Commercial Generator and ATSs.

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PCBelarge

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Large generator to a commercial building. There are 6 ATS. GFPE protection to the 5 Service disconnects. Two of the ATS are 4-pole. The others are 3-pole. The 3-pole loads do not have a neutral. Is this compliant with the NEC?
 
I don't see a need for a neutral at the transfer switches that do not supply any line to neutral loads.
Since the other transfer switches are 4 pole that would indicate that the neutral is bonded at the generator and the 3 pole switches will need a fault clearing path back to the generator.
 
augie47 said:
It might just be over my head but I don't see how you can satisfy the Code having the neutral switched in some switches and not the others,.
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Though if what the op is saying is correct, there would be no neutral to switch on the others. But looks like there would be circulating currents on the egc. I would have to draw it out to see if there was a path that would affect the proper operation of the ground fault mains.
 
augie47 said:
It might just be over my head but I don't see how you can satisfy the Code having the neutral switched in some switches and not the others,.
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Because there is no neutral to switch. Think ATS feeding a delta-wye transformer or 3 phase, 3 wire distribution.
 
From the OP I deducted the 3 pole ATS don;t have a neutral but the 4 pole one do. Perhaps I misread.
 
augie47 said:
From the OP I deducted the 3 pole ATS don;t have a neutral but the 4 pole one do. Perhaps I misread.
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Right. So if there is a neutral it is switched by the 4-pole ATS. If the load doesn't have a neutral, then it is fed from a 3-pole ATS. Therefore, every neutral is switched and the whole system is a separately derived system with the neutral and ground bonded at the utility service and at the generator.
 
d0nut said:
Right. So if there is a neutral it is switched by the 4-pole ATS. If the load doesn't have a neutral, then it is fed from a 3-pole ATS. Therefore, every neutral is switched and the whole system is a separately derived system with the neutral and ground bonded at the utility service and at the generator.
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makes sense..
 
If there are no L-N loads for the 3-pole ATSs, and the generator has a N-G bond (and the utility does too), then no issue with the situation.

No need to waste neutral conductors and a 4th pole on an ATS if there are no L-N loads. The GF current will happily find its way back to the source's neutral without it.
 
Here is my thought on this.

1. Non-separately derived systems, use the premises wired grounding electrode system of the building - all the way back at the building system

2. Separately derived systems do not us the premises wired grounding electrode system of the building.

So, how does one ground/bond the generator that is supplying both systems?


I see this as the issue to contend with.
 
PCBelarge said:
Here is my thought on this.

1. Non-separately derived systems, use the premises wired grounding electrode system of the building - all the way back at the building system

2. Separately derived systems do not us the premises wired grounding electrode system of the building.

So, how does one ground/bond the generator that is supplying both systems?


I see this as the issue to contend with.
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A separately derived system will have its own GEC to the grounding electrode system. Otherwise you are incorrect, most separately derived systems use the same building electrode system as the service.

Your generator is not supplying 'both systems'. It is its own system and supplies what is connected to it.

If you are switching the neutral in some ATS then the generator needs to be set up as a separately derived system for all ATS. But that does not mean you need to run neutrals to the ATS where the load does not use them.
 
jaggedben said:
Are all the ATS connected to the same service? Same grounded system?
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For Normal Power, yes all are supplied At the service.
Yet...There are 3 separate - 4000A, 480V service disconnects from 3 separate utility transformers in the Utility transformer vault. Each service is separated from the other service disconnects. They all share the same premises building ground from cold water and structural steel, tied into a large copper ground detail, oversized conductors as per specifications from EOR.
 
jaggedben said:
A separately derived system will have its own GEC to the grounding electrode system. Otherwise you are incorrect, most separately derived systems use the same building electrode system as the service.

Your generator is not supplying 'both systems'. It is its own system and supplies what is connected to it.

If you are switching the neutral in some ATS then the generator needs to be set up as a separately derived system for all ATS. But that does not mean you need to run neutrals to the ATS where the load does not use them.
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I have not had the experience of having separately derived and Non-separately derived systems on the same generator that is separately derived. As mentioned in another response, I am going to draw this out on paper to try and figure out if this is permitted as wired (off a set of stamped drawings).
 
PCBelarge said:
I have not had the experience of having separately derived and Non-separately derived systems on the same generator that is separately derived. As mentioned in another response, I am going to draw this out on paper to try and figure out if this is permitted as wired (off a set of stamped drawings).
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That is because it isn't possible. As soon as you install a solid neutral ATS on a generator system, you have a non-separately derived system even if all of the other transfer switches are switching the neutral or do not have a neutral present. If your system was intended to be a separately derived system, you now have a system with two neutral-ground bonds (one at the utility and one at the generator) and you will have issues with current dividing between the neutral and equipment grounding conductors.
 
PCBelarge said:
Large generator to a commercial building. There are 6 ATS. GFPE protection to the 5 Service disconnects. Two of the ATS are 4-pole. The others are 3-pole. The 3-pole loads do not have a neutral. Is this compliant with the NEC?
Click to expand...
Are these actual service disconnects? Neutral bonded at the disconnect? That’s where you would have a problem with paralleled neutrals, not the loads that do not have a neutral. Once the neutrals leave the generator main, if it is an sds, must remain isolated from the egc and gec. But if they are just distribution disconnects, and the neutral is isolated, then there is not a problem.
 
PCBelarge said:
For Normal Power, yes all are supplied At the service.
Yet...There are 3 separate - 4000A, 480V service disconnects from 3 separate utility transformers in the Utility transformer vault. Each service is separated from the other service disconnects. They all share the same premises building ground from cold water and structural steel, tied into a large copper ground detail, oversized conductors as per specifications from EOR.
Click to expand...
PCBelarge said:
I have not had the experience of having separately derived and Non-separately derived systems on the same generator that is separately derived. As mentioned in another response, I am going to draw this out on paper to try and figure out if this is permitted as wired (off a set of stamped drawings).
Click to expand...

You haven't described any non-separately derived systems. You have described a generator and three utility powered transformers. You stated that each ATS that supplies loads that use a neutral is 4-pole. That makes four separately derived systems. The fact that the other ATS are 3-pole does not make a non-separately derived system if there are no neutral conductors to those ATS; the neutral being solidly connected is what makes it not-separately-derived, and you don't have that.

I know of no NEC rule that prohibits such a setup, but perhaps others know more.
 
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