4 Wire system and without landing the Neutral

[Lady Engineer]

I have an existing plant where we have a 800A, 480V, 4 wire system. The system is fed from a 3ph, 4W 500KVA pad mount. We also have a generator rated 4 Wire. The reason for it is because the generator and main are GFP breakers, and ATS is 4 pole (contactor type). The generator is a separately derived system, at least I hope it's grounded or bonded, correctly.

The client's engineer is saying that we don't need the neutral because we can only connect 3 phase loads to the main switchboard and used step downs for single phase, single pole loads, which I agree. I'm replacing the four pole ATS with a 3 pole, eliminating the GFP, replacing the switchboard, replacing the grounding on both the service and generator.

Question: Since the generator is also 3ph, 4W, do you see an issue? I'm trying to trouble shoot, even though I can't think of anything. He's insisting on a 3W service, and I'm not one argue. It's hard dealing with EEs with a PE. Hahaha...(yes, I'm one!)

 

[texie]

I have an existing plant where we have a 800A, 480V, 4 wire system. The system is fed from a 3ph, 4W 500KVA pad mount. We also have a generator rated 4 Wire. The reason for it is because the generator and main are GFP breakers, and ATS is 4 pole (contactor type). The generator is a separately derived system, at least I hope it's grounded or bonded, correctly.

The client's engineer is saying that we don't need the neutral because we can only connect 3 phase loads to the main switchboard and used step downs for single phase, single pole loads, which I agree. I'm replacing the four pole ATS with a 3 pole, eliminating the GFP, replacing the switchboard, replacing the grounding on both the service and generator.

Question: Since the generator is also 3ph, 4W, do you see an issue? I'm trying to trouble shoot, even though I can't think of anything. He's insisting on a 3W service, and I'm not one argue. It's hard dealing with EEs with a PE. Hahaha...(yes, I'm one!)

It is imperative that the neutral be brought to the first means of disconnect and bonded when a system is grounded at any point. To not do so would leave you with no fault return path. The old argument that the neutral is not required to this point because there is no line to neutral load(s) is false and dangerous for obvious reasons. See 250.24(C).

 

[Lady Engineer]

It is imperative that the neutral be brought to the first means of disconnect and bonded when a system is grounded at any point. To not do so would leave you with no fault return path. The old argument that the neutral is not required to this point because there is no line to neutral load(s) is false and dangerous for obvious reasons. See 250.24(C).

Thanks texie. I hadn't thought of that, and I'll look into that. I didn't think there's a way to ground the wye side (the system is delta/wye) without using a neutral as a path, correct? This is not a delta system system on the secondary.

 

[david luchini]

Thanks texie. I hadn't thought of that, and I'll look into that. I didn't think there's a way to ground the wye side (the system is delta/wye) without using a neutral as a path, correct? This is not a delta system system on the secondary.

If the conductors from the transformer are Service conductors, and the system is grounded, 250.24(C) requires that grounded conductor be routed with the ungrounded conductors and connected to the service disconnecting means.

 

[charlie b]

Once you have settled on bringing the service neutral into the party, I think that the generator's neutral should be brought in as well, and that the ATS remain a 4-pole. I don't believe that is a requirement, but it is how I would design the system.

 

[Lady Engineer]

Once you have settled on bringing the service neutral into the party, I think that the generator's neutral should be brought in as well, and that the ATS remain a 4-pole. I don't believe that is a requirement, but it is how I would design the system.

That's interesting, Charlie. I used to do that as well, but then I learned that it's usually only used for GFP. Do you see other benefits to using a 4 pole?

 

[Phil Corso]

Lady Engineer...

Has the problem arisen because the Fault-duty, with two sources, exceeds equipment rating?

Regards, Phil Corso

 

[Lady Engineer]

Lady Engineer...

Has the problem arisen because the Fault-duty, with two sources, exceeds equipment rating?

Regards, Phil Corso

No actually. I'm not sure what his reasoning is with this.

 

[steve66]

Lady Engineer...

Has the problem arisen because the Fault-duty, with two sources, exceeds equipment rating?

Regards, Phil Corso

That would only apply if the two sources were connected at the same time. For example, if the ATS was closed transition.

 

[steve66]

With an 800 amp service, I'm not sure why it ever had GFP. The requirement doesn't kick in until 1000 amps, and GFP on the service seems to cause a lot of coordination issues.

With no GFP, and for a single service with a single ATS, I would tend to keep things simple, and use a 3 pole ATS.

Does anything in the NEC require the generator neutral to be connected to the ATS or the service switchboard??

 

[capybara]

The advantage of a 4 pole ATS or MTS is that the neutral is a current carrying conductor and you dont want neutral current feeding back from the Genset to the Utility Transformer or vice versa. as it would with a 3 pole ATS or MTS with a solid unswitched neutral. Its a safety thing.

 

[ActionDave]

The advantage of a 4 pole ATS or MTS is that the neutral is a current carrying conductor and you dont want neutral current feeding back from the Genset to the Utility Transformer or vice versa. as it would with a 3 pole ATS or MTS with a solid unswitched neutral. Its a safety thing.

There is nothing dangerous about a non SDS set up. How do you back feed the utility with your generator using only the neutral?
Answer is, you can't.

 

[ron]

Where I don't need the neutral for L-N loads immediately, I like to bond the N-G at the first legal opportunity, then not bring the neutral downstream to ATS's. It avoids the cost of the neutral, but it also avoids the age old argument for a 4 pole ATS, which is should the neutral be open transition, or overlapping (closed) transition switching.

 

[GoldDigger]

Where I don't need the neutral for L-N loads immediately, I like to bond the N-G at the first legal opportunity, then not bring the neutral downstream to ATS's. It avoids the cost of the neutral, but it also avoids the age old argument for a 4 pole ATS, which is should the neutral be open transition, or overlapping (closed) transition switching.

As long as the neutral breaks after hot and makes before hot it is not nearly as important whether the neutral transition itself is open or closed. (Assuming of course that the hit wires are open transition.)

 

[Bang]

From a maintenance standpoint or outlook for upgrading the future utility, I would keep the 4-pole disconnect and keep everything separate. I don't think there's anything against wiring the ATS as non-SDS but for an outlook for the maintenance guy years down the road, again, I'd keep the 4-pole with separate neutrals tying back to their respective grounding source.

 

[Flapjack]

I agree with ron.

I would design similar to NEC 2014 Exhibit 250.13 except the transfer switch would be a 3-pole and the neutral would only be brought from the transformer to the service. There wouldn't be a neutral from the service to the ATS, the generator to the ATS, or from the ATS to load. There's no reason to add cost if it won't be needed and is not required.

 

[kwired]

The advantage of a 4 pole ATS or MTS is that the neutral is a current carrying conductor and you dont want neutral current feeding back from the Genset to the Utility Transformer or vice versa. as it would with a 3 pole ATS or MTS with a solid unswitched neutral. Its a safety thing.

When neutral is bonded to EGC at both generator and at service - you put the EGC in a parallel path with the neutral and the EGC as well as frames of equipment will be carrying neutral current - if you don't switch the neutral in the transfer switch.

With a non separately derived generator the only bond is at the service equipment - so it doesn't matter if the neutral gets interrupted by the transfer switch or not there is no parallel current in non desired paths to deal with.

 

[GoldDigger]

When neutral is bonded to EGC at both generator and at service - you put the EGC in a parallel path with the neutral and the EGC as well as frames of equipment will be carrying neutral current - if you don't switch the neutral in the transfer switch.

With a non separately derived generator the only bond is at the service equipment - so it doesn't matter if the neutral gets interrupted by the transfer switch or not there is no parallel current in non desired paths to deal with.

Actually, if the generator is non SDS, with no bond at the generator, I do not think that you would be allowed to use a four wire ATS since it could leave the generator running without a ground at times.

 

[kwired]

Actually, if the generator is non SDS, with no bond at the generator, I do not think that you would be allowed to use a four wire ATS since it could leave the generator running without a ground at times.

Is that really a problem if it is not supplying any load? it is simply an ungrounded system with no current flowing. Fault one of the lines and you now have a grounded system, fault a second line and you have high current flow and should open overcurrent device(s).

All metal components are still bonded together as well as to the EGC.

 

[GoldDigger]

Is that really a problem if it is not supplying any load? it is simply an ungrounded system with no current flowing. Fault one of the lines and you now have a grounded system, fault a second line and you have high current flow and should open overcurrent device(s).

All metal components are still bonded together as well as to the EGC.

True in terms of hazard, but by code you would (depending on voltage) either be required to ground it or at a minimum incorporate ground detectors.
And there could well be local outlts and loads at the generator.