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Bought a 1960s home with aluminum wiring.

I noticed that I have two sets multiwire branch circuit? (also known as "multiwire circuit", "multiwire branch circuit", "shared neutral", "two circuits fed from one 12-3 or 14-3 NM-B cable", "shared neutral circuit", etc).

Both hot feeds are connected to same side of the breaker panel (thus presumably NOT a split-phase MWBS, but a same-phase). I would think that this overloads the neutral line.

Sure enough, one single-gang SPST switch box started clicking and then smoldering (had to turn two non-adjoining breakers off before accessing).

enter image description here

Purple wire nut got burned thru that was connecting all the neutral (white) AWG10 wires together.

After much long-wire continuity tests to identify each 3-wire plus ground Romex cables going to other boxes, I've resisted repacking properly with purple Alumicon steel buses because ...

The lone neutral feed (AWG10 aluminum) wire would be computationally overloaded with 2x15A loads.

I've often wonder which approach is better repair method:

  1. switch one of the two breakers covering the MWBS from same phase as the other to an adjoining slot having inverted phase.

  2. drop two separate but new AWG12 3-wire + ground copper Romex

Also get a deeper single-gang box to accommodate the clunky 3-connector Alumicon steel lugs.

I obviously cannot pack this box until this resolution has been taken.

Old construction, mostly aluminum AWG10, some contractor re-work in AWG12 copper.

General Electric TX1612F Breaker Panel, CTL, 1966 panel

Slot Position 10 (phase A) and 18 (phase A) have shared neutral.

GE TX1612 panel box.

If one side is same phase and other side is inverted, I would swap 15 (left side) and 10 (right side).

If one of those fancy zigzag alternating phase on one side (Edited: every two rows, yes, starting at row having slot 7-8).

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    Photos? Particularly of the panel? Generally both phases are available on both sides of a panel so the two legs of a MWBC going to adjacent spaces is normal. Two-pole 240V breakers would be rather awkward if they had to span both sides (not to mention such a design would make it impossible to have “single column” panels with the same bus design).
    – nobody
    Feb 22, 2023 at 3:17
  • added photo and comments as requested. Feb 22, 2023 at 3:56
  • assuming that my panel has both phase on alternating slot on either side, then 20 and 28 are on same phase and this needs to change. Feb 22, 2023 at 4:08
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    The alternation happens on the scale of each 1” breaker space (which can hold two half-inch breakers). So 17 and 19 are on the same phase, 21 and 23 are on the other. 20 and 28 are on the same phase and need to be moved if they share a neutral.
    – nobody
    Feb 22, 2023 at 4:27
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    @JohnGreene can you post a photo of the label on the inside of your breaker box's door please? Feb 22, 2023 at 4:44

2 Answers 2

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The MWBC mistake is terrible, and would explain your troubles. Take extra time to ensure the phasing is correct, as this can get very confusing on Zinsco or GE panels, and is also bewildering where tandems are involved. The voltmeter (measured hot to hot) will tell.

The purple "twister" wire nuts also have a poor track record. (but so do regular wire nuts when Johnny Limp-wrist doesn't tighten them enough).

The key to making aluminum connections work is to use terminals rated for aluminum (CO-ALR receptacles or Alumiconns) and torque the terminals to spec. So have a torque driver.

Which kind of 1966 GE box I have?

Oh, you have a split-bus (Rule of Six) panel. Your four double breakers up top are the main breakers. You must have no more than six; you're legal now except that 30A breaker needs a handle tie, and it's obsolete.

This Rule of Six panel has no main breaker. Those five breaker throws are your main breaker. These have been outlawed because nothing prevents them from drawing more than your service size and setting your service wires on fire. If you upgrade to a meter-main that corrects the aforementioned problem, however you also don't have a ton of extra spaces, so a panel upgrade may be a better long-term plan.

If one side is same phase and other side is inverted, I would swap 25 (left side) and 20 (right side).

If one of those fancy alternating phase on one side (same in 1st slot, inverse on next slot down) then I am thinking of exchanging 21 and 28 (both right side, but puts 28 closer to 20), assuming split-phase

Pop off breakers and look, but you almost certainly have interleaved bus stabs.

enter image description here

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    of course, i should invest in a replacement of two adjoining breakers with a handle-tie dual breaker out of being nice to the next homeowner (or me, should I get forgetful). Feb 22, 2023 at 4:14
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    @John you can just use listed handle-ties, you don't need to buy a whole $12 breaker. Whatever level of care suits your safety needs, go for it! Feb 22, 2023 at 4:19
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    @JohnGreene Interesting, but I wouldn't use it as a substitute for complying with NEC 110.14. Alumiconns are made of zinc plated aluminum, as are your breaker panel lugs and neutral bar, because they play well with copper and aluminum wire. Feb 22, 2023 at 22:51
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    @John sure, that'll work fine. That's how GE intends these panels be used. Feb 24, 2023 at 21:12
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    Update: contracted out for a new outdoor panel opposite this panel. Also got 200A main top breaker, a transfer breaker for 60A outdoor generator complete with male NEMA outsoor plug box for easier hookup of generator. Now I can rest easy knowning I can get solar panel set and a swimming pool in the future. Dec 26, 2023 at 0:52
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All the bottom right-side 1/2-size breakers are 15A, so it should be no problem moving circuits around to get it right, as long as you make sure you aren't messing up another MWBC in the process. There are 2 keys to the selection of breakers for an MWBC:

  • Opposite legs. In this case, consider 18/20, 26/28 to be "A", 22/24, 30/32 to be "B".
  • Adjacent, so they can be tied together.

So if you can swap 28 with 22 or 20 with 30 then you're all set.

Note: My counting is based on each full-size breaker as 2 rows, odd on left, even on right. In other words, top left 60A is 1/3,5/7; top right 50A is 2/4, 6/8.

The last piece is a handle tie. That may be these from Home Depot but you need to make sure that you get exactly the right thing for your breakers.

The aluminum wire is a big problem. You might not have even had a real neutral overload - it may simply have been a bad connection. But once you know you have an MWBC problem like this, you do need to fix it. (I had one improper MWBC and didn't know it until panel replacement, but fortunately it had never led to any problems.)

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    I do think that this is not an overload issue but a case of aluminum oxide manifesting once copper-based (white) neutral was twisted with other 3 aluminum neutral. All lightings have gone down to LED so load was even less of an issue there. Feb 22, 2023 at 4:32
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    It is also interesting to note that an inspection of one other switch box revealed a fallen orange twist nut at the bottom of the box. It was to cover four neutral wires together (a lead-in to a 3-way 200W lamp circuit). Feb 22, 2023 at 4:34
  • @John That shouldn't happen. Wire nuts are NOT insulating caps. If you are installing them properly, i.e. VERY tight... the wire nut is actually twisting the wire. Even if you pre-twisted in the wrong direction, the properly applied wirenut would correct it by brute force lol. The idea of one 'falling off" is impossible. It sounds like you had an amateur ahead of you, and every wire nut and screw should be checked. Feb 24, 2023 at 23:05

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