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 OF SECURITIES

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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(Print or Type Responses)
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 (Month/Day/Year)
03/31/2024
(Street)

SUMMIT,, NJ 07901		 4. If Amendment, Date Original Filed (Month/Day/Year)
 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 (Month/Day/Year) 2A. Deemed Execution Date, if any (Month/Day/Year) 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 03/31/2024   M   18,640 A $ 0 (1) 18,640 I See footnote (2)
Common Stock               25,738,988 I See footnotes (3) (4)

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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 (Month/Day/Year) 3A. Deemed Execution Date, if any (Month/Day/Year) 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
(Month/Day/Year) 		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
Restricted Stock Unit (1) 03/31/2024   M     18,640   (5)   (5) Common Stock 18,640 $ 0 0 I See footnote (2)
Restricted Stock Unit (1) 04/01/2024   A   22,218     (6)   (6) Common Stock 22,218 $ 0 22,218 I See footnote (2)

Reporting Owners

Reporting Owner Name / Address Relationships
 Director  10% Owner  Officer  Other
ECP ControlCo, LLC
40 BEECHWOOD ROAD,
SUMMIT,, NJ 07901		   X   X    
ECP Management GP, LLC
40 BEECHWOOD ROAD,
SUMMIT,, NJ 07901		   X   X    
Energy Capital Partners Management, LP
40 BEECHWOOD ROAD,
SUMMIT,, NJ 07901		   X   X    

Signatures

 ECP ControlCo, LLC By: /s/ Rahman D'Argenio, Managing Member   04/02/2024
**Signature of Reporting Person Date

 ECP Management GP, LLC By: ECP ControlCo, LLC, its sole member By: /s/ Rahman D'Argenio, Managing Member   04/02/2024
**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   04/02/2024
**Signature of Reporting Person Date

Explanation of Responses:

* If the form is filed by more than one reporting person, see Instruction 4(b)(v).
** Intentional misstatements or omissions of facts constitute Federal Criminal Violations. See 18 U.S.C. 1001 and 15 U.S.C. 78ff(a).
(1) Each restricted stock unit represents a contingent right to receive one share of the Issuer's common stock.
(2) 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. ECP ControlCo, LLC ("ECP ControlCo") is 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 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.
(3) Includes 21,238,988 shares held by NESCO Holdings, LP and 4,500,000 shares held by ECP Cardinal Holdings, LP ("ECP Cardinal"). The general partner of each of ECP Cardinal and NESCO Holdings, LP is NESCO Holdings GP, LLC. ECP Starlight Guarantor (Public), LP ("ECP Starlight") and Energy Capital Partners III (NESCO Co-Invest), LP are the sole members of NESCO Holdings GP, LLC.
(4) ECP ControlCo is the sole member of Energy Capital Partners III, LLC, which is the general partner of (i) Energy Capital Partners GP III, LP, which is the general partner of each of 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"), which are the sole members of ECP Starlight Public GP, LLC, which is the general partner of ECP Starlight, 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 and NESCO Holdings, LP.
(5) The restricted stock units vested in full on March 31, 2024.
(6) The restricted stock units will vest on March 31, 2025 and have no expiration date.

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