Generator Ground

[hbendillo]

Situation: 277/480V/1200 KW Generator mounted outdoors in a weatherproof enclosure. 3-pole, transfer switch.

Question: We have a detail that we use in our office that shows a ground rod driven at the generator and a conductor from the ground rod bonded to the generator frame. I do not necessarily think this is required but what size bonding conductor should be used? I am thinking minimum #6 AWG copper but no larger is required. The detail shows the bonding conductor the same size as the equipment grounding conductor originating from the generator.

 

[dereckbc]

Re: Generator Ground

Originally posted by hbendillo:
The detail shows the bonding conductor the same size as the equipment grounding conductor originating from the generator.

You answered your own question.

 

[volt101]

Re: Generator Ground

Since you referenced a 3-pole transfer I am guessing that this is a non-separately derived system. That is one that does not require the use of a grounding electrode conductor at the generator unless the gen's manufacture requires it.

250.53(E) Supplemental Electrode Bonding Connection Size. Where the supplemental electrode is a rod, pipe, or plate electrode, that portion of the bonding jumper that is the sole connection to the supplemental grounding electrode shall not be required to be larger than 6 AWG copper wire or 4 AWG aluminum wire.

 

[don_resqcapt19]

Re: Generator Ground

It is my opinion that even when the generator is not SDS, that 250.32 requires the installation of a grounding electrode system at the generator.
Don

 

[volt101]

Re: Generator Ground

Is the generator a service?

250.32 refers to two or more buildings or structures that are supplied from a common ac service

Service. The conductors and equipment for delivering electric energy from the serving utility to the wiring system of the premises served.

 

[don_resqcapt19]

Re: Generator Ground

volt,
Look at the wording in 250.32. It doesn't say that the service directly supplies the second building or structure.

Where two or more buildings or structures are supplied from a common ac service by a feeder(s) or branch circuit(s), ...

The generator building or enclosure will most likely be fed by a feeder from the main building to provide power for the battery charger, engine block heater, lights and other associated equipment. If there is a feeder serving the generator a grounding electrode system is required. For some smaller installations, only a single branch circuit may supply normal power to the generator and in that case a grounding electrode system would not be required, but I doubt that a single branch circuit is supplying the normal power for a 1200 KW installation.
Don

 

[volt101]

Re: Generator Ground

Ah Ha,
I see what you are getting at, and I couldn't agree more with that statement. It would be required for a installation as such. But here he is talking about a bond from a grounding rod to the frame of the generator. He never stated that he does or doesn't have a feeder from a common ac service to the gen set.

 

[dereckbc]

Re: Generator Ground

Maybe I am missing the point here. The specification calls for a bonding jumper that meets or exceeds NEC requirements. What is the problem?

 

[tom baker]

Re: Generator Ground

Don you are saying the generator is a structure, and every structure requires a grounding electrode system (unless served by a single or multiwire branch circuit). So its not really the generator that needs a ground rod or =, its the feeder panel at the generator.

 

[don_resqcapt19]

Re: Generator Ground

Tom,
True, the code does not require a GEC to the generator frame, but it does require an EGC.

The bonding in the original post is required by the specs and is not required nor prohibited by the NEC.
Don