My electrical panel has a double-pole breaker that looks like it's for a MWBC, but actually the attached wires are from different cables.
In the below picture the black wire with the yellow rubber bands goes out the side of the panel (to a sump pump). The red and black wires with blue rubber bands are from the same cable going out the top and to the kitchen. (The dishwasher is on the red wire; as far as I can tell nothing is on the black wire and it's probably a provision for a garbage disposal, but I haven't moved the dishwasher to get to the outlet and verify it's a split.)
How bad is this? I understand that one would have to be extra careful about turning off breakers to the kitchen to not leave half the circuit live, and I can't turn off just the dishwasher or just the sump pump, but is there any other danger here? I guess if the dishwasher trips the breaker it will take out the sump pump, but I feel like I'd notice that before my basement would flood.
My current thought is "some day I'll have an electrician out for something else and I'll ask him to look at this too." Should it be a higher priority?
Best Answer
Well as you know, 120/240V power is delivered in 2 opposite poles, with neutral in the middle. Neutral carries only differential current between the 2 poles. The same applies to a multi-wire branch circuit (MWBC) - neutral only carries differential current as long as the 2 hots are on opposite poles. (if same poles, neutral overloads - neutrals don't have breakers!)
In your photo we see a 2-pole breaker immediately above a tandem. The poles are 1 space wide, and alternate. The tandem puts 2 breakers on the same pole.
The good news is it really looks like the 1 red wire you flagged is on the opposite phase from the 2 black wires you flagged. So you're not setting the neutral on fire.
However, what's weird is why the 2-pole breaker for these loads? The only conceivable use for a 2-pole breaker here is correctly protecting a MWBC. Clearly the original installer was conscientious and did the right thing, but confused the two black wires. OR NOT. OK, so... 2 step process.
Re-trace those wires one more time and make sure this isn't your error: preclude the possibility that "the last guy" actually did get it right. No offense.
They're all 15A, so exchange those 2 black wires so the MWBC is on the 2-pole and the 120V pump is on half a tandem.
Never worry about it again lol.