Wiring – Using a three-wire cable across two breakers

wiring

I recently had a new panel installed and asked for a couple new runs to be installed at the same time.

The electrician used a 14/3 cable (red-black-white) for each of these new runs, informing me that 1A = Red / 1B = Black and 2A = Red / 2B = Black. I think he did this as a favor to me as I told him I wanted to get my basement fridge on its own circuit. I've wired up the circuit; breaking the tab in the outlet with Red on top, black on bottom and each outlet can be control by their own 15amp breaker from the box.

I've read a few other posts, specifically this answer about this situation and I'm trying to understand about how/why/when the neutral could end up in a situation of 30amps. I was going to take my second run and split it at a box with A (Red) using 2 wire for my overhead lights and B (Black) 2 wired going to the outlets. This isn't terribly different than the first scenario with the split outlet, just that the two loads aren't beside each other in the final outlet. I just want to ensure I'm not wrong to wire it up this way or could get into the situation of the 30amp neutral.

Per requests, here is an image of the breakers in the panel. Bottom left 2 groups. I currently have one off as they aren't hooked up.

Edit/update: Testing with a multimeter shows:

  • red/white = 120v
  • black/white =120v
  • red/black = 0v –per comments this is a problem.

enter image description here

Best Answer

DANGER!

This is totally wrong. You have 2 pairs of 1/2-size breakers - a.k.a., "double stuff" or "tandem". If you measure voltage between black and red (i.e., between a pair of hots) you will find 0V instead of 240V. If this was done by an actual licensed electrician, he should have his license revoked. If it was done by a handyman-acting-as-electrician, this is why most places don't allow that.

Note that this would be perfectly fine with two separate cables - i.e., each breaker getting its own black & white wires. But sharing (MWBC) only works if the pair of breakers are (a) on opposite poles/legs so that you get 240V between the hots and (b) the breakers are physically paired (handle tie or double-breaker) like the 40A pair across from these breakers.

And now for the really crazy part. The usual reason to go for 1/2 size breakers is because you don't have enough space to put full size breakers. But you have more space right below above (picture is upside down, or panel is upside down) the new breakers! You could easily have 2 pairs (i.e., 2 double breakers) of 15A full-size breakers and everything would be just fine.

There is actually one other alternative. A quadplex breaker:

QT 15A Quadplex

The beauty of a quadplex is that the inner and outer pairs each function like a traditional double breaker. Red and black to the outer breakers + white neutral. Red and black to the inner breakers + white neutral. Done.

Difference in cost between 1 quadplex and 2 tandem breakers at Home Depot? Less than $11. Probably even less of a difference at an electrical supply house.

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