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I'm renovating a house and am finding lots of questionable electrical work. My recent finding is a 12/3 cable that carries two circuits from my electrical panel that branches off to two different chains of receptacles. To clarify, in my panel the red and black wires go to seperate 20A breakers, white to the neutral bar. At the other end of the cable, the red wire goes to an old BX cable, the black wire another, and all three neutrals together.

I've never seen this and logic tells me it's bad, because that doubles the current being carried on the neutral wire. But before I tear it out I wanted a sanity check. This is bad, right?

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  • In fact, the neutral carries LESS current in a MWBC. – Ecnerwal Jul 29 '19 at 3:12
  • If it's wired right. It's easy to not wire right, and going to separate breakers is the gateway drug. – Harper - Reinstate Monica Jul 29 '19 at 9:03
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Meet the Multi-Wire Branch Circuit (MWBC)

It's your friend if you do it right - but it's easy to do wrong. It has several very important requirements that you have to get right:

  • Neutral must be pigtailed. This is so removing a device won't sever neutral for the other half of the MWBC.
  • It's absolutely critical the MWBC be powered from opposite poles - and the #1 mistake there is tying it to a "double-stuff" duplex/twin type breaker (whose 2 circuits are actually on the same pole).
  • It needs to be supplied from a single 2-pole breaker, for 2 reasons:
    • to assure the two legs are powered from opposite poles.
    • to provide common maintenance shutoff, so both/all legs of the MWBC are shut off together when someone is attempting to service part of the circuit.
  • The wires need to be marked as being grouped, but that happens automatically with cable

Technically speaking, you only need common maintenance shutoff, not common trip as a 2-pole breaker also provides. However, we advise 2-pole breakers because you do common maintenance shutoff (only) with handle-ties, and properly listed handle-ties are often hard to find, and they can be installed wrong. It's hard to go wrong with a 2-pole breaker that obviously has factory-linked handle action.

As far as GFCI, you can protect the entire MWBC with a 2-pole GFCI breaker, which includes all 3 wires. Or, you can fit GFCI outlets that serve only themselves. Or, you can fit GFCI devices that protect their half of the circuit after it splits.

As far as AFCI, that has to be done at the breaker, and methods vary depending on brand and need for GFCI also. Consult with either the manufacturer of your service panel, or maker of breakers which are UL-classified for your panel. You must not use random brands of breaker, even if they seem to fit.

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This is a MWBC - a multi-wire branch circuit. It is perfectly fine, provided that:

  • The two circuits are on different legs - i.e., 240V between the two hots. If so, the neutral is the difference between the circuits instead of the sum of the circuits. In other words, if you have 15A on one circuit and 10A on the other, the neutral will actually have 15 - 10 = 5A, not 25A.
  • The two circuits have common shutoff - i.e., if you turn off the breaker for one circuit, the breaker for the other circuit always turns off at the same time. That is important because otherwise the shared neutral could have current on it from the other circuit when you think everything is off because the breaker for one circuit is off.

There are also complications when it comes to wiring up GFCI on an MWBC, but if installed correctly an MWBC is perfectly safe. Depending on the configuration, you may want to install a dual-pole GFCI in the panel (advantage: one location and "done"; disadvantage: long way to go to reset) or you may want to install receptacle/GFCI devices at the point of use (advantage: easy reset when needed; disadvantage: depending on how/where the MWBC is split, this may not be practical or may require other changes to the wiring).

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  • Thank you. My setup is missing a handle tie (currently each breaker throw is separate), which is easy enough to correct. However, they did this for kitchen countertop outlets w/o GFCI's, so I'll need to figure out how to add GFCI's with this setup. – DrTarr Jul 29 '19 at 11:58
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    Depending on your panel (or what you replace that with, depending how bad things are) a dual-pole GFCI breaker might be the easiest solution. Kitchen is probably the most common place to find an MWBC, due to the "2 20Amp countertop circuits" rule. – Ecnerwal Jul 29 '19 at 14:39

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