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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?

Breaker Panel

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    @riffin-rich there is a code requirement for wiring to be done in a neat and workmanship like manor NEC 110.12. Some take this to an extreme and cause other issue like not enough wire to balance a panel, that panel is nothing close to what I would call a birds nest and having small service loops at each breaker pays off big time later when remodeling or balancing the panel load.. what bothers me is the big black wire going to the neutral / ground buss iChat should have been reidentified with tape because we know it is not a ungrounded conductor.
    – Ed Beal
    Dec 24, 2020 at 13:56
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    Welcome IceGlasses! I’m new here too. These folks are fantastic and I’m looking forward to responses given your awesome question. Selfishly speaking, your question is timely because I just scratched the surface learning about MWBC from the smart guys in this forum that are so willing to share their knowledge. Welcome!
    – riffin-rich
    Dec 24, 2020 at 14:58
  • On those 2P breakers terminations are hard to see. Is the blue banded black wire terminated in the lower half of the split breaker directly below the breaker where the red wire is terminated?
    – NoSparksPlease
    Dec 24, 2020 at 18:01
  • @NoSparksPlease Yes
    – IceGlasses
    Dec 24, 2020 at 18:28

3 Answers 3

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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.

  1. 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.

  2. 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.

  3. Never worry about it again lol.

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You may think there is nothing on that circuit but it needs to be verified. It is a simple fix to turn both breakers and swap the wires. Why is this important? With both hots on the same leg this overloads the neutral and this can cause a fire. Many years ago MWBC’s did not require handle ties or double pole breakers. People moving breakers around would split the pair and caused problems on the neutral. I have found panels where the neutral had gotten hot enough to melt its insulation and I was called when it melted into another live connector causing that breaker to trip. Since you understand the cable has 2 hot conductors that are sharing a neutral turn both breakers off and correct the problem by swapping the wires. Both are 15 amp so the wire size should be correct.

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It doesn't appear that the red is on the same leg as the black, so the neutral should not be in danger of overload.

Prior to 1998 the NEC only required simultaneous disconnect for MWBC's if the two circuits were on the same yoke (like a split receptacle) or if it also fed line to line loads. So if the second circuit isn't in use it might not be a violation or dangerous if not altering electrical circuits. Because of the dangers that caused the Code to make these changes it is certainly worthwhile to have an electrician update your configuration.

You mentioned the 2P breaker fed a sump, I have done this intentionally with freezer and sumps circuits that weren't multiwire branch circuits to serve as a passive alarm, but a dishwasher seems poorly suited for that purpose. It looks like the bottom breaker is only half used, you could possibly replace it with a full size breaker and use a handle tie or two pole breaker and re-align circuits to maintain that passive alarm.

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  • I have done the same with freezers and sump pumps, not MWBC’s but a double pole or handle tied independent circuit so we would know if the freezer lost power. I got the idea after a friend lost 1/2 an elk and asked me to make an alarm (this was the cheapest and easiest method I could think of at the time). I have been doing this with sumps & freezers on double pole or handle tied circuits for decades. I have been questioned on it several times by inspectors. there is nothing in code preventing this. Each circuit is still a dedicated circuit (my state I can omit the GFCI requirement on both).
    – Ed Beal
    Dec 24, 2020 at 21:20

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