Minimum ampacity for a single family dwelling

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pkhanan

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Grass Valley, CA.
I am taking over a new construction 1200 s/f single family dwelling that is nearly complete. The (exterior mounted) panelboard is fed from a 200 amp service located 230 away on the "main house". At the main house 200 amp service entrance, a 100 amp breaker feeds 2 #4 Copper conductors through 2" PVC conduit. The conduit also house a #6 neutral and a #8 ground. The demand load for the new house is only 42 amps (all gas appliances). When I perform a voltage drop calculation, I find I that at 260' feet, the voltage loss for the #4 conductors to be about 3.11%
Assuming all that to be correct, and that the voltage drop is within an acceptable range, can I install a 50 amp breaker (at the main house panel) to supply the new dwelling or does NEC 230.79 (c) imply that I must supply the new dwelling with 100 amps?
 
IMO, 230.79 would apply, but I think 310.15(B)(6) might well apply also so your #4 on the 100 amp breaker should be O.K.
 
I don't believe you are required to have even the 100A breaker. This isn't the main disconnect for a SFR. It is the disconnect for a panel being fed from the main panel which already meets 230.79 with its 200a disconnect. I believe the building is a 2nd dwelling on the same parcel and therefore is only required to have 60a as per 230.79(d). It's essentially a duplex at this point unless there are two separate parcels in which case they shouldn't be sharing the service.
 
ceb58 said:
The way I read it is what he has is feeders to the "new" SFD. Not service conductors.
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I agree and I'm sure there is room for discussion, but 310.15(B)(6) {'08} references feeders as well as services and this is a feeder that supplies the entire load of a dwelling so, IMHO, it applies in the same way an apartment feeder would apply of a multi-family,
 
sfav8r said:
I don't believe you are required to have even the 100A breaker. This isn't the main disconnect for a SFR. It is the disconnect for a panel being fed from the main panel which already meets 230.79 with its 200a disconnect. I believe the building is a 2nd dwelling on the same parcel and therefore is only required to have 60a as per 230.79(d). It's essentially a duplex at this point unless there are two separate parcels in which case they shouldn't be sharing the service.
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Yes, 2nd dwelling on same parcel, they call them "granny" units here. My sensibilities tell me I need to comply whith 230.79(d) at the minimum. Sounds like I may need to pull #3 in place of #4 to keep voltage drop within 3%.
Thanks, help is appreciated!
 
augie47 said:
I agree and I'm sure there is room for discussion, but 310.15(B)(6) {'08} references feeders as well as services and this is a feeder that supplies the entire load of a dwelling so, IMHO, it applies in the same way an apartment feeder would apply of a multi-family,
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If you look at it 225.39(C) it says the same as 230.79(D). 310.15(B)(6) has #4 Cu good for 100 amps so ether way he should be good to go with the #4.
 
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