310.15(b)(4)a,b,c Derating multiwire branch

[gregr]

I feel a little confused about what I have read on a few of the other respones concerning this area.

310.15(b)(4)a
I always thought this is refering to a multiwire branch circuit. Whether it be one two or three phases that supply only line to neutral load. Sharing the same neutral.

310.15(b)(4)b
I thought there is a difference between a 3 wire circuit and a multi wire circuit.

310.15(b)(4)c
I also thought there is a difference between a 4 wire three phase circuit and a multi wire circuit.

The reason I bring all of this up is Derating seems to have a lot of unanswered questions.

9-20A circuits 1,3,N,2,4,6,N,8,10,12,N,7,N all of which feed receptacles. What size wire should be used? Also which of the above neutrals carry unbalanced current of the same circuit, since this would be the ones that arent counted?

 

[roger]

Re: 310.15(b)(4)a,b,c Derating multiwire branch

Greg,

310.15(b)(4)a
I always thought this is refering to a multiwire branch circuit. Whether it be one two or three phases that supply only line to neutral load. Sharing the same neutral.

When only one phase or leg is present the grounded conductor can not be a neutral, this would also be true of two phases of a wye configuration.

310.15(b)(4)b
I thought there is a difference between a 3 wire circuit and a multi wire circuit.

Not in this case, see above.

310.15(b)(4)c
I also thought there is a difference between a 4 wire three phase circuit and a multi wire circuit.

No, this is a Multi-Wire circuit.

In your last sentence since we are assuming a wye system; 1,3, and N would all be current carrying conductors, 2,4,6, would be current carrying conductors, 8,10,12, would be current carrying conductors, 7, and N, would be current carrying conductors, this means you would derate the conductors in the conduit to 50% since you have 11 current carrying conductors.

Roger