United States securities and exchange commission logo





                                March 3, 2021

       John Erhard
       Chief Executive Officer
       ArcLight Clean Transition Corp.
       200 Clarendon Street, 55th Floor
       Boston, Massachusetts 02116

                                                        Re: ArcLight Clean
Transition Corp.
                                                            Registration
Statement on Form S-4
                                                            Filed February 3,
2021
                                                            File No. 333-252674

       Dear Mr. Erhard:

              We have reviewed your registration statement and have the
following comments. In
       some of our comments, we may ask you to provide us with information so
we may better
       understand your disclosure.

              Please respond to this letter by amending your registration
statement and providing the
       requested information. If you do not believe our comments apply to your
facts and
       circumstances or do not believe an amendment is appropriate, please tell
us why in your
       response.

              After reviewing any amendment to your registration statement and
the information you
       provide in response to these comments, we may have additional comments.

       Registration Statement on Form S-4 filed on February 3, 2021

       General

   1.                                                   Please disclose the
anticipated use of the funds from the trust and PIPE.
       Cover Page

   2.                                                   We note that your cover
page is three pages long. Please revise to limit your cover page
                                                        in accordance with Item
501(b) of Regulation S-K.
       Questions and Answers for Shareholders of ArcLight, page x

   3.                                                   Please include a
section that discusses the interests that the current officers and directors
                                                        have in the business
combination. Your disclosure should quantify the return that they
 John Erhard
FirstName  LastNameJohn
ArcLight Clean  Transition Erhard
                           Corp.
Comapany
March      NameArcLight Clean Transition Corp.
       3, 2021
March2 3, 2021 Page 2
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         will receive on their initial investments by listing the value of the
securities as of the most
         recent practicable date. Include the PIPE investors in your
discussion. Please make
         similar changes to your disclosure on page 15 and throughout the proxy
         statement/prospectus. For example, replace the statement that the
"securities will have a
         significantly higher value at the time of the Business Combination"
with the actual value.
Summary of the Proxy Statement/Prospectus
Company Overview, page 1

4.       Please address Proterra's history of net losses in the Summary, and
indicate whether
         Proterra intends to use any of the proceeds from the transaction to
address its accumulated
         deficit of $575.6 million.
Our limited history of selling battery systems..., page 26

5.       We note your disclosure in the third paragraph that Proterra's battery
systems,
         electrification and charging solutions, fleet and energy management
software and electric
         transit buses have in the past contained product defects. Please
separate this disclosure
         into its own risk factor with separate heading that sufficiently
describes the risk to
         investors. Please discuss any specific product defects that have
materially impacted your
         business. As an example, we note news reports that Proterra   s
electric buses were taken
         out of service in Philadelphia in 2020.
Risk Factors, page 26

6.       Please revise the introductory language to indicate that the risk
factors listed reflect the
         material factors that make an investment in the registrant or offering
speculative or risky.
         Refer to Item 105(a) of Regulation S-K.
Provisions in our charter documents..., page 64

7.       We note your disclosure here that the forum provision will not apply
to suits brought to
         enforce a duty or liability created by the Exchange Act. Please ensure
that you include
         similar disclosure where the forum provision is referenced throughout
the proxy
         statement/prospectus.
Related Agreements, page 102

8.       It does not appear that each of the agreements referenced in this
section have been
         attached as annexes to the proxy statement/prospectus. You may not
qualify by reference
         to the text of agreements that have not been included with the
registration statement.
         Please revise to disclose that all material terms have been discussed.
Background to the Business Combination, page 105

9.       We note your disclosure on page 106 that you were initially introduced
to Proterra by one
         of your board members who knew Proterra's Chief Technology Officer.
Please tell us
 John Erhard
ArcLight Clean Transition Corp.
March 3, 2021
Page 3
         what consideration was given to the board members' potential conflict
of interest, and to
         what extent he or she took part in the negotiations and decision to
proceed with the
         business combination.
10.      Please revise to include specific information with respect to the
negotiation of the terms of
         the transaction, particularly the 0.8925 exchange ratio.
11.      We note your disclosure on page 107 that ArcLight terminated business
combination
         discussions with all other targets. Please elaborate on the extent to
which negotiations
         with the other targets progressed.
GOVERNING DOCUMENTS PROPOSAL D, page 128

12.      Please revise the fourth paragraph to clearly state that the exclusive
forum provision in
         your certificate of incorporation will not apply to claims arising
under the Exchange Act
         or Securities Act.
U.S. Federal Income Tax Considerations, page 145

13.      Please have counsel file a tax opinion pursuant to Item 601(b)(8) of
Regulation S-K.
         Please revise your prospectus disclosure to provide a firm conclusion
regarding treatment
         of the transaction under Section 368(a) and remove language stating
         that "generally" certain material tax consequences will apply. Please
revise disclosure that
         the discussion is    a summary only    and revise to state that this
section constitutes
         counsel's opinion. Refer to Section III of Staff Legal Bulletin No. 19
for guidance.
Notes to Unaudited Pro Forma Condensed Combined Financial Information
3. Adjustments to Unaudited Pro Forma Condensed Combined Financial Information,
page 168

14.      We refer to your disclosures in Note 3. It appears the unaudited pro
forma condensed
         combined financial information has been prepared under the legacy
rules of Article 11 of
         Regulation S-X. Please be advised the amended rules to Article 11 of
Regulation S-
         X became effective on January 1, 2021 for calendar year-end
registrants. In this regard,
         please revise your pro forma presentation to comply with the
requirements under the
         amended rules.
INFORMATION ABOUT PROTERRA, page 190

15.    We note your disclosure that you have over 550 vehicles on the road
across 43 states,
       territories and provinces spanning a wide spectrum of climates,
conditions, altitudes, and
       terrains. We also note your disclosures regarding Proterra's competitive
strengths. Please
FirstName LastNameJohn Erhard
       balance this disclosure with a discussion of the challenges Proterra's
products face. For
Comapany    NameArcLight
       example,              Cleanchallenges
                  please disclose  TransitionorCorp.
                                                limitations associated with
your products in
March different  terrains
       3, 2021 Page   3 and in areas with extreme high or low temperatures.
FirstName LastName
 John Erhard
FirstName  LastNameJohn
ArcLight Clean  Transition Erhard
                           Corp.
Comapany
March      NameArcLight Clean Transition Corp.
       3, 2021
March4 3, 2021 Page 4
Page
FirstName LastName
Proterra's Management's Discussion and Analysis of Financial Condition and
Results of
Operations
Key metrics and select financial data
Deliveries, page 215

16.      We note your disclosure with respect to your vehicle deliveries in
recent periods. We note
         also that the image on page 213 references the sale of your 1,000th
bus in 2020. Please
         revise to clarify the difference between vehicle sales and deliveries,
and consider adding a
         section that specifically addresses vehicle sales.
        We remind you that the company and its management are responsible for
the accuracy
and adequacy of their disclosures, notwithstanding any review, comments, action
or absence of
action by the staff.

       Refer to Rules 460 and 461 regarding requests for acceleration. Please
allow adequate
time for us to review any amendment prior to the requested effective date of
the registration
statement.

       You may contact Effie Simpson at 202-551-3346 or Jean Yu at 202-551-3305
if you have
questions regarding comments on the financial statements and related matters.
Please contact
Erin Purnell at 202-551-3454 or Asia Timmons-Pierce at 202-551-3754 with any
other
questions.



                                                               Sincerely,

                                                               Division of
Corporation Finance
                                                               Office of
Manufacturing