Shared neutral question.

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guschash

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Ohio
I had a shared neutral question put to me the other day,not sure I answer right. Each circuit draws 12 1/2 amps, and I said the shared neutral needs to be able to handle 25 amps. Double pole 20 amp breaker. So he needed a #10 wire. He wants to use romax. Not sure if I answer right.
 

GoldDigger

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Placerville, CA, USA
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If it is a full sized double pole breaker, with 1/2 fed from L1 and 1/2 fed from L2, then there will be no net current on the neutral.
That assumes 120/240 single phase 3-wire.
If it is two poles of a three phase panel, then the neutral will carry the vector sum, 12 1/2 amps.
If you had a duplex breaker where both circuits fed from the same stab, you would have 25A on the neutral.
 

ActionDave

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I had a shared neutral question put to me the other day,not sure I answer right. Each circuit draws 12 1/2 amps, and I said the shared neutral needs to be able to handle 25 amps. Double pole 20 amp breaker. So he needed a #10 wire. He wants to use romax. Not sure if I answer right.
It's called a multi-wire branch circuit. The neutral only carries the imbalance between the two hots.

Think about the feeder to a panel. Assume 100A single phase. If there is 75A on one leg and 75A on the other leg is there 150A on the feeder neutral?
 

Barbqranch

Senior Member
Location
Arcata, CA
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Plant maintenance electrician Semi-retired
Naturally, all this assumes the two wires are not on the same leg (I have seen where people shift breakers around and end up w/ that happening). The reason why mwbc are now required to be on a multipole breaker.
 

don_resqcapt19

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Illinois
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retired electrician
Naturally, all this assumes the two wires are not on the same leg (I have seen where people shift breakers around and end up w/ that happening). The reason why mwbc are now required to be on a multipole breaker.
Multi-wire branch circuits are not required to be on a multipole breaker, they are required to have a common means of disconnect. That can be accomplished with single pole breakers and handle ties. Also possible overloading of the neutral was not part of the reason for this code change. The reason was safety issues if you do not disconnect all of the ungrounded conductor of a MWBC, and you open the grounded conductor of that branch circuit.
 

charlie b

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Lockport, IL
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The reason why mwbc are now required to be on a multipole breaker.
Not quite, I would say. The very definition of mwbc includes the requirement that the ungrounded conductors come from different phases. The reason for the multipole breaker (or handle ties) is related to the shock hazard that could arise if you only turn one breaker off and work on the circuit.

 
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