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I've just opened up my main panel with the plan of adding a new circuit, but have temporarily aborted that idea after squinting for a while, followed by getting worried.

annotated panel

I put some pictures from more angles at https://imgur.com/a/TZkSZQi.

There's quite a lot of 14/3 (or 12/3) wires running running in the top of the panel and fewer 14/2 (or 12/2). The double pole breakers at the bottom seem to be reasonable MWBCs to me, but I'm particularly worried about the ones I marked in red and yellow (a fairly large proportion of them!)

My admittedly limited understanding is that each full row of the panel would correspond to a leg of the incoming power, so MWBC should only share a neutral when they're on double pole breakers or otherwise spanning two rows by being part of a quad breaker.

So my questions are:

  1. Should I be worried about the circuits I marked in red? They seem to have two problems: a) they're not tied but are likely sharing neutrals, and b) both hots are on the same phase, so possibly overloading the neutral.

  2. Should I be worried about the circuits I marked in yellow? They aren't physically tied so the neutral could be unexpectedly live, but they at least seem to be on opposite phases.

I'm also in BC, Canada and the home was built in 2002 in case that's relevant.

I'm also still just assuming that the smaller gauge red/blacks are part of 3 conductor bundles, I haven't yet turned off the main power and started trying to deduce which white/red/blacks are going to the same place, but I suppose some stranger configuration is possible too. e.g. that the various 15A black/reds are indeed on opposite phases (though untied) but that's been accomplished by splitting across rows "manually" and keeping track of what's on which phase... somehow (could that have been Code in 2002?).

(Editing to add this pictures that shows cables, indeed, not conduit):

Wider view showing cables entering

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    The yellow-marked pair on the left seems odd to me because it looks like the black wire is a different gauge to its red mate 3 wires below. Do you feel comfortable tracing all the suspect pairs from the breakers to the actual cables where they exit the panel to see which ones are actually grouped together with a single neutral?
    – brhans
    Dec 13 '21 at 23:48
  • Being not tied is the main problem. The phases usually switch for every pair(#1 be on right side, #2 right below would be connected to left side).
    – crip659
    Dec 13 '21 at 23:49
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    @crip659 The problem is that a pair of half size breakers are on the same leg unless they are "inner pair" or "outer pair" of a quadplex. The yellow groups are quadplexes, so if wired correctly the only issue is common shutoff. The red pairs are likely a problem because the usual reason for black + red wires going to a pair of breakers is 240V appliance or MWBC. A 240V appliance wired with black & red on the same leg simply won't work. An MWBC will work but can overload the neutral. Dec 13 '21 at 23:59
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    This is all based on the assumption that "all wires black, red, white" means "lots of /3 cables". If in fact these are randomly red & black wires with separate white neutrals in conduit then there may not be any problem at all. But that would be unusual. Dec 14 '21 at 0:00
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    @manassehkatz-Moving2Codidact Thanks that matches my understanding. At the imgur link you can see all the incoming cables and they are indeed a bunch of /3, not separate conductors in conduit.
    – Scott
    Dec 14 '21 at 0:08
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You appear to be correct to be concerned.

The "tandem breakers" in red (same phase half-width dual breakers) wired as MWBCs are the bigger concern and more difficult to resolve, since they will put up to 30 amps onto a shared 14Ga neutral wire. I guess it's possible that our hero who doesn't like handle ties wired them as if two of them side by side were quads, in which case the neutral current would be OK but the lack of common shutoff is bad, and I don't think any handle ties are listed to be used on a pair of tandems, rather than a quad.

The yellow ones would only need a handle tie. Depending when they were installed it might not technically be needed, (it became a requirement eventually) but it's certainly safer. Hmm, 2002, I guess it wasn't required here down-below until 2008 if the MWBC was not feeding devices on the same yoke. Not sure what the up-there date would be.)

Given the full panel, you probably need to move a bunch of stuff to a sub-panel, or a panel replacement with a bigger panel to actually resolve this. Sub-panel is generally going to be easier. Don't skimp on the size (number of spaces) of the sub-panel, and don't be concerned about the amperage rating of the sub-panel being higher than the breaker feeding it, if that happens to be how you get a lot of spaces in it.

I guess you might be able to replace pretty much all the tandems with handle-tied quads to make those proper MWBCs and replace one of the non-tandem pairs with another quad to add your new circuit, but that's a whole lot of quads and leaves you no room for the next thing.

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    Thank you! And $#@^% dang it. :)
    – Scott
    Dec 14 '21 at 0:10
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    As Harper will come along shortly and say, somebody saved the price of a latte on a panel that was "just big enough". Then someone got improperly creative making it bigger than it was the wrong way. Spaces are cheap when you buy a panel, expensive (and a pain in the %#$@!) when you run out of them in a panel.
    – Ecnerwal
    Dec 14 '21 at 0:15
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    Maybe it could be saved with a bunch of quads and handleties??? I haven't really added that all up. Ditch all the tandems that are MWBCs and replace them with 15/15/15/15 quads, if those can be had. Since you want to add a circuit, probably also going to need to ditch a couple of non-tandems for another quad, too.
    – Ecnerwal
    Dec 14 '21 at 0:19
  • Yeah, I was wondering about squeezing things in with those double tied 15 quads, but then that's probably just adding more complexity and kicking the can down the road a few years. I think I'd probably better bite the bullet on a subpanel or bigger panel now.
    – Scott
    Dec 14 '21 at 0:38
  • @Scott the quads are a fine solution, if you can obtain outer-handles-tied Siemens breakers, and can tolerate the complexity. You can't use other makes' breakers. Dec 14 '21 at 2:19
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See my post on panel design. Here is your panel with phases highlighted.

enter image description here

Since you have not chased each red-black pair from their respective cables, we don't know if they're improperly phased. That will tell the tale. Improperly phased breakers means the neutral is being overloaded. You'll want to fix that PDQ.

The colors I use for phases suggests a solution: position all black wires on odd rows, and all red wires on even rows. This would allow confirming at a glance that they are at least phased properly.

Other than that, you need listed handle-ties for each MWBC circuit. With tandems, that's a bit complicated.

The obvious way is to use quadplex breakers. Those come in two varieties: like your 15/40/15 with the outer breakers not tied, or with the outer breakers handle-tied. Unless you can find plenty of plain 1-pole circuits to fill the outer positions, you need quadplex with handle-ties.

The other way is to use handle-ties between tandem breakers. You can have an unlimited stack of those, with the tandems in any 2 slots having a handle-tie between them. You end up with single breakers at top and bottom that you'll need to find plain circuits for. It's a real nightmare to assemble many in a row, though.

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  • Thanks, makes sense and your panel design post was very helpful. It's been like this for at least 2+ years, so maybe since the house hasn't burnt down (?) @Ecnerwal's guess is right and it's been wired as if they were already quads. I'll work on some tracing and then try to mock up a layout to see how complicated it'd with a bunch of Siemens Q21515CT2.
    – Scott
    Dec 14 '21 at 3:54
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    Assuming the problems are as severe as we suspect they are, the reason the house hasn't burned down is because nobody was running (on improperly configured circuits) more than one at a time of: space heater, big computer (mining), grow lights, etc. Most typical loads are either much < 1/2 circuit capacity (ordinary computer, ordinary lighting, TV, refrigerator, etc.) or are short-term (toaster, hair dryer, etc.). But it can be a disaster waiting to happen, especially with space heaters because you can easily have two rooms sharing a messed up MWBC. Dec 14 '21 at 4:30
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    @manassehkatz-Moving2Codidact I see, thanks. Fark! By dumb luck, the only almost-full-almost-continuous load (that I can think of) is on one of the 15A circuits of the "yellow" quad breaker (charging an EV). I guess as a temporary safety measure, I should turn off one of the two circuits in every tandem pair that looks suspicious until the mess can be investigated.
    – Scott
    Dec 14 '21 at 6:41
  • As suggested, I've now traced all the of /3 red/black pairs and convinced myself that they are indeed connected across separate legs (all something like 15A/13B never 13A/13B), with single pole /2 "fillers" at the top and bottom. So at least I'm not casually melting neutrals!
    – Scott
    Dec 15 '21 at 3:34
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    @Scott I don't know how handle ties work in the Siemens line, they might make a field-applied handle-tie for a 15-15-15 quadplex. That is fine; MWBCs do not require a common trip mechanism. The handle tie is purely for the protection and education of a maintainer. If they defeat an outer handle tie by forcing the handles to opposite positions, they have no one to blame but themselves... Dec 15 '21 at 3:43

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