Sizing the Neutral

[finhead]

The last part of Example D3(a) in Annex D of the 2005 NEC, under the heading "Feeder Neutral Conductor" states that when sizing the neutral [for this particular example] the entire lighting load must be considered the maximum imbalance. Since 210.11(B) does not apply to these buildings it cannot be assumed that the load will be evenly distributed.

The implication here is that if the load is calcultated on the basis of VA per square feet, it would be mandatory to evenly divide the load.
However, all of the examples in Annex D use the entire load as the maximum imbalance - even those computed on the basis of VA per square feet.

Any insights would be appreciated
Brian Dolan

 

[finhead]

Re: Sizing the Neutral

It seems to me there is no sound basis to overule the specific procedures of the Examples.

 

[charlie b]

Re: Sizing the Neutral

The code is recognizing that a neutral of a 120/240 single phase 3-wire system is not likely to carry the same current as either leg. You are free to make the neutral the same size as the ungrounded conductors, or even bigger, if you like. But if you want to save some money by making it smaller, the code doesn't want you to go too far.

If the loads are perfectly balanced, the neutral current will be zero. That does not mean it is safe to not install the neutral. Why not? Because as soon as the homeowner turns off one load, the total loads will no longer be balanced.

210.11(B) does require that loads be evenly proportioned among multi-wire branch circuits. But it does not prohibit us from putting all the lights on one leg and other stuff (receptacles, fans, heaters, dishwashers, etc.) on the other leg. So if all you have on is all the lights, then the neutral will carry all of current from the lighting loads. The code is just being conservative, and not allowing us to reduce the size of the neutral by too much.

p.s. The examples are not enforceable.