Grounded (neutral) conductor sizing

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robg

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I am a low-voltage residential systems integrator. For a recent project, I asked the line-voltage electrician to provide two 20-amp circuits to power the the equipment that I am supplying to the customer (entertainment, network, security, etc.). Rather than pulling in two 12-2 wires, the contractor pulled in a 12-3 wire to supply these two circuits (a 12 gauge black "hot" for one circuit, a 12 gauge red "hot" for the other circuit, and a 12 gauge white "neutral" for both circuits, plus a 12 gauge bare "ground" for both circuits).

It seems to me that for both circuits to be able to supply the requested 20 amps (or 80% thereof), that there should either be two 12 gauge grounded (neutral) conductors or a heavier gauge grounded (neutral) conductor.

Can anyone shed some light on this for me. Thanks.
 
robg said:
I am a low-voltage residential systems integrator. For a recent project, I asked the line-voltage electrician to provide two 20-amp circuits to power the the equipment that I am supplying to the customer (entertainment, network, security, etc.). Rather than pulling in two 12-2 wires, the contractor pulled in a 12-3 wire to supply these two circuits (a 12 gauge black "hot" for one circuit, a 12 gauge red "hot" for the other circuit, and a 12 gauge white "neutral" for both circuits, plus a 12 gauge bare "ground" for both circuits).

It seems to me that for both circuits to be able to supply the requested 20 amps (or 80% thereof), that there should either be two 12 gauge grounded (neutral) conductors or a heavier gauge grounded (neutral) conductor.

Can anyone shed some light on this for me. Thanks.
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This install is fine. If you have 16A on the black and 12A on the red you will have 4A on the white.

EDIT: Given the black and red are not on the same phase.
 
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If this is a single phase 120/240 system then look no further than Chris's answer. The neutral current will never be greater than the unbalance between the two circuits.
 
rob, you can learn more about this if you search for "multi wire branch circuit". as indicated by posters above, the two "phases" cancel out on the neutral. residential services are generally referred to as split phase, although its disputed that there are actually two phases (semantics), the phases cancel on the neutral due to their inverted waveforms, and except in special circumstances only the difference shows on the neutral as indicated above.
 
chris kennedy said:
This install is fine. If you have 16A on the black and 12A on the red you will have 4A on the white.

EDIT: Given the black and red are not on the same phase.
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A very quick way to determine that both hots are on seperate phases.. put a voltment on them - if you get 240v, the install is compliant. If you get 0v the electrician needs to but the black and red on different phases.

~Matt
 
Quick note: these posts also apply to two circuits originating in a 208Y/120v 3ph panel, too.

The neutral will carry more than "imbalance" current, but it will never exceed 20a.
 
LarryFine said:
Quick note: these posts also apply to two circuits originating in a 208Y/120v 3ph panel, too.

The neutral will carry more than "imbalance" current, but it will never exceed 20a.
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That's true, but in Chris's example the system voltage could not be 208Y/120.
 
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