NEC load calculation SFD with sub-feed to subpanel

W

WD40

Senior Member
Location
Philly
Occupation
Electrician
Greetings group I have two code questions.
I have a large single family dwelling that I am going to upgrade the service on. The existing meter can will be swapped with a new meter / main with two new branch breakers for an AC unit and a GFCI and feed thru lugs for the feeder.
The existing SE cable feed to the old panel will be replaced with 4-wire and the house renovated so I am going thru to the 'optional load calculation' in 220.82 for a dwelling unit. However the feeder will not have the 'total connected load' for the dwelling, so do I need to do a general calculation for the feeder? I was told I cant use the 83% rule in 310.12 for the feeder as its not the 'entire dwelling' load.
What calculation methods do I have for a sub-panel load that is not the 'entire load' of a single family house? Just the general?
Is it true I cant use the residential 83% for a sub-panel?
Thanks everyone sorry for such a basic question.
 
As to 310.12, neither (A) or (B) will apply to the feeder, but (C) does. So for example, if your optional load calc for the service comes out to 140A, and you decide to provide a 150A service, your service conductor ampacity only has to be 150A * 83% = 124.5A; if nothing requires derating, it could be #1 Cu or #2/0 Al. As your feeder is supplied by feed-thru lugs, it is perforce also a 150A feeder. 310.12(C) means that the feeder can be #1 Cu or #2/0 Al as well.

For the sub-panel/feeder load, you must use the standard load calculation, as it is not carrying the entire dwelling unit load. For the service you can use the optional load calculation for dwelling units. However, given that the feeder has the same rating as the service because of the feed-thru lugs, and given 310.12(C), you can basically skip a load calc for the feeder.

Cheers, Wayne
 
Thank you, I was also informed by the inspection agency I could use 220.83 and I actually can't use 310.12(C) for the feeder.
 
You must log in or register to reply here.
Top