Minimum ampacity for a single family dwelling

[pkhanan]

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?

 

[augie47]

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.

 

[ceb58]

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.

The way I read it is what he has is feeders to the "new" SFD. Not service conductors.

 

[sfav8r]

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.

 

[augie47]

The way I read it is what he has is feeders to the "new" SFD. Not service conductors.

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,

 

[pkhanan]

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.

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!

 

[ceb58]

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,

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.