in a global security to those persons. In addition, because DTC can act only on behalf of its participants, who in turn act on behalf of persons who hold interests through participants, the ability of a person having an interest in a global security to pledge or transfer that interest to persons or entities that do not participate in DTC’s system, or otherwise to take actions in respect of that interest, may be affected by the lack of a physical definitive security in respect of that interest.

So long as DTC or its nominee is the registered owner of a global security, DTC or that nominee will be considered the sole owner or holder of the notes represented by that global security for all purposes under the applicable indenture and under the notes. Except as described below, owners of beneficial interests in a global security will not be entitled to have notes represented by that global security registered in their names, will not receive or be entitled to receive the notes in the form of a physical certificate and will not be considered the owners or holders of the notes under the applicable indenture or under the notes, and may not be entitled to give the trustee directions, instructions or approvals. For that reason, each holder owning a beneficial interest in a global security must rely on DTC’s procedures and, if that holder is not a direct or indirect participant in DTC, on the procedures of the DTC participant through which that holder owns its interest, to exercise any rights of a holder of notes under the applicable indenture or the global security.

Neither we nor the trustee will have any responsibility or liability for any aspect of DTC’s records relating to the notes or relating to payments made by DTC on account of the notes, or any responsibility to maintain, supervise or review any of DTC’s records relating to the notes.

We will make payments (including principal, premium, if any, and interest) on the notes represented by the global securities to DTC or its nominee, as the registered owner of the notes, by wire transfer of immediately available funds to the accounts specified by the registered owner of the notes. We expect that when DTC or its nominee receives any payment on the notes represented by a global security, DTC will credit participants’ accounts with payments in amounts proportionate to their beneficial interests in the global security as shown in DTC’s records. We also expect that payments by DTC’s participants to owners of beneficial interests in the global security held through those participants will be governed by standing instructions and customary practice as is now the case with securities held for the accounts of customers registered in the names of nominees for such customers. DTC’s participants will be responsible for those payments.

DTC has advised us that it will take any action permitted to be taken by a holder of notes only at the direction of one or more participants to whose account DTC has credited the interests in the global securities and only in respect of such portion of the aggregate principal amount of the notes as to which such participant or participants has or have given such direction. However, if there is an event of default under the notes, DTC reserves the right to exchange the global securities for certificated securities, and to distribute such securities to its participants.

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Initial settlement for the notes will be made in immediately available funds. Transfers between participants in DTC will be effected in accordance with DTC’s procedures, and transfers between participants in Euroclear and Clearstream will be effected in accordance with their respective rules and operating procedures. The notes represented by the global securities are expected to be eligible to trade in DTC’sSame-Day Funds Settlement System, and any permitted secondary market trading activity in such notes will, therefore, be required by DTC to be settled in immediately available funds.

Cross-market transfers between the participants in DTC, on the one hand, and Euroclear or Clearstream participants, on the other hand, will be effected through DTC in accordance with DTC’s rules on behalf of Euroclear or Clearstream, as the case may be, by its depository; however, such cross-market transactions will require delivery of instructions to Euroclear or Clearstream, as the case may be, by the counterparty in such system in accordance with the rules and procedures and within the established deadlines (European time) of such system. Euroclear or Clearstream, as the case may be, will, if the transaction meets its settlement requirements, deliver instructions to its respective depository to take action to effect final settlement on its behalf by delivering or receiving interests in the relevant global security in DTC, and making or receiving payment in accordance with normal procedures forsame-day funds settlement applicable to DTC. Euroclear participants and Clearstream participants may not deliver instructions directly to the depositories for Euroclear or Clearstream.

Because of time zone differences, the securities account of a Euroclear or Clearstream participant purchasing an interest in a global security from a participant in DTC will be credited, and any such crediting will be reported to the relevant Euroclear or Clearstream participant, during the securities settlement processing day (which must be a business day for Euroclear and Clearstream) immediately following the settlement date of DTC. DTC has advised us that cash received in Euroclear or Clearstream as a result of sales of interests in a global securities by or through a Euroclear or Clearstream participant to a participant in DTC will be received with value on the settlement date of DTC but will be available in the relevant Euroclear or Clearstream cash account only as of the business day for Euroclear or Clearstream following DTC’s settlement date.

Although DTC, Euroclear and Clearstream have agreed to the foregoing procedures to facilitate transfers of interests in the global securities among participants in DTC, Euroclear and Clearstream, they are under no obligation to perform or to continue to perform such procedures, and may discontinue such procedures at any time. Neither we nor the trustee nor any of our or their respective agents will have any responsibility for the performance by DTC, Euroclear or Clearstream or their respective direct or indirect participants of their respective obligations under the rules and procedures governing their operations.

Certificated Notes

We will issue certificated notes to each person that DTC identifies as the beneficial owner of notes represented by the global securities upon surrender by DTC of the global securities only if:

• DTC notifies us that it is no longer willing or able to act as a depository for the global securities, and we have not appointed a successor depository within 90 days of that notice;

• an event of default with respect to the notes has occurred and is continuing; or

• we decide not to have the notes represented by a global security.

Neither we nor the trustee will be liable for any delay by DTC, its nominee or any direct or indirect participant in identifying the beneficial owners of the related notes. We and the trustee may conclusively rely on, and will be protected in relying on, instructions from DTC or its nominee, including instructions about the registration and delivery, and the respective principal amounts, of the notes to be issued.

We will make all payments of principal, interest and premium, if any, with respect to certificated notes by wire transfer of immediately available funds to the accounts specified by the holders of the certificated notes or, if no such account is specified, by mailing a check to each such holder’s registered address. We expect that secondary trading in any certificated notes will also be settled in immediately available funds.

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DESCRIPTION OF WARRANTS

The following summary of certain provisions of the warrants does not purport to be complete and is subject to, and is qualified in its entirety by reference to, all provisions of the applicable warrant agreement.

We may issue warrants for the purchase of debt securities, common stock or other securities. Warrants may be issued independently or together with debt securities, common stock or other securities offered by any prospectus supplement and may be attached to or separate from any such offered securities. Each series of warrants will be issued under a separate warrant agreement to be entered into between us and a bank or trust company, as warrant agent, all as will be set forth in the prospectus supplement relating to the particular issue of warrants. The warrant agent will act solely as our agent in connection with the warrants and will not assume any obligation or relationship of agency or trust for or with any holders of warrants or beneficial owners of warrants.

Reference is made to the prospectus supplement relating to the particular issue of warrants offered pursuant to such prospectus supplement for the terms of and information relating to such warrants, including, where applicable:

• the designation, aggregate principal amount, currencies, denominations and terms of the series of debt securities purchasable upon exercise of warrants to purchase debt securities and the price at which such debt securities may be purchased upon such exercise;

• the number of shares of common stock purchasable upon the exercise of warrants to purchase common stock and the price at which such number of shares of common stock may be purchased upon such exercise;

• the designation and number of units of other securities purchasable upon the exercise of warrants to purchase other securities and the price at which such number of units of such other securities may be purchased upon such exercise;

• the date on which the right to exercise such warrants will commence and the date on which such right will expire;

• U.S. federal income tax consequences applicable to such warrants;

• the number of warrants outstanding as of the most recent practicable date; and

• any other terms of such warrants.

Warrants will be issued in registered form only. The exercise price for warrants will be subject to adjustment in accordance with provisions described in the applicable prospectus supplement.

Each warrant will entitle the holder thereof to purchase such principal amount of debt securities or such number of shares of common stock or other securities at such exercise price as will in each case be set forth in, or calculable from, the prospectus supplement relating to the warrants, which exercise price may be subject to adjustment upon the occurrence of certain events as set forth in such prospectus supplement. After the close of business on the expiration date, or such later date to which such expiration date may be extended by us, unexercised warrants will become void. The place or places where, and the manner in which, warrants may be exercised will be specified in the prospectus supplement.

Prior to the exercise of any warrants to purchase debt securities, common stock or other securities, holders of such warrants will not have any of the rights of holders of debt securities, common stock or other securities, as the case may be, purchasable upon such exercise, including the right to receive payments of principal of, premium, if any, or interest, if any, on the debt securities purchasable upon such exercise or to enforce covenants in the applicable indenture, or to receive payments of dividends, if any, on the common stock purchasable upon such exercise, or to exercise any applicable right to vote.

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DESCRIPTION OF STOCK PURCHASE CONTRACTS AND STOCK PURCHASE UNITS

The following summary of certain provisions of the stock purchase contracts and stock purchase units does not purport to be complete and is subject to, and is qualified in its entirety by reference to, all provisions of the applicable stock purchase contracts or stock purchase units.

We may issue stock purchase contracts, including contracts obligating holders to purchase from us, and obligating us to sell to the holders, a specified number of shares of common stock or other securities at a future date or dates. The price per security and the number of securities may be fixed at the time the stock purchase contracts are issued or may be determined by reference to a specific formula set forth in the stock purchase contracts. The stock purchase contracts may be issued separately or as part of stock purchase units consisting of a stock purchase contract and debt securities, warrants, other securities or debt obligations of third parties, including U.S. treasury securities, securing the holders’ obligations to purchase the securities under the stock purchase contracts. The stock purchase contracts may require holders to secure their obligations under the stock purchase contracts in a specified manner. The stock purchase contracts also may require us to make periodic payments to the holders of the stock purchase units or vice versa, and those payments may be unsecured or refunded on some basis.

The stock purchase contracts, and, if applicable, collateral or depositary arrangements relating to the stock purchase contracts or stock purchase units, will be filed with the SEC in connection with the offering of stock purchase contracts or stock purchase units. The prospectus supplement relating to a particular issue of stock purchase contracts or stock purchase units will describe the terms of those stock purchase contracts or stock purchase units, including the following:

• if applicable, a discussion of material U.S. federal income tax considerations; and

• any other information we think is important about the stock purchase contracts or the stock purchase units.

If we issue stock purchase units where debt obligations of third parties are used as security for your obligations to purchase or sell shares of common stock or other securities, we will include in the prospectus supplement relating to the offering information about the issuer of the debt obligations. Specifically, if the issuer has a class of securities registered under the Exchange Act and is either eligible to register its securities on FormS-3 under the Securities Act of 1933, as amended, or the Securities Act, or meets the listing criteria to be listed on a national securities exchange, we will include a brief description of the business of the issuer, the market price of its securities and how you can obtain more information about the issuer. If the issuer does not meet the criteria described in the previous sentence, we will include substantially all of the information that would be required if the issuer were making a public offering of the debt obligations.

SELLING SHAREOWNERS

We may register shares of common stock covered by this prospectus forre-offers and resales by any selling shareowners to be named in a prospectus supplement. We may register these shares to permit selling shareowners to resell their shares when they deem appropriate. A selling shareowner may resell all, a portion or none of such shareowner’s shares at any time and from time to time. Selling shareowners may also sell, transfer or otherwise dispose of some or all of their shares of our common stock in transactions exempt from the registration requirements of the Securities Act. We do not know when or in what amounts the selling shareowners may offer shares for sale under this prospectus and any prospectus supplement. We will not receive any proceeds from any sale of shares by a selling shareowner under this prospectus and any prospectus supplement. We may pay all expenses incurred with respect to the registration of the shares of common stock owned by the selling shareowners, other than underwriting fees, discounts or commissions which will be borne by the selling shareowners. We will provide you with a prospectus supplement naming the selling shareowners, the amount of shares to be registered and sold and any other terms of the shares of common stock being sold by each selling shareowner.

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PLAN OF DISTRIBUTION

We may sell our securities, and any selling shareowner may sell shares of our common stock, in any one or more of the following ways from time to time: (1) through agents; (2) to or through underwriters; (3) through brokers or dealers; (4) in “at the market offerings” within the meaning of Rule 415(a)(4) under the Securities Act, to or through a market maker or into an existing trading market, on an exchange or otherwise; (5) directly by us or any selling shareowners to purchasers, including through a specific bidding, auction or other process; or (6) through a combination of any of these methods of sale. The applicable prospectus supplement will contain the terms of the transaction, name or names of any underwriters, dealers, agents and the respective amounts of securities underwritten or purchased by them, the initial public offering price of the securities, and the applicable agent’s commission, dealer’s purchase price or underwriter’s discount. Any selling shareowners, dealers and agents participating in the distribution of the securities may be deemed to be underwriters, and compensation received by them on resale of the securities may be deemed to be underwriting discounts. Additionally, because selling shareowners may be deemed to be “underwriters” within the meaning of Section 2(11) of the Securities Act, selling shareowners may be subject to the prospectus delivery requirements of the Securities Act.

Any initial offering price, dealer purchase price, discount or commission may be changed from time to time.

The securities may be distributed from time to time in one or more transactions, at negotiated prices, at a fixed price or fixed prices (that may be 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 prevailing market prices.

Offers to purchase securities may be solicited directly by us or any selling shareowner or by agents designated by us from time to time. Unless otherwise indicated in the prospectus supplement, any such agent will use its commercially reasonable efforts to solicit purchases for the period of its appointment or to sell securities on a continuing basis. Agents may receive compensation in the form of commissions, discounts or concessions from us. Agents may also receive compensation from the purchasers of the securities for whom they sell as principals. Each particular agent will receive compensation in amounts negotiated in connection with the sale, which might be in excess of customary commissions. Any such agent may be deemed to be an underwriter, as that term is defined in the Securities Act, of the securities so offered and sold. Accordingly, any commission, discount or concession received by them and any profit on the resale of the securities purchased by them may be deemed to be underwriting discounts or commissions under the Securities Act. We have not entered into any agreements, understandings or arrangements with any underwriters or broker-dealers regarding the sale of their securities. As of the date of this prospectus, there are no special selling arrangements between any broker-dealer or other person and us. No period of time has been fixed within which the securities will be offered and sold.

If underwriters are utilized in the sale of any securities in respect of which this prospectus is being delivered, such securities will be acquired by the underwriters for their own account and may be resold from time to time in one or more transactions, including negotiated transactions, at fixed public offering prices or at varying prices determined by the underwriters at the time of sale. Securities may be offered to the public either through underwriting syndicates represented by managing underwriters or directly by one or more underwriters. If any underwriter or underwriters are utilized in the sale of securities, unless otherwise indicated in the applicable prospectus supplement, the obligations of the underwriters are subject to certain conditions precedent, and the underwriters will be obligated to purchase all such securities if they purchase any of them.

If a dealer is utilized in the sale of the securities in respect of which this prospectus is delivered, we will sell such securities, and any selling shareowner will sell shares of our common stock to the dealer, as principal. The dealer may then resell such securities to the public at varying prices to be determined by such dealer at the time of resale. Transactions through brokers or dealers may include block trades in which brokers or dealers will attempt to sell shares as agent but may position and resell as principal to facilitate the transaction or in cross trades, in which the same broker or dealer acts as agent on both sides of the trade. Any such dealer may be deemed to be an underwriter, as such term is defined in the Securities Act, of the securities so offered and sold.

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In addition, any selling shareowner may sell shares of our common stock in ordinary brokerage transactions or in transactions in which a broker solicits purchases.

Offers to purchase securities may be solicited directly by us or any selling shareowner, and the sale thereof may be made by us or any selling shareowner, directly to institutional investors or others who may be deemed to be underwriters within the meaning of the Securities Act with respect to any resale thereof.

Agents, underwriters and dealers may be entitled under relevant agreements with us or any selling shareowner to indemnification by us against certain liabilities, including liabilities under the Securities Act, or to contribution with respect to payments which such agents, underwriters and dealers may be required to make in respect thereof. The terms and conditions of any indemnification or contribution will be described in the applicable prospectus supplement.

We may pay all expenses incurred with respect to the registration of the shares of common stock owned by any selling shareowners, other than underwriting fees, discounts or commissions, which will be borne by the selling shareowners. We or any selling shareowner may also sell shares of our common stock through various arrangements involving mandatorily or optionally exchangeable securities, and this prospectus may be delivered in connection with those sales.

We or any selling shareowner may enter into derivative, sale or forward sale transactions with third parties, or sell securities not covered by this prospectus to third parties in privately negotiated transactions. If the applicable prospectus supplement indicates, in connection with those transactions, the third parties may sell securities covered by this prospectus and the applicable prospectus supplement, including in short sale transactions and by issuing securities not covered by this prospectus but convertible into, exchangeable for or representing beneficial interests in securities covered by this prospectus, or the return of which is derived in whole or in part from the value of such securities. The third parties may use securities received under derivative, sale or forward sale transactions or securities pledged by us or any selling shareowner or borrowed from us, any selling shareowner or others to settle those sales or to close out any related open borrowings of stock, and may use securities received from us or any selling shareowner in settlement of those transactions to close out any related open borrowings of stock. The third party in such sale transactions will be an underwriter and will be identified in the applicable prospectus supplement.

Additionally, certain selling shareowners may engage in hedging transactions with broker-dealers in connection with distributions of shares or otherwise. In those transactions, broker-dealers may engage in short sales of shares in the course of hedging the positions they assume with such selling shareowner. Certain selling shareowners also may sell shares short and redeliver shares to close out such short positions. Certain selling shareowners may also enter into option or other transactions with broker-dealers which require the delivery of shares to the broker-dealer. The broker-dealer may then resell or otherwise transfer such shares pursuant to this prospectus. Certain selling shareowners also may loan or pledge shares, and the borrower or pledgee may sell or otherwise transfer the shares so loaned or pledged pursuant to this prospectus. Such borrower or pledgee also may transfer those shares to investors in our securities or the selling shareowner’s securities or in connection with the offering of other securities not covered by this prospectus.

Underwriters, broker-dealers or agents may receive compensation in the form of commissions, discounts or concessions from us or any selling shareowner. Underwriters, broker-dealers or agents may also receive compensation from the purchasers of shares for whom they act as agents or to whom they sell as principals, or both. Compensation as to a particular underwriter, broker-dealer or agent will be in amounts to be negotiated in connection with transactions involving shares and might be in excess of customary commissions. In effecting sales, broker-dealers engaged by us or any selling shareowner may arrange for other broker-dealers to participate in the resales.

Any securities offered other than common stock will be a new issue and, other than the common stock, which is listed on the New York Stock Exchange, will have no established trading market. We may elect to list

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any series of securities on an exchange, and in the case of the common stock, on any additional exchange, but, unless otherwise specified in the applicable prospectus supplement and/or other offering material, we shall not be obligated to do so. No assurance can be given as to the liquidity of the trading market for any of the securities.

Agents, underwriters and dealers may engage in transactions with, or perform services for, us or our subsidiaries or any selling shareowner in the ordinary course of business.

Any underwriter may engage in overallotment, stabilizing transactions, short covering transactions and penalty bids in accordance with Regulation M under the Exchange Act. Overallotment involves sales in excess of the offering size, which create a short position. Stabilizing transactions permit bids to purchase the underlying security so long as the stabilizing bids do not exceed a specified maximum. Short covering transactions involve purchases of the securities in the open market after the distribution is completed to cover short positions. Penalty bids permit the underwriters to reclaim a selling concession from a dealer when the securities originally sold by the dealer are purchased in a covering transaction to cover short positions. Those activities may cause the price of the securities to be higher than it would otherwise be. If commenced, the underwriters may discontinue any of the activities at any time. An underwriter may carry out these transactions on the New York Stock Exchange, in theover-the-counter market or otherwise.

The place and time of delivery for securities will be set forth in the accompanying prospectus supplement.

WHERE YOU CAN FIND MORE INFORMATION

We file annual, quarterly and current reports, proxy statements and other information with the SEC(File No. 1-9894). We also filed a registration statement on FormS-3, including exhibits, under the Securities Act, with respect to the securities offered by this prospectus. This prospectus is a part of the registration statement, but does not contain all of the information included in the registration statement or the exhibits. You may read and copy the registration statement and any other document that we file with the SEC at the SEC’s Public Reference Room at 100 F Street, N.E., Washington, D.C. 20549. You can call the SEC at1-800-SEC-0330 for further information on the operation of the Public Reference Room. You can also find our public filings with the SEC on the internet at a web site maintained by the SEC located at http://www.sec.gov.

We are “incorporating by reference” specified documents that we file with the SEC, which means:

• incorporated documents are considered part of this prospectus;

• we are disclosing important information to you by referring you to those documents; and

• information we file with the SEC will automatically update and supersede information contained in this prospectus.

We incorporate by reference the documents listed below and any future filings we make with the SEC under Sections 13(a), 13(c), 14 or 15(d) of the Securities Exchange Act of 1934, after the date of this prospectus and before the end of the offering of the securities pursuant to this prospectus:

• our Annual Report on Form10-K for the year ended December 31, 2016;

• our Quarterly Reports on Form10-Q for the quarters ended March 31, 2017, June 30, 2017 and September 30, 2017;

• our Current Reports on Form8-K filed May 9, 2017, May 25, 2017, August 8, 2017, August 21, 2017, October 10, 2017, November 15, 2017 and December 11, 2017;

• the description of our common stock contained in our Registration Statement on Form8-B, dated April 1, 1988, and any amendment or report updating that description; and

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• the description of our common share purchase rights contained in our Registration Statement onForm 8-A, dated January 20, 1999, as amended by Amendment No. 1 on Form8-A/A, dated December 11, 2008, and any other amendment or report updating that description.

Notwithstanding the foregoing, information furnished under Items 2.02 and 7.01 of any Current Report on Form8-K, including the related exhibits under Item 9.01, is not incorporated by reference in this prospectus.

You may obtain copies of documents incorporated by reference in this prospectus, at no cost, by request directed to us at the following address or telephone number:

Alliant Energy Corporation

4902 North Biltmore Lane

Madison, Wisconsin 53718

Attention: James H. Gallegos, Senior Vice

President, General Counsel and Corporate

Secretary

(608)458-3311

You can also find these filings on our website at www.alliantenergy.com. However, we are not incorporating the information on our website other than these filings into this prospectus.

Any statement contained herein or in a document incorporated or deemed to be incorporated by reference herein will be deemed to be modified or superseded for purposes of this prospectus to the extent that a statement contained herein, in any other subsequently filed document which also is or is deemed to be incorporated by reference herein or in any prospectus supplement, modifies or supersedes such statement. Any such statement so modified or superseded will not be deemed, except as so modified and superseded, to constitute a part of this prospectus.

LEGAL MATTERS

Unless otherwise specified in a prospectus supplement accompanying this prospectus, the validity of the securities offered in this prospectus and certain legal matters will be passed upon for us by Perkins Coie LLP. If the securities are being distributed in an underwritten offering, certain legal matters will be passed upon for the underwriters by counsel identified in the related prospectus supplement.

EXPERTS

The consolidated financial statements, and the related financial statement schedules, incorporated in this prospectus by reference from Alliant Energy Corporation’s 2016 Annual Report on Form10-K, and the effectiveness of Alliant Energy Corporation and subsidiaries’ internal control over financial reporting have been audited by Deloitte & Touche LLP, an independent registered public accounting firm, as stated in their reports, which are incorporated herein by reference. Such financial statements and financial statement schedules have been so incorporated in reliance upon the reports of such firm given upon their authority as experts in accounting and auditing.

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$175,000,000

Alliant Energy Corporation

Common Stock

PROSPECTUS SUPPLEMENT

Barclays

BNY Mellon Capital Markets, LLC

J.P. Morgan

May 18, 2018