310.15(B)(6) vs 310.16. Residential or Commercial?

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Which chart should be used in the following situation? 310.15(B)(6) or 310.16?

I installed a 35Kw (146.5 amp) generator with a 175 amp breaker on the generator. The generator goes to a ATS that is connected by a main disconnect that is fed from the meter. The generator was installed at a single family home and the estimator (commercial and residential) used 310.15(B)(6) to size the wire at 1/0 AWG copper. The NEC Code Handbook ( i know, just use the code book) says:
....one-family...the reduced conductor size permitted by 310.15(B)(6) is applicable to the service-entrance conductors, service-lateral conductors, or any feeder conductors that supply the main power feeder to a dwelling unit (i.e. the generator becomes the main power feeder).

My head Commercial manager says we should have used 310.16 and the lug temperature rating (which i am more acclimated with) and in this case with the 75 degree lugs would have been 2/0 AWG copper.

I understand where both are coming from and have more of a background in commercial. The residential chart does have the ability to be used for reduced conductor size and seems to throw out the lug temperature (usually the weakest point) out the window. On the other hand, this house will never see 175 amps considering the generator will never produce more that 146.5 amps. Some experienced EC want to help another EC figure this one out? What if I really needed all 175amps or had to rate this to its potential to produce based on the breaker? Please clarify these charts. Give code references if possible.
 

dnem

Senior Member
Location
Ohio
Re: 310.15(B)(6) vs 310.16. Residential or Commercial?

innersurffer said:
Please clarify these charts. Give code references if possible.
I think you should ask your head Commercial manager to provide references.
innersurffer said:
My head Commercial manager says we should have used 310.16 and the lug temperature rating (which i am more acclimated with) and in this case with the 75 degree lugs would have been 2/0 AWG copper.
Find out from him why he thinks you can't use Table310.15(B)(6) on that installation in a single family home.

David
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
Is your Commercial Manager taking the stand that the utility power is the "main service" and that the generator is just a back-up (i.e., not "main")? If I had designed what you have described, and if an Inspector or Plan Reviewer had taken that stand, I don't think I could win the argument. The text of 30.15(B)(6) does not recognize the possibility that the "main service" might come from either the utility or a generator, depending on the status of the ATS.
 

haskindm

Senior Member
Location
Maryland
Whether these conductors are considered Service Entrance Conductors or Feeders, you are still allowed to use 310.15(B)(6). It seems that they would have to be one or the other. What else could they be called? The only criteria is that they are used in a "dwelling unit". I don't see any requirement that you use the ampacity from 310.16 but I have been wrong before.
 
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