Grounded conductor size for ATS

[electrofelon]

I have a 1600A feeder leaving a switchboard, going thru an ATS, then back to the switchboard. The generator/ATS is NOT an SDS. The generator output breaker is 1200A. The conductors to (and back from) the ATS and switchboard are 6 sets of 500 AL, 2 sets in each pipe. The conductors from the gen to the ATS are 6 total sets of 350 AL, in 3 pipes. Max demand over 3 years is 380 amps. What size grounded conductors should I use?

My thoughts: Its temping to go real small, maybe minimum size. Under normal operation, the neutral to/from the ATS wont be carrying anything. Minimum size per 215.2(A)(2) is 350MCM AL, which would be a 2/0 in each pipe. Also was kinda thinking that (3) 2/0 exceeds the demand figure (assuming perfectly balanced), but not sure that makes any logical sense to compare those two that way. I guess I could say absurd worst case is the 136 KW all on one phase which would be like 1100A This is large facility, im not doing a neutral calc, it would take half a week.

 

[Birken Vogt]

Under normal operation, the neutral to/from the ATS wont be carrying anything.

Theoretically, it could be "forced" to carry its normal amount of current, if the imbalance is large. I don't know the proper terminology, but the wires all run together act as a transformer of sorts that would like to remain a zero sum. If you have a worst case, all imbalanced current on one phase, then the neutral would sort of automatically flow to cancel that out.

I would still size it to a minimum size. Whatever minimum size for the whole building is, or could reasonably be calculated or measured per code. The max it could carry would be the total 120 volt imbalance, which is usually not all that much.

 

[wwhitney]

Isn't the minimum size based on the calculation in 220.61, for the case that the generator is powering the loads? (I.e. so any L-N loads automatically shed when on generator could be ignored). Since when the loads are on grid power, there will be no current on this grounded conductor.

Cheers, Wayne

 

[electrofelon]

Isn't the minimum size based on the calculation in 220.61, for the case that the generator is powering the loads? (I.e. so any L-N loads automatically shed when on generator could be ignored). Since when the loads are on grid power, there will be no current on this grounded conductor.

Cheers, Wayne

220.87 isnt really clear on if I can use it to size the grounded conductor. Now every utility demand figure I have seen doesnt cover imbalance or neutral current, but 220.87 also doesnt get specific about what parameters are measured.