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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?
 

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Copied from my reply on the other forum:

It is actually the opposite. The white will only carry the imbalance of the two circuits of a multi-wire circuit.
For instance, if the black is carrying 10 amps, and the red is carrying 7 amps, the white is only carrying 3 amps.

Understand?

This is provided the multi-wire circuit is wired correctly. Meaning the two hots are on different "phases" in the panel.
 

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Speedy Petey is right about having the "hot" wires on 2 different phases in the panel. To check this, you should have 240 volts (or close to it) from black to red. If you have nothing from black to red, then both "hots" are from the same phase...and you would be overloading the neutral (white) wire, as you first mentioned. "Edison 3-Wire Circuit"...http://www.bobvila.com/BBS/3_wire-Electrical-1-T1593.html
 
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