Generator neutral grounding

[ladengr]

We have a emergency generator grounding challange. The system has two transfer switches, one with 4 pole and second with 3 pole. How do we ground the generator neutral? Can you give me NEC Article references?

 

[davidbeach]

Is there ground fault sensing on the normal side ahead of either of the transfer switches?

 

[ladengr]

There is no GFP on either of the transfer switches.

 

[rbulsara]

Does the 3 pole switch serve 3 wire load (no neutral) or 4-wire (solid neutral) load?

If 3P ATS is serving 3 wire load, you must ground the gen N as the system will be ungrounded while on the gen. The 4P ATS will disconncet the N to N conncetion between the gen system and the utiltiy (assuming the other source is utiltiy).

If the 3P ATS has a solid neutral, adequately sized for the full load service of the generator capacity (this is very important) AND is connceted to the the utility system N which is effectively grounded, you need not ground the generator, in fact you should not as you can not ground the utitliy N down stream of the main device per NEC.

If your 3P ATS has solid neutral but it is not sized for the generator's full load rating (equivalent service size) then you need to upgrade that N.

Refer to NEC article 250 and IEEE Green Book for reference.

Having GFP adds to the complication but not having it does not mean that you can ground the neutral multiplie times (other than at the main device of the source)

You may also choose to "jumper" the N terminals in the 4P ATS and treat the system as not-separately derived system and not ground the generator. Still each of the solid N must be sized for the service equivalent to generator rating.

Too much to digest?..review again...

 

[davidbeach]

If there is no GFP, you can leave the generator neutral ungrounded at the generator, and it becomes grounded via its solid connection at the 3-pole transfer switch and the bonding connection at the service. The 4-pole switch buys you nothing.

The other option would be to replace the 3-pole with a second 4-pole and then ground the generator neutral at the generator.

 

[peebee]

Davidbeach -- you need to qualify your statement -- there may not be a solid connection at the 3-pole switch. There may not be any neutral conductors run to the 3-pole switch at all. In that case, the generatator would be required to be bonded at the generator.

If neutrals HAVE been run to the 3-pole switch & solidly bonded together, then it would be illegal to put an N-G bond at the generator.

 

[davidbeach]

peebee -- you are correct. I assumed a neutral and should have clarified that point. rbulsara (who was typing his response as I was typing mine) is also correct in stating that the neutral to the 3-pole would have to be large enough, all the way from the generator to the service, to be usable. My preference for any system with GFP or multiple transfer switches is 4-pole switches and the generator as a separately derived system.