ECP ControlCo SC 13D/ACustom Truck One Source / ECP ControlCo ownership change

United States

Securities and Exchange Commission

Washington, D.C. 20549

SCHEDULE 13D

(Rule 13d-101)

Information to be Included in Statements Filed Pursuant to § 240.13d-1(a) and

Amendments Thereto Filed Pursuant to § 240.13d-2(a)

Under the Securities Exchange Act of 1934

(Amendment No. 3)*

Custom Truck One Source, Inc.

(Name of Issuer)

Common Stock

(Title of Class of Securities)

64083J104

(CUSIP Number)

Christopher M. Leininger, Esq.

c/o ECP

40 Beechwood Road

Summit, NJ 07901

(973) 671-6100

(Name, Address and Telephone Number of Person Authorized to Receive Notices and Communications)

December 20, 2021

(Date of Event Which Requires Filing of this Statement)

If the filing person has previously filed a statement on Schedule 13G to report the acquisition that is the subject of this Schedule 13D, and is filing this schedule because of §§240.13d-1(e), 240.13d-1(f) or 240.13d-1(g), check the following box.  ☐

Note: Schedules filed in paper format shall include a signed original and five copies of the schedule, including all exhibits. See § 240.13d-7 for other parties to whom copies are to be sent.

The information required on the remainder of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of 1934 (“Act”) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

Explanatory Note

This Amendment No. 3 to Schedule 13D (this “Amendment No. 3”) amends and supplements the Statement on Schedule 13D filed with the United States Securities and Exchange Commission on August 7, 2019, as amended to date (the “Statement”), relating to the common stock, par value $0.0001 per share (the “Common Stock”) of Custom Truck One Source, Inc. (the “Issuer”). Capitalized terms used herein without definition shall have the meaning set forth in the Statement.

Item 2. Identity and Background.

Item 2 of the Statement is hereby amended and restated in its entirety by the following:

This Statement is filed on behalf of the following (each a “Reporting Person” and collectively the “Reporting Persons”):

(15) ECP Cardinal Holdings, LP

Each of the Reporting Persons is organized under the laws of the State of Delaware. The business address of each of the Reporting Persons is c/o ECP, 40 Beechwood Road Summit, New Jersey 07901. The Reporting Persons are principally engaged in the business of investing in securities, including of the Issuer.

Information with respect to the managing members and officers of ECP ControlCo, LLC (collectively, the “Related Persons”), including name, business address, present principal occupation or employment and the organization in which such employment is conducted, and citizenship is listed on the attached Schedule A, which is incorporated herein by reference.

Certain of the Reporting Persons (collectively, “ECP”), certain of the investment entities affiliated with The Blackstone Group (collectively, “Blackstone”), Capitol Acquisition Management IV, LLC and Capitol Acquisition Founder IV, LLC (together, “Capitol”), Platinum

Equity Advisors, LLC (“Platinum”) and certain other stockholders of the Issuer (collectively with ECP, Blackstone, Capitol and Platinum, the “Stockholder Parties”) may be deemed to constitute a group for purposes of Rule 13d-3 under the Act. Shares beneficially owned by the Stockholder Parties, other than by ECP, are not the subject of this Schedule 13D and accordingly, none of the other Stockholder Parties is included as a reporting person. For a description of the relationship between the Reporting Persons and the other Stockholder Parties, see Item 4 below.

During the last five years, none of the Reporting Persons or Related Persons (i) has been convicted in any criminal proceeding (excluding traffic violations or similar misdemeanors) or (ii) was a party to a civil proceeding of a judicial or administrative body of competent jurisdiction and as a result of such proceeding was or is subject to a judgment, decree or final order enjoining future violations of, or prohibiting or mandating activities subject to, federal or state securities laws or finding any violation with respect to such laws.

Item 4. Purpose of Transaction.

Item 4 of the Statement is hereby amended and supplemented by adding the following:

Pledge Agreement

On December 20, 2021, ECP Cardinal Holdings, LP and NESCO Holdings, LP (together, the “Borrowers”) each entered into a loan agreement (together, the “Loan Agreements”) with JPMorgan Chase Bank, N.A. (the “Lender”). As security for their obligations under the Loan Agreements, the Borrowers pledged 25,738,988 shares of Common Stock (collectively, the “Pledged Securities”). All voting rights and rights to receive dividends or distributions with respect to the Pledged Securities will remain with the Borrowers unless an event of default under the Loan Agreements has occurred and is continuing.

Also on December 20, 2021, 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 collectively transferred an aggregate of 4,500,000 shares of Common Stock to ECP Cardinal Holdings, LP in order to facilitate the foregoing pledge transaction.

Item 5. Interest in Securities of the Issuer.

(a) – (b)

The following sets forth, as of the date of this Schedule 13D, the aggregate number of shares of Common Stock and percentage of shares of Common Stock beneficially owned by each of the Reporting Persons, as well as the number of shares of Common Stock as to which each Reporting Person has the sole power to vote or to direct the vote, shared power to vote or to direct the vote, sole power to dispose or to direct the disposition of, or shared power to dispose or to direct the disposition of, as of the date hereof, based on 247,311,494 shares of Common Stock outstanding as of November 17, 2021.

ECP Cardinal Holdings, LP and NESCO Holdings, LP are the record holders of 4,500,000 and 21,238,988 shares of Common Stock, respectively. Energy Capital Partners Management, LP is the beneficial owner of 107,148 shares of Common Stock and 13,631 shares of Common Stock underlying restricted stock units.

The general partner of each of ECP Cardinal Holdings, LP and NESCO Holdings, LP is NESCO Holdings GP, LLC. 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.

ECP Management GP, LLC is the general partner of Energy Capital Partners Management, LP and may be deemed to share beneficial ownership of the securities beneficially owned by Energy Capital Partners Management, LP.

ECP ControlCo, LLC is the sole member of Energy Capital Partners III, LLC and the sole member of ECP Management GP, LLC and may be deemed to share beneficial ownership of the securities beneficially owned be each of Energy Capital Partners III, LLC and ECP Management GP, LLC. The managing members of ECP ControlCo, LLC 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, LLC. Each such individual disclaims beneficial ownership of such shares except to the extent of his pecuniary interest therein.

As discussed in Item 2 above, the other Stockholder Parties are not included as reporting persons in this Statement, and the Reporting Persons expressly disclaim beneficial ownership of the shares of Common Stock held by the other Stockholder Parties.

Item 7. Materials to be Filed as Exhibits

Item 7 of the Statement is amended and supplemented by the following:

SIGNATURES

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this Statement is true, complete and correct.

Date: December 29, 2021

SCHEDULE A

The name, present principal occupation or employment and citizenship of each of the executive officers and directors of the Reporting Persons are set forth below. The business address of each individual is c/o ECP, 40 Beechwood Road Summit, NJ 07901.

ECP ControlCo, LLC

Managing Members