1600 Amp 480 VAC Service with GFI and ATS

[wirestu]

Happy New Year All.

We are involved with a project designed by a consulting engineer that has a 1600 Amp 480/277 VAC 3-phase 4-wire Y utility service. From the utility transformer to the 1600 Amp Ground Fault Protection Main Circuit Breaker 4 sets of 4-500 MCM are installed in four isolated phase conduits. The utility neutral is connected to the system grounding electrodes and bonded to the ground conductor in the 1600 Amp Main Circuit Breaker section. The utility service is grounded as a separately derived service. From the load side of the Main Circuit Breaker, the same size and number of conductors are used to power the normal side of a 1600 Amp 3-Pole Automatic Transfer Switch. The neutral conductor from the Main Circuit Breaker connected to a non switched pole (solid busbar) in the 3-pole ATS. The load of the ATS feeds a 1200 Amp MLO panel that has a 1200 Amp circuit breaker with other breakers under 1000 Amps. There is a 25 kVA 480-120/240 VAC step down transformer for all of the single-phase and low voltage 120/240 VAC loads powered from the load of the ATS. All of the 480 VAC loads are three-phase. Does this 1200 Amp branch circuit breaker in the distribution panelboard need to be GFP? I am under the impression that since the 2014 code change, this branch circuit breaker also needs to be GFP.

This project also has a 450 kW emergency generator with a generator mounted 800 Amp non-GFP circuit breaker installed on the unit. . The engineer's drawings call for two sets of 3-500 MCM three-phase conductors and a 1/0 ground wire in two 4" conduits to be installed from the emergency generator to the ATS, without a neutral wire. My understanding is with a three-pole ATS the neutral of the emergency generator must be isolated from the frame at the emergency generator, and the wiring should not be treated as a separately derived service. I believe this presents an issue as without the neutral conductor the only path for a ground fault to return to the emergency generator when it is supply power to the load would be over the ground conductor which is not connected to the emergency generator neutral. This ground fault would eventually return to the utility transformer source. I feel this would cause the GFP of the Main Circuit Breaker to nuisance trip and the generator circuit breaker may not open on a ground fault.

Please comment with your thoughts.

Thanks in advance.

 

[augie47]

Immediate thought is not one you questioned but (4) 500s on a 1600 amp M/B is not compliant.
Can we assume you have no neutral load since the generator is 3 wire ?
Normally with a GF main your would be using a 4 pole T/S but with no neutral load I don't know why the neutral extends beyond the service.

 

[wirestu]

Thanks for the reply.

The generator is 4-wire but the engineer has only specified 3-wire and ground for connection to the ATS.

I agree about the (4) sets of 500 MCM not being compliant. (as I said, did not design it, just wiring it). Any reference that I can point the engineer to?

My concern is with 3-wires coming from the generator to the ATS is how a ground fault current would return to the generator when the load is connected to it and a ground fault occurs.

 

[jim dungar]

My concern is with 3-wires coming from the generator to the ATS is how a ground fault current would return to the generator when the load is connected to it and a ground fault occurs.

If the neutral is not bonded to the ground at the generator, you would not have a complete path for ground faults.

If the neutral is banded to ground at the generator it is possible the ground fault path could involve the GFP sensor in the main breaker resulting in unexpected trips.

Ideally the engineer would provide a single-line diagram of the grounding circuit showing the normal generator paths.

 

[wirestu]

Thanks Jim.

I will try to put up a one-line.

Do you think the second downstream1200 AMP circuit breaker needs to be GFP?

 

[jim dungar]

Thanks Jim.

I will try to put up a one-line.

Do you think the second downstream1200 AMP circuit breaker needs to be GFP?

You need to look st the one-line for the grounding, not the power one-line. My guess is one has not been created, as they rarely are.

IMO the downstream breaker should have GFP but I know many people, even AHJs, that say otherwise.

 

[augie47]

Opps.. i guess I;'m one of the "otherwise"
If I understand correctly, the 1200 amp would be fed by the 1600 amp main which has GF, or the 800 amp generator.
If that's correct, it does not need GF per 215.10 Exception...

added:
(take waht I say with a grain of salt as an opinion,. Jim is more knowledgeable)

 

[jim dungar]

Opps.. i guess I;'m one of the "otherwise"
If I understand correctly, the 1200 amp would be fed by the 1600 amp main which has GF, or the 800 amp generator.
If that's correct, it does not need GF per 215.10 Exception...

I like the selective coordination typically possible with multiple levels of GFP.

 

[dy11]

I believe the generator should be bonded since the neutral doesn’t leave the generator. A 3 or 4 pole ATS doesn’t matter here since the generator neutral isn’t brought to the ATS.

The service main would not see any ground fault current if the generator is serving the load, regardless of whether or not the generator is bonded. In the unbonded case, there is not a complete circuit and no ground fault current would flow. In the bonded case, since the neutral doesn’t leave the generator, the only possible path is through the generator bond alone.

 

[electrofelon]

Unless I am misunderstanding, I Am seeing a 408.36 violation feeding the 1200 amp panel with a 1600A OCPD - unless the 1200 panel is actually a switchboard which would not have that restriction.

 

[electrofelon]

I agree about the (4) sets of 500 MCM not being compliant. (as I said, did not design it, just wiring it). Any reference that I can point the engineer to?

Standard ampacity table 310.15(B)(16) noting that 240.4(B) doesn't apply. Also if this is isophase, you will have a 80% derating.