Generator ATS

[twayne]

Does anyone know whether or not a four blade ATS is necessary on a generator circuit and if so why break the neutral? This emergency generator back up project involves gens from 60 kw to 850 kw, some ahead of the main to support the entire facility and some for specific circuits on the load side.

Thanks, Wayne

 

[dereckbc]

Re: Generator ATS

Whether or not to make a generator SDS is a design decision, not NEC. If you have GFP in the main, you better configure the generator SDS or else big trouble is waiting in the form of false tripping.

IMHO with the size of generators I work with, I always use SDS ATS configuration, it provides the lowest ground fault impedance possible, plus avoids GFP issues.

 

[charlie b]

Re: Generator ATS

If you do not break the neutral, then the generator will not be classified as the source of a ?Separately Derived System? (SDS). In that case, there are some extra steps that you must take, in order to provide a proper path for fault currents to trip the generator breaker, and in order to avoid having parallel paths for current to flow (i.e., along the equipment grounding conductor and along exposed metal parts). If you can get your hands on a copy of the 2002 NEC Handbook (try the local library, if your employer does not have a copy), Exhibits 250.6 and 250.7 may help you see the issue.

 

[necbuff]

Re: Generator ATS

Charile, There is another scenario one would have to consider. Actually I would like your opinion. In a case where you fed two buildings from the same generator, you would have to have switched neutrals in the ATS' at each building to prevent tieing the two buildings grounded conductor together. This actually came up not long ago. The GFP issue is a good one. I haven't ran into it yet. If I am thinking correctly, the above would also require a true SDS. Each building had it's own service but shared a generator. I actually had think this one out.

[ April 15, 2005, 07:56 PM: Message edited by: necbuff ]