Grounding and bonding of portable generators

November 5, 1991

Mr. Lawrence R. Roche
Senior Project Engineer
WINCO, Inc.
[Recipient address omitted]

Dear Mr. Roche:

This in response to your letter of July 16 requesting written clarification regarding the safety of employees in the workplace where portable generators are used as power sources in separately derived systems. Thank you for follow-up information forwarded by your letter of August 9. The three questions you asked and the corresponding replies follow. Please accept our apology for the delay in response.

Question 1: "Is bonding the neutral of a portable generator to the general frame optional?"

Reply: a. If a portable generator is to be used as an electrical power source in a separately derived system, as opposed to a service supplied system such as is installed permanently in a building, a neutral (ground) conductor must be bonded to the portable generator frame; it is not optional.

b. Given a. above, the frame of a portable generator is not required to be grounded when supplying electricity to equipment mounted on the generator if the noncurrent carrying metal parts of the equipment are bonded to the generator frame.

c. Given a. and b. above, the frame of a portable generator is not required to be grounded when supplying electricity to equipment through receptacles mounted on the generator by way of directly-connected or plug-connected conductor cables if the grounding conductor terminals of the receptacles are bonded to the generator frame. Grounding continuity from equipment through the conductor cable to the receptacle is required.

d. A separately derived system ceases to exist when the neutral of its portable generator (or other alternating current (AC) power source) is interconnected to the neutral of a service-supplied system.

e. For grounding of portable generators supplying fixed wiring systems reference is made to the National Electrical Code (NEC), section 250-5(d).

Question 2: "If the portable generator as defined by NEC Section 250-6(a)(i) and (2) is a three wire 120/240 volt unit, is bonding of the neutral to the generator frame still optional?"

Reply: The preceding response to question 1 also applies to three wired 120/240 volt portable generators used as electrical power sources in separately derived systems.

Question 3: "Does NEC Section 250-6(c), describing neutral bonding of a separately derived system apply even to portable generators as defined above?"

Reply: Yes, Section 250-6(c), as clarified by the reply to question number 1, applies to three wire 120/240 volt portable generators used as electrical power sources in separately derived systems.

We appreciate your interest in employee safety and health. If we can be of further assistance, please do not hesitate to contact us.

Sincerely,
Patricia K. Clark, Director
Directorate of Compliance Programs