Current Carrying Conductors

[jeff43222]

I saw this one on a closed-book exam recently:

Q: Suppose there are two multiwire circuits in a conduit supplying single-phase lighting at 115/230. How many of the six wires are counted for derating purposes?

A: 2, 3, 4, 5, or 6.

I chose 6, as I figured that the two neutrals, while shared between the four circuits, would be current-carrying unless they were balanced. Nothing in the problem indicated that the circuits were balanced, so I figured they should all count.

I know in some cases neutrals are not counted for derating purposes, but since it was a closed-book exam, I couldn't look this up.

Anyone have an idea if I guessed correctly?

 

[charlie b]

Re: Current Carrying Conductors

If the neutral only carries the unbalanced current from the phase conductors, then it does not count. Reference 310.15(B)(4). If the source had been a 3-phase WYE, the neutral would have counted. Reference 310.15(B)(4)(b). But your source is a single phase 3-wire, so that rule does not apply. My answer would be ?4.?

 

[bphgravity]

Re: Current Carrying Conductors

I feel the answer is 4, for the ungrounded conductors only. I don't feel the neutral conductors in this case need to be counted as current-carrying.

 

[jeff43222]

Re: Current Carrying Conductors

I had a feeling the neutrals might not count, but I wasn't sure. Thanks for the responses.