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This may be an obvious question, but I haven't been able to find a direct answer.

I believe I have a MWBC - I have two hot wires connected to two separate circuit breakers, sharing a neutral and ground wire. The two circuit breakers are not adjacent to one another. they're on slot 24 and slot 28, respectively. Slot 26 is occupied with another circuit.

This is bad, right? This means that my two circuits are both on bus B and in the same phase, rather than in opposite phases like they should be, correct?

Is there some other reason a MWBC would be set up like this?

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  • Can we presume that your panel is numbered "odds on the left, evens on the right"? – ThreePhaseEel Nov 26 '20 at 4:34
  • Yes, odds on the left, evens on the right – Mwinn Nov 26 '20 at 4:34
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Yes, this does sound like a problem. Assuming your panel is like most, the breakers are on the same phase and you have two problems:

No Common Shutoff

MWBC is required to have common maintenance shutoff. That means if you turn off one breaker to work on the circuit, the other is turned off too. That is important for safety. This can be done with a handle-tie or by using a double-breaker. It is quite possible that a violation of this rule is what caused the problem in the first place! Without a handle-tie, and without realizing that the breaker is part of an MWBC, someone might move a breaker in order to accommodate other things, such as double-breakers for 240V circuits (dryer, oven, etc.). Once they do that, 50% chance that the 2 MWBC breakers will be on the same phase instead opposite phases.

When two breakers are side-by-side, a handle tie can be added. When the breakers are not side-by-side (with the special exception of the outer breakers of some quadplex breakers), you have to move at least one of the breakers to solve the problem.

Overloaded Neutral

Putting the two breakers on different phases (legs, poles, terminology varies and I'll leave it to the pros to argue that) means that the neutral will carry the difference between the two hots.

Putting the breakers on the same phase means the neutral will carry the sum of the two hots.

If you have a 15A circuit with 12A in use on each 1/2 of the MWBC, with different phases neutral carries nothing at all. With the same phase, neutral carries 24A, which is way too much for a 14 AWG wire. Neutral does not (normally) have a circuit breaker, so there is nothing to keep the neutral wire from overheating and starting a fire.

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To add to the other answer, your two phases are 180 degrees out of phase so that it creates what is called destructive interference

Destructive Interference

Now this doesn't totally fit because your electricity is all flowing in the same direction, but the same principle holds. Note that wave A and wave B are completely canceled out. Even with the two waves running in the same direction, they null one another out.

When you look inside your electrical panel it typically looks like this

inside your electrical panel

Note how each bus zig-zags. This is so you can put 240v double-breakers in. Note how slots 1 and 3 each hit a different phase. It sounds like they're using something like slots 1 and 5 which creates constructive interference

Constructive interference

Thus your neutral now gets that double load manassehkatz talks about

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    Interesting viewpoint on it. Except AC mains "signals" are continuous not pulses, so you are in the "during interference" state 100% of the time. – Harper - Reinstate Monica Nov 26 '20 at 19:51

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