Existing same-phase multi-wired branch circuit

Question

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).

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

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).

Answer 1

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.

Answer 2

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.)