ATS 3-pole vs 4-pole

[nhee2]

Maybe a dumb question - a new ATS (3-pole) is being installed to switch a 3 wire generator feed and a 480V 1200 A utility feed with GFP. The service disconnect is upstream of the ATS, and N-G bond is at the service disconnect. The generator MCB is 1000A, also with GFP capabilities. Feeders from the service disconnect and from the generator are each 3-wire+G with no neutral pulled (no 277 VAC loads).

3-wire feeds should only require a 3-wire switch. Is there any scenario where i'd need to pull the neutral and install a 4-pole to ensure GFP at each breaker functions properly under all scenarios? I'm saying no, but am going around in circles a bit concerned I am missing something.

 

[CoolWill]

Seems to me that if the generator is providing GFP, you would need to open the neutral from the service to sever the N-G bond at the disconnect. I'm assuming that the neutral is bonded at the generator to provide the EGC.

 

[JoeStillman]

There has to be a path back to the generator neutral point for ground-fault currents. If you don't run a neutral with the generator output and you don't bond the generator neutral to ground, there is no path and your GFP is defeated. So you have to either run the neutral to a solid neutral 3P ATS or bond the generator neutral. But if you do both, then you will need a 4P ATS to prevent "objectionable neutral currents".

 

[nhee2]

Thanks.

In this case, I believe if generator neutral is bonded AND service is bonded, and neutrals are isolated (not run from service or generator to the ATS), then the 3P is fine, GFP from both sources will detect/protect against ground faults, and it'll be code-compliant.

 

[JoeStillman]

Thanks.

In this case, I believe if generator neutral is bonded AND service is bonded, and neutrals are isolated (not run from service or generator to the ATS), then the 3P is fine, GFP from both sources will detect/protect against ground faults, and it'll be code-compliant.

Agreed.

 

[Strathead]

I asked an Engineer for a hospital when generator needs to be a separately derived system and hence requires a 4 wire transfer switch. His answer was only when the amperage was high enough to require GFP. Then it went over my head, but something about their operation. So based on that, I think you need a 4 wire transfer switch and separate bonding and grounding.

 

[nhee2]

I asked an Engineer for a hospital when generator needs to be a separately derived system and hence requires a 4 wire transfer switch. His answer was only when the amperage was high enough to require GFP. Then it went over my head, but something about their operation. So based on that, I think you need a 4 wire transfer switch and separate bonding and grounding.

agreed, for cases when the neutral is used. As mentioned in the posts above, for my case where neutral is not brought to the ATS, only 3P required (provided generator neutral is bonded)

 

[ron]

If there are no neutral loads, there is no need for a 4 pole ATS and no neutrals brought to the ATS (unless the ATS was functioning as the service disconnect).

 

[Strathead]

agreed, for cases when the neutral is used. As mentioned in the posts above, for my case where neutral is not brought to the ATS, only 3P required (provided generator neutral is bonded)

makes sense.