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While moving some wires around in my garage (to make space for attic storage), I discovered an electrical issue that might be concerning: In my garage there are two circuits delivered as a MWBC, but the two circuits are on the same bus (0V between the red and black, 120V between each of the hots and the neutral).

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The two circuits are sitting completely separate in the panel (see attached image -- I highlighted the two wires in the MBWC). Interestingly, when I called a local electrician for advice he said it's nonsense and should work fine just as it is, but from everything I am reading it is not only a code violation but also a fire hazard... What would be the easiest way to fix this issue?

One quick fix I could think of was to swap one of those tandem breakers with one below or above it, so at least the two circuits will be on different phases. My concern with that one is that I'd first need to confirm that the other wire on that tandem breaker isn't also a part of another MBWC... And I'm not sure how to do that. Any better ideas?

I have to admit I am quite surprised: We purchased this house new from a builder and have never modified anything, so the home was built this way, which seems incredibly careless.

Update: Just occurred to me that SolarCity (now Tesla) installed the PV system years ago and likely moved circuits around on this panel, so I suppose it's possible they created this situation.

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    It is quite possible that someone who didn't know about the MWBC moved things around. That is actually the other benefit (besides common shutoff) of handle-ties (or double breakers): It makes it real obvious to someone working on the panel later that the pair should stay together. MWBCs have not always had that requirement.
    – manassehkatz-Moving 2 Codidact
    Commented Jan 14, 2022 at 2:13
  • Are you sure all 20A breakers are wired with 12AWG wire?
    – P2000
    Commented Jan 14, 2022 at 5:16
  • Yeah, solar is a real "gold rush" market, and as a result a lot of the workers are "bottom of the barrel" shall we say.
    – Harper - Reinstate Monica
    Commented Jan 14, 2022 at 6:56
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    @P2000 That is a good question. I noticed these sleeves on each lead, presumably from the wire it's from -- they seem to match the breakers (white on 15A breakers, yellows on 20A)... Not sure if I can trust this, but at least it shows someone was TRYING to make sure it all matches.
    – ldoogy
    Commented Jan 15, 2022 at 6:48
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    Aha, ok, now I understand what's going on there. Well, if you find it necessary you can check by unscrewing the wire (breaker off) and checking the copper lead gauge with a wire stripper tool. You can also confirm the cable jacket colour at panel entry. Yellow = 20A NMWU, but some 20A is also white. Maybe you knew this, but I was concerned if the previous installer mixed up the MWBC then perhaps they mixed up wire gauge.
    – P2000
    Commented Jan 15, 2022 at 16:42

1 Answer 1

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That should be an easy fix.

  • Swapping wires: Swap garage and laundry in the "Circuit 2" breaker pair. Swap the garage wire from the "Circuit 1" breaker pair with the first wire in the pair below the "Circuit 2" breaker pair. That puts the two garage wires next two each other but on different phases.
  • Get a handle tie to connect the two garage breakers. I believe that is HOMTHTCP. That connects the two breakers together so that you have common shutoff which is required for MWBC for safety.

A handle tie is "just a little piece of metal" - it does not carry any current or have any active components. But rules are rules, so don't just stick a nail in there. As for the cost, this is a relatively rarely needed component for the average DIY/handyman. So big box stores will sell them at a premium, because they can. They have to cover the loss leaders and low margin bulk items somewhere. When you need it, $6 is not a big deal. I would expect an electrical supply house to sell them much cheaper, but you can't beat the big box stores for convenience, and they list everything on web sites open to everyone. Buy it wherever works best for you.

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  • Thanks! That sounds easy enough, just hope I have enough slack in the wire to move those around without having to extend them.
    – ldoogy
    Commented Jan 14, 2022 at 2:08
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    Key thing to remember is that your MWBC is 20 so you can only move around with the other 20A breakers/circuits. Extending a couple of wires is not a big deal - that's what wire nuts are for.
    – manassehkatz-Moving 2 Codidact
    Commented Jan 14, 2022 at 2:10
  • Wow... is it normally $6.50 for a pack of three? $2 for a bit of metal... another victim of supply chain issues?
    – LShaver
    Commented Jan 14, 2022 at 15:49
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    Thanks @manassehkatz-Moving2Codidact! It's ridiculously overpriced for what it is, but I'll pay it (in fairness, I thought I would need a brand new breaker, so this is actually a smaller expense than I was expecting). Will try to get this rearrangement done over the weekend.
    – ldoogy
    Commented Jan 15, 2022 at 6:46
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    Just a quick update: Got it done without much issue. Moved the two circuits so they're no longer on the same bus, and installed one of those little tandem handles. Thanks again for the advice!
    – ldoogy
    Commented Feb 1, 2022 at 17:55

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