When I opened up my walls for a kitchen remodel I discovered what I'm pretty sure is a multi-wire branch circuit. It has a 12/3 (black + red + white + ground) NM cable going back to the breaker. I think the black wire is attached to breaker 6 (20-amp) and the red to breaker 8 (20-amp) which are both on the same side of the panel stacked one on top of the other. From reading about this it sounds like the white neutral wire is shared for both circuits. I also read that unless this is wired correctly to a double pole breaker so that the current in each circuit is on different phase there could be a fire hazard on the shared neutral.
What are some simple tests I can do to make sure this is wired safely and correctly? The house was built in 1986 so it has been this way all this time and I assume is wired safely and correctly but I want to make sure before closing everything back up. It sounds like the key to this is that the two circuits are on a double pole breaker and each one is wired to a separate pole.
Best Answer
Use a multimeter to measure voltage between the two hots (black to red). You should get somewhere around 240 volts. If you get 0 volts, the breakers are on the same leg.
An empty service panel looks like this
I've drawn rectangles where breakers would go, and labeled the legs A and B. As you can see, breakers across from each other (left/right) are on the same leg, while breakers next to each other (up/down) are on different legs.
If you measured between breaker 1 and 2, you'd get 0 Volts. This is because they're on the same leg, so they're at the same potential. If you measured between 1 and 3, you'd get ~240 Volts. These breakers are on separate legs, so they're not at the same potential.