How much current draw

[drhowarddrfine]

I understand I can run 20amps on a 12 guage wire but I have a cable that is 12/3, ie, red, white, black, green. So my question is, can I run 20amp on both the red and black?

I'm confused because I would think the return white wire would still only be rated for 20amps total but if the other two ran 20 then that would put a 40amp load on the white wire.

What am I misunderstanding?

[Speedy Petey]

See replies on the other forums.

[HayZee518]

the ruling on a multiwire branch circuit is that the white carries the unbalanced load of the circuit - ac is changing polarity 120 times a second.

[Kidd6488]

what you're thinking does hold true, but only if the red and black are on different circuits, that are on the opposite bussbars in the panel box. This is called multiwire circuit, I believe. For example, if you had 2 circuits running in a 12/3 cable, and the circuits were on the same bus bar, they are both at + at the same time, and both at - at the same time. So while the Black and Red are each only carrying 20 amps, that current is returning thru the white wire, and would ADD, which means a 12AWG wire is carrying 40 amps, and would fail.

But if the Black and Red are on opposite bussbars, then one is at + while the other is at - . Then, one is pulling 20A (+) while the other is pulling 20A (-). Therefore, these cancel out, and the white wire sees 0A. However, the loads are never perfectly ballanced, so if one pulls 19A and the other pulls 10A, the current on the white wire would be 9A.

Hope this answers your question. Post back if it doesn't.

[HayZee518]

Jeez Kidd you're smart!

[Kidd6488]

ha, thanks HayZee...

[HayZee518]

In any multiwire circuit the white when used as a return path to ground carries the unbalanced load of both circuits. As you stated, if both #12's carry a full 20 amps then the white is seeing zero amps, theoretically. If one line is 20 and the other is 15 then the white is carrying 5 amps. flexible cord's nomenclature states all the conductors within the jacket. 12/2 means two #12 wires. 12/3 means a black, white and green. 12/4 means a red, a black, a white and a green. Look closely at the printing on the jacket. sometimes it says "with ground" in that case there will be a green ground cable inside. Romex on the other hand almost always includes a ground wire even if they don't say it.

[Speedy Petey]

In any multiwire circuit the white when used as a return path to ground carries the unbalanced load of both circuits.

Actually the neutral (grounded conductor) carries the current imbalance back to it's source, not to ground as is commonly thought.