OSHA requirements are set by statute, standards and regulations. Our interpretation letters explain these requirements and how they apply to particular circumstances, but they cannot create additional employer obligations. This letter constitutes OSHA's interpretation of the requirements discussed. Note that our enforcement guidance may be affected by changes to OSHA rules. Also, from time to time we update our guidance in response to new information. To keep apprised of such developments, you can consult OSHA's website at https://www.osha.gov.

June 28, 1993

Mr. Colin Iwasa
Yamaha Power Equipment
6555 Katella Avenue
Cypress, California 90630

Dear Mr. Iwasa:

This is in response to your December 20, 1991, letter requesting clarification of Occupational Safety and Health Administration (OSHA) standards concerning portable and vehicle mounted generators. I apologize for the excessive delay in responding to your inquiry.

In regard to whether portable and vehicle mounted generators on a construction job site are required to be listed, labeled or certified, please be advised that all electrical conductors and equipment are required to be "approved," according to 1926.403(a). Section 1926.449 includes definitions of terms used in the Electrical Standards for Construction, contained in Subpart K of 29 CFR Part 1926. The definitions of "approved" and "acceptable" indicate what constitutes approval under Subpart K of the Construction Standards.

An installation or equipment is acceptable to the Assistant Secretary of Labor, and approved within the meaning of this Subpart K:

(a) If it is accepted, or certified, or listed, or labeled, or otherwise determined to be safe by a qualified testing laboratory capable of determining the suitability of materials and equipment for installation and use in accordance with this standard; or

(b) With respect to an installation or equipment of a kind which no qualified testing laboratory accepts, certifies, lists, labels, or determines to be safe, if it is inspected or tested by another Federal agency, or by a State, municipal or other local authority responsible for enforcing occupational safety provisions of the National Electrical Code, and found in compliance with those provisions; or

(c) With respect to custom-made equipment or related installations which are designated, fabricated for, and intended for use by a particular customer, if it is determined to be safe for its intended use by its manufacturer on the basis of test data which the employer keeps and makes available for inspection to the Assistant Secretary and his authorized representatives.

Electric equipment (i.e., generators) meeting (a), (b) or (c) above will be considered approved by OSHA.

In regard to whether portable and vehicle mounted generators are included in the definition of "equipment" for purposes of Subpart K, please be advised that "equipment" is defined as follows:

A general term including material, fittings, devices, appliances, fixtures, apparatus, and the like, used as a part of, or in connections with, an electrical installation.

This definition clearly includes an electric generator supplying power to other electric equipment used on a jobsite.

In regard to a separately derived system, please be advised that the National Electrical Code (National Fire Protection Association, NFPA 70) contains information useful in determining whether a portable or vehicle-mounted electric generator is being used as part of a separately derived system. Section 250-5(d) of this code contains the following explanation:

(d) Separately Derived Systems. A premises wiring system whose power is derived from generator, transformer, or converter windings and has no direct electrical connection, including a solidly connected grounded circuit conductor, to supply conductors originating in another system...

(FPN No. 1): An alternate alternating-current power source such as an on-site generator is not a separately derived system if the neutral is solidly interconnected to a service-supplied system neutral.

Thus, a separately derived system has no direct electrical connection to another system supplying power at the jobsite, while a non-separately derived system would be directly connected to another system supplying power to the site. A non-separately derived system at the jobsite would not need to have its neutral grounded, because the service-supplied system would already provide this function. Portable and vehicle-mounted generators are typically used to provide primary power for utilization equipment on construction sites and are normally separately derived systems for the purposes of OSHA's electrical standards for construction.

In the OSHA standards for portable vehicle-mounted generators, equipment grounding conductor bonding is mentioned in 1926.404(f)(3)(i)(B) and in 1926.404(f)(3)(ii)(C). Neutral conductor bonding is mentioned in 1926.404(f)(3)(iii).

The intent of 1926.404(f)(3)(iii) becomes very clear when one considers that the term "neutral" is used in its technical sense. A 120/240 volt system has a neutral and therefore must be bonded to the generator frame. A 2-wire 120 volt system has no neutral and therefore bonding is optional. Recall that neither side of a 2-wire derived system is a neutral and when one grounds either side, it becomes a grounded terminal or conductor, but it is not a neutral.

If we can be of any further assistance, please contact me or Mr. Dale R. Cavanaugh of my staff in the Office of Construction and Maritime Compliance Assistance at (202) 219-8136.


Roy F. Gurnham, P.E., Esq. Director
Office of Construction and Maritime
Compliance Assistance

December 20, 1991

Patricia K. Clark
Directorate of Compliance Programs
U.S. Department of Labor
Washington, D.C. 20210

Dear Ms. Clark:

Your letter of October 1, 1991 pertaining to portable and vehicle mounted generators left me somewhat less than clear on OSHA's standards.

The following is my understanding of the information which you provided.

Labeled, listed, certified.

Portable and vehicle mounted generators on a job sit are not required to be listed, labeled or certified.

As defined in subpart K, portable and vehicle mounted generators are not "electrical conductors" or "equipment".

Neutral Bonding

Neutral bonding is required when the generator is a component of a separately derived system (29 CFR 1926.404(f)(3)(iii).

A generator is not a component of a separately derived system when it only supplies equipment mounted on the generator and/or cord and plug connected equipment through the generator receptacles.

A generator is a component of a separately derived system when it supplies conductors originating in another system.

29 CFR 1926.404(f)(3)(i) does not refer to neutral bonding.

29 CFR 1926.404(f)(1), (4) and, (5) do not refer to neutral bonding.

If my understanding of your letter is incorrect, inaccurate or incomplete please contact me.


Colin Iwasa
Outdoor Power Equipment