Okay, this kind of blew my mind because I had no idea that anyone ever wired house circuits this way. Part of me loves it… but only part of me. Let’s see what is awesome about using 3-conductor wires to share a neutral between two circuits.

A typical American home is fed by 3 large wires:

1) Hot phase A
2) Hot phase B
3) Neutral

*ground is not supplied by the utility

All 3 of those feeds are in your breaker panel. Now, if you run some 2-conductor 12/2 Romex from the breaker panel to, say, a circuit of outlets, those outlets will be connected to the neutral and one of the hot phases. Any energy that goes out on the hot MUST return on the neutral.

The next thing to understand is that our homes are supplied with Alternating Current (AC), which delivery’s energy in waves, not a continuous stream. Not just that, but just as waves on a beach push water up and then pull water back before the next wave, AC current “pulls” just as much as it “pushes” energy.

Finally, we get to the “phase A” and “phase B” topic. In residential electrical systems, there are “two sets of waves crashing on the beach” and they are perfectly opposite each other. When one is pushing energy, the other is pulling, and vise versa.

Now, let’s say you PROPERLY run some 3-conductor 12/3 Romex from the breaker panel as shown in the video. You install a 20-amp 2-pole breaker and are now using both hot phases from the utility and feeding them into the same neutral. Think about 4 possible scenarios:

1. If you have no loads on the black or red hot wires, the neutral will carry 0 amps back to the breaker panel.

2. If you max out the red wire with 20 amps, the neutral will have to return 20 amps to the breaker panel.

3. If you max out the black wire with 20 amps, the neutral will have to return 20 amps to the breaker panel.

4. If you max out both the red and black wires with 20 amps each, the neutral will actually carry 0 amps back to the breaker!

How is #4 possible? It’s because the red and black wires are perfectly opposing each other, so when one is trying to push 20 amps into the neutral wire, the other is trying to pull 20 amps from the neutral wire. These efforts cancel out and the two hot wires actually end up supplying each other. The neutral gets to just sit idle.

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