Neutral grounding requirements for generator--- Use of 3

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kuchenig

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I have a solidly grounded, 480/277-volt 1200 ampere service with ground
fault protection on the incoming main. I wish to add a 480/277 volt,
stand-by generator that will be connected as a alternate source of power to
selected loads via an 800-ampere automatic transfer switch. The generator
is intended to operate only during utility power outages. It is not a
legally required emergency supply. My question is whether or not the NEC
specifically requires that there be a 4-pole, switched neutral ATS and
separate grounding of the generator neutral for this installation. It is
realized that if the generator neutral were not separately grounded, and
that a ground fault were to occur while the generator were running, the main
disconnect would trip and would not isolate a low level fault. But because
the generator would only be in operation if the utility supply were out, I
don't understand if that, per se, although an irritation, is, or is not a
violation of code.
 
Re: Neutral grounding requirements for generator--- Use of 3

By code you are allowed to configure your generator either as SDS or not. NEC is not concerned with operation, only safety. But I advise you to configure the generator SDS since you have GFP.
 
Re: Neutral grounding requirements for generator--- Use of 3

Well, Mr Campbell, Thank you for your answer, although I was a little surprised by your response in saying this is a case of operational preference rather than a safety issue. I have seen a couple of articles recently stating that if GFP is required on the main service, then generator SDS and 4-pole neutral IS required, but I gather you're saying it is not required?

Had the generator been a 1200 ampere alternate supply would then the generator be required to be a SDS ---as then GFP on the generator service is assumed required (230-95). Or is that not considered an 'electrical service'?

Am I correct in saying that had this been a legally required standby installation THEN using a SDS for the generator would have been absolutely required by code in order to acheive selective ground fault protection?

I'm just trying to get a thorough understanding of this...............Thanks
 
Re: Neutral grounding requirements for generator--- Use of 3

Q1. I have seen a couple of articles recently stating that if GFP is required on the main service, then generator SDS and 4-pole neutral IS required, but I gather you're saying it is not required?

A1. SDS is not required.

Q2. Had the generator been a 1200 ampere alternate supply would then the generator be required to be a SDS ---as then GFP on the generator service is assumed required (230-95). Or is that not considered an 'electrical service'?

A2. Generator is not a service.

Q3. Am I correct in saying that had this been a legally required standby installation THEN using a SDS for the generator would have been absolutely required by code in order to acheive selective ground fault protection?

A3. No. To my knowledge there is no NEC requirement for a generator to be SDS for any of the three systems (emergency, legal, or optional). The chioce is a design issue, not NEC.
 
Re: Neutral grounding requirements for generator--- Use of 3

N.E.C. 1999 ART.'S 700-702 POINT OUT THAT GFP,EVEN THOUGH ON THE MAIN SERVICE IS NOT REQUIRED ON THE STAND-BY SYSTEM,OR EITHER OF THE OTHER TWO SYSTEMS.BUT GENERATOR HAS TO BE GROUNDED,SO A SOLIDLY TIED NEUTRAL IS NOT AN OPTION-THAT LEAVES THE SDS/SWITCHED NEUTRAL AS ONLY CHOISE. GOOD LUCK HOWEVER YOU CHOSE TO GO.
 
Re: Neutral grounding requirements for generator--- Use of 3

kuchenig:

My understanding is that if you use a 3 pole transfer switch, and permenatly tie the generator neutral to the service neutral, when your generator kicks in the main GFP may falsely trip. No actual ground fault is required.

I think the articles you have read a 4 pole transfer switch is required for practical operation (but not code required). In other words, if tie the neutrals together, it will be such a hassle, you will probably wind up buying another transfer switch and redoing the whole installation.

And Pierre is correct, a 1200A generator would require GFP per 210.15.

One possible option: Run your service to a 1200 service entrance panel, and install two disconnects each rated less than 1000A. (One could feed a panel that would have all the loads backed up by the generator on it.) Then no GFP would be required.
Steve

[ March 23, 2004, 09:20 AM: Message edited by: steve66 ]
 
Re: Neutral grounding requirements for generator--- Use of 3

kuchenig, do you have any 277 loads? I typically bring in the 4-wire service to the MSS, terminate the grounded conductor to the neutral bus, and bring the GEC to the neutral bus to satisfy code requirements. Then from the MSS I only use 3-wire distribution, and if 277 loads are needed use a isolation transformer to derive a grounded circuit conductor. This allows a 3-pole transfer switch and an SDS generator configuration with GFP. No false trips, low harmonic distortion, no common mode noise, and easy segregation of loads.

The design drives the code requirements, rather than the code requirements driving the design IMO.
 
Re: Neutral grounding requirements for generator--- Use of 3

Well, That all made for some lively discussion. ---Thank you all.

I gather from all this that although the code does not require a generator with under a 1000 ampere disconnect to be SDS with switched neutral, it is required over 1000 amperes per 215.10, and, for all practical purposes is needed ---either because the service main may trip due to gremlins or due to actual ground faults.

I particularly liked dereckbc's comment regarding limiting the 4-wire system to isolated areas of 277 volt usage employing a separate neutral deriving transformer.

Thanks again
 
Re: Neutral grounding requirements for generator--- Use of 3

kuchenig, I hope I was able too help, but I do not understand how you interpet 215.10 or 230.95 to require a generator to be SDS. IMO it means if the generator is 1000-amps or more, and solidly grounded it is required to have GFP.

Proper design techniques should require it to be SDS, but not the code IMO. If someone knows something different or interpets this differently please point it out.
 
Re: Neutral grounding requirements for generator--- Use of 3

Dereckec: I did not mean to imply the code in any way generally REQUIRES the generator be SDS. What I think I wrote is that for all PRACTICAL PURPOSES for installations where the primary utility service has GFP, the generator connnected on that system then generator NEEDS to be SDS because of the concern for nusiance or ground fault tripping of the main as per Steve66's comments.

And for installations where the generator 480 v 4W supply itself exceeds 1000 amperes, 215.10 requires GFP (as pointed out by pierre) and hence must become a SDS to achieve the ground fault detection and tripping.

So although the code may not specifically state that a generator be SDS there are cases, i.e., WHENEVER the utility service contains GFP, that the generator NEEDS to be configurated as a SDS; and if the 480 volt generator supplies a 1000 amperes or more on a wye grounded system, it MUST contain GFP and thus be configuraed as a SDS.

Since there are instances where the generator supply must have GFP, (215.10) there are instances where the generator must be locally grounded and thus must be a SDS.

Have I got it right?
 
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