Branch circuit minimum neutral size

[texie]

This thread: http://forums.mikeholt.com/showthread.php?t=163459 and the the one the other day regarding a 120 volt motor starter coil fed from the motor branch circuit has me wondering...

We know what the minimum size could be for a neutral used in a service or for a feeder based on code rules. Assume a branch circuit supplying a piece of equipment with both line to line load(s) and minimal line to neutral load. What would the minimum neutral size be? It would seem obvious that it would have to be a least sized to 250.122 for safety reasons in order to trip the OCPD in the case of a line to neutral short. For example, what if the branch OCPD was was 100 amp and the neutral only needed to be a #14 based on the load, what in the NEC would prevent this?

 

[charlie b]

It's an interesting question, and I am not certain I could provide a code article that supports my answer. But my answer (for the moment, at least) is that the neutral wire must match the ungrounded conductor wire size. My basis is that 220.61 gives us a method of calculating a smaller neutral wire size for feeders and service conductors, but there is no similar rule for branch circuits.

 

[david luchini]

I read 210.19(A)(1) as requiring the minimum branch circuit conductor size, including any neutral, as having an allowable ampacity not less than 100% of the non-continuous load plus 125% of the continuous load.

So if you had a 50A continuous load (with only 10A on neutral) on a 4 wire branch circuit, the conductors (including the neutral) must be at least #6. If the branch OCPD was 80A, requiring the ungrounded conductors to be increased to #4, I think the neutral could stay at #6 to meet the requirements 210.19(A)(1).