Sec Form 4 Filing - ECP ControlCo, LLC @ Custom Truck One Source, Inc. - 2021-05-17

FORM 4 Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940

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1. Name and Address of Reporting Person ECP ControlCo, LLC 2. Issuer Name and Ticker or Trading Symbol Custom Truck One Source, Inc. [ CTOS] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) X __ Director X __ 10% Owner _____ Officer (give title below) _____ Other (specify below) (Last) (First) (Middle) 40 BEECHWOOD ROAD, 3. Date of Earliest Transaction (MM/DD/YY) 05/17/2021 (Street) SUMMIT,, NJ07901 4. If Amendment, Date Original Filed (MM/DD/YY) 6. Individual or Joint/Group Filing (Check Applicable Line) _____ Form filed by One Reporting Person __ X __ Form filed by More than One Reporting Person (City) (State) (Zip)

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned 1.Title of Security (Instr. 3) 2. Transaction Date (MM/DD/YY) 2A. Deemed Execution Date, if any (MM/DD/YY) 3. Transaction Code (Instr. 8) 4. Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5) 5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4) 6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4) 7. Nature of Indirect Beneficial Ownership (Instr. 4) Code V Amount (A) or (D) Price Common Stock 05/17/2021 M 80,000 A $ 4 140,000 I See footnotes( 1 )( 5 ) Common Stock 05/17/2021 F 32,852 D $ 9.74 107,148 I See footnotes( 1 )( 5 ) Common Stock 04/01/2022 M 13,631 A $ 0( 2 ) 120,779 I See footnotes( 1 )( 5 ) Common Stock 25,738,988 I See footnotes( 3 )( 4 )( 5 )

Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned ( e.g. , puts, calls, warrants, options, convertible securities) 1. Title of Derivative Security (Instr. 3) 2. Conversion or Exercise Price of Derivative Security 3. Transaction Date (MM/DD/YY) 3A. Deemed Execution Date, if any (MM/DD/YY) 4. Transaction Code (Instr. 8) 5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4, and 5) 6. Date Exercisable and Expiration Date (MM/DD/YY) 7. Title and Amount of Underlying Securities (Instr. 3 and 4) 8. Price of Derivative Security (Instr. 5) 9. Number of Derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4) 10. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 4) 11. Nature of Indirect Beneficial Ownership (Instr. 4) Code V (A) (D) Date Exercisable Expiration Date Title Amount or Number of Shares Stock Option $ 4 05/17/2021 M 80,000 04/01/2021 06/11/2030 Common Stock 80,000 $ 0 0 I See footnotes( 1 )( 5 ) Restricted Stock Unit ( 3 ) 04/01/2022 M 13,631 ( 6 ) ( 6 ) Common Stock 13,631 $ 0 0 I See footnotes( 1 )( 5 )

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other ECP ControlCo, LLC 40 BEECHWOOD ROAD, SUMMIT,, NJ07901 X X ECP Management GP, LLC 40 BEECHWOOD ROAD, SUMMIT,, NJ07901 X X Energy Capital Partners Management, LP 40 BEECHWOOD ROAD, SUMMIT,, NJ07901 X X Signatures ECP ControlCo, LLC By: /s/ Rahman D'Argenio, Managing Member 04/05/2022 Signature of Reporting Person Date ECP Management GP, LLC By: ECP ControlCo, LLC, its sole member By: /s/ Rahman D'Argenio, Managing Member 04/05/2022 Signature of Reporting Person Date ECP ControlCo, LLC By: ECP Management GP, LLC, its general partner By: ECP ControlCo, LLC, its sole member By: /s/ Rahman D'Argenio, Managing Member 04/05/2022 Signature of Reporting Person Date

Explanation of Responses:

( 1 )Represents securities held directly by Energy Capital Partners Management, LP ("ECP Management"). ECP Management GP, LLC is the general partner of ECP Management and may be deemed to share beneficial ownership of the securities held by ECP Management. ( 2 )Each restricted stock unit represents a contingent right to receive one share of the Issuer's common stock. ( 3 )Represents securities held directly by 21,238,988 shares held by NESCO Holdings, LP and 4,500,000 shares held by ECP Cardinal Holdings, LP ( 4 )The general partner of each of ECP Cardinal Holdings, LP and NESCO Holdings, LP is NESCO Holdings GP, LLC. Energy Capital Partners III, LP, Energy Capital Partners III-A, LP, Energy Capital Partners III-B, LP, Energy Capital Partners III-C, LP, and Energy Capital Partners III-D, LP (together, the "ECP III Funds") and Energy Capital Partners III (NESCO Co-Invest), LP are the sole members of NESCO Holdings GP, LLC. Energy Capital Partners III, LLC is the general partner of (i) Energy Capital Partners GP III, LP, which is the general partner of each of the ECP III Funds, and (ii) Energy Capital Partners GP III Co-Investment (NESCO), LLC, which is the general partner of Energy Capital Partners III (NESCO Co-Invest), LP. As a result, each of the foregoing entities may be deemed to share beneficial ownership of the shares of Common Stock held by ECP Cardinal Holdings, LP and Nesco Holdings LP. ( 5 )ECP ControlCo, LLC ("ECP ControlCo") is the managing member of ECP III GP LLC and the sole member of ECP Management GP, LLC. As a result, ECP ControlCo may be deemed to share beneficial ownership of the securities beneficially owned by ECP III GP LLC and ECP Management GP, LLC. The managing members of ECP ControlCo are Douglas Kimmelman, Andrew Singer, Peter Labbat, Tyler Reeder and Rahman D'Argenio all of whom collectively share the power to vote and dispose of the securities beneficially owned by ECP ControlCo. Each such individual disclaims beneficial ownership of such shares except to the extent of his pecuniary interest therein. ( 6 )The restricted stock units vested in full on April 1, 2022.

Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.

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