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#93096 - 05/02/05 04:48 PM 100 amp service
spark1051 Offline
Junior Member

Registered: 04/28/05
Posts: 5
Loc: Dickson,TN
According to table 310.15(B)(6) You can use #4 copper for 100 amp service entrance. I would not think that would also be the case for eguipment such as Air condition and Heat. Am I right on this?

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#93097 - 05/02/05 05:44 PM Re: 100 amp service
e57 Offline

Registered: 05/27/03
Posts: 2837
Loc: S.F.,CA USA
If you have AC on a 100a service and it made it through a load calc for a service that size, it is one small house! Or they have few if any other loads.

As far as that table, if it were a single family, with no way to go up, or out, and the calc was ok, maybe. But you would not be doing your customer a favor by sticking to the minimum. Think about what could be done with the house if it had a 125, or 150A.
Mark Heller
"Well - I oughta....." -Jackie Gleason

#93098 - 05/02/05 05:52 PM Re: 100 amp service
spark1051 Offline
Junior Member

Registered: 04/28/05
Posts: 5
Loc: Dickson,TN
No e57 that was not my question. I was not asking if I could feed the equipment off a 100amp service. I was just wondering if I could feed a load such as heat and air with #4 copper. Load at 100 amp.

#93099 - 05/02/05 06:24 PM Re: 100 amp service
Roger Offline

Registered: 05/18/02
Posts: 1779
Loc: N.C.
Sparks, no you can't use the table for a heat or an A/C unit.
Read 310.15(B)(6), this explains the use of the table, notice the wording "that serve as the main power feeder to a dwelling unit


#93100 - 05/02/05 06:48 PM Re: 100 amp service
spark1051 Offline
Junior Member

Registered: 04/28/05
Posts: 5
Loc: Dickson,TN
Thanks Roger for clearing that up for me.

#93101 - 05/02/05 08:31 PM Re: 100 amp service
markp Offline

Registered: 04/12/04
Posts: 60
Loc: Kent, WA, USA
I think you could argue that all residential feeders can use that table, but certainly not branch circuits. The definition of main power feeder says "between the main disconnect and the lighting and appliance panelboards" (note the plural on panelboards). Article 215 is even more broad (as written) saying dwelling feeders can use 310.15(B)(6). It doesn't say if you still have to use the L&A panelboard or main power feeder restrictions though.

If you have a feeder to a panelboard that has 100A worth of airconditioning equipment for a dwelling, that would be one large dwelling (and I know of no single AC made in 240V single phase that is this large). But you also have to have at least 1 circuit with a neutral load that is less than or equal 30A in order to have a lighting and appliance panelboard. So run the circuit for the 120V mandatory servicing receptacle that must be near the AC from this same panel and you've now got a 100A feeder with a 100A branch circuit (the 120V service outlet has no load calculation value). The branch circuit wires would have to be #3.

I think you see this rule because a single 100A load is almost unheard of in a dwelling, and they're assuming some diversity.

This all being said, I wouldn't do a 100A feeder with #4 if it had two or more large loads on it (40A or more).
Kent, WA


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