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Question

I will be using 12/3 Romex to make two separate circuits. As you know, 12/3 is 20 amp wire, for two separate circuits. There is a red wire (hot) for one circuit. There is a black wire (hot) for another circuit. There is a white wire (neutral) for both circuits. In other words, the black wire and the red wire both share the common white wire. Let’s say that one circuit draws 15 amps (red wire) and the second circuit draws another 15 amps (black wire). That is 30 amps. However, they both share the common white wire. The white wire is now carrying substantially in excess of 20 amps. In this case, it is carrying 30 amps and at full load from both circuits, it would have to carry 40 amps. Add it up. It means that the white wire is carrying the voltage from both individual 20 amp circuits (40 amps). The white wire would melt and catch fire instantly. Hence, 12/3 circuits and 12/3 Romex seem dangerous --- but they are used all the time. I don’t understand.
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Answer 1

The other people who answered have no idea what they are talking about, and you are missing a key part of this also. If you aren't sure what you are doing, you should have an electrician do it.

You are talking about a multiwire branch circuit. To use that, the two hot wires MUST be connected to opposite phases in the panel. If you don't know how to do that, don't do this yourself. Once you have opposite phases, the current in the neutral is the difference between the hots. So if both hots had 15A, the neutral current would be 0. If one was 10 and the other was 15, the neutral current would be 5A. Thus the neutral current cannot be larger than the phase current, so you will not overload the neutral.

A couple side notes. You can use a larger wire at any time, meaning 12AWG on a 15A circuit, except wire terminals are limited in the wire size, so you can't get too carried away.

This multiwire circuit concept is the same for the service to your house. You get 2 hots and a neutral from the utility. The neutral does not need to be bigger than the hots.

Answer 2

First of all, you cannot use 12/3 Romex, as you planned, in this instance. You should use two 12/2 wires for each circuit separate! The gauge of the wires determined by current consumption of each circuit, and, accordingly the breaker size. The 20A breaker required 12 Ga wire, and the 15A breaker goes with 14Ga wire. And these rates are for SUMMARY current consumptions of all loads in the circuit. Be sure not to overload! Also , the type of wires you run (Romex or ...) depends of where your wires are located (inside, outside, environment, etc).
Consult specialist for any special circumstances!
Good luck.

Answer 3

12 3 is generally used for switch legs, mostly fan/lights, so there is only one wire in the box, you usually never use 12 3 off of the panel, cause then your using it as 220. Not theisable... if its 2 circuits, use 2- 12 2 wires.

Answer 4

You just did the math. Use 10 ga. wire, or run two totally separate circuits with two 20 amp breakers.

Answer 5

the neutral wire does not carry amperage--it goes back to ground and would short every time you put power to it. it is return leg and just completes the circut. your power(amps) are being used up at the appliance