2011 NEC Annex D1 (a)...

[eric dolphy]

hello

please clarify an example given for a one-family dwelling, specifically, the calculation for neutral for feeder and service...

to recap: the example gives the net calculated load thus:

18000 va, divided by 240 = 78 amps...bumped up to 100 amps as per 230.79 C

the calculated load for neutral figures the range and dryer at 70% thus:

14550 va divided by 240v = 61 a.

beyond the fact that the feeders needed to be rated for 100 amps, has it occurred to you pros that a one-family dwelling service used a neutral ampacity other than that sized for the two ungrounded conductors?

said another way, in determining the service for a small residential dwelling, has it occurred that the neutral calculation indicated that the neutral ampacity conductor differed from that determined for the two ungrounded conductors?

attempting to fathom the practical need for the Annex D1 (a) neutral calculation--is it fairly rare that neutral and hot would differ in ampacity for a small dwelling?

thank you

 

[ActionDave]

....said another way, in determining the service for a small residential dwelling, has it occurred that the neutral calculation indicated that the neutral ampacity conductor differed from that determined for the two ungrounded conductors?

attempting to fathom the practical need for the Annex D1 (a) neutral calculation--is it fairly rare that neutral and hot would differ in ampacity for a small dwelling?

thank you

The current on the neutral is always less than the current on the two ungrounded conductors since the neutral only carries the imbalance.

 

[Dennis Alwon]

Think of the loads on a residence. Often there are loads rated 240V such as water heaters, electric heat, a/c units etc. No neutral involved. So yes the load on a neutral can be significantly less than the ungrounded conductors.