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I'm thinking about running a single 12/3 cable to my garage to wires all of the outlets (8 or 9). (This is old construction, with a finished ceiling.) I understand that the black and red wires will create two circuits fed by a double-pole breaker. Will I still be able to install GFCI outlets at the beginning of each wall run? I've read that a GFCI breaker may be needed. I haven't decided if I'll do every other receptacle or the tops of all receptacles on one circuit and the bottoms on the other. By the way, each wall will likely be fed by its own cable, going back to a junction box.

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This applies to all MWBCs

That type of configuration is called a multi-wire branch circuit (MWBC). In this arrangement, you connect a receptacle to one hot and the shared neutral. It is a Code requirement that neutral be pigtailed (i.e. don't use the receptacle as a splice block for neutral). The rule is to assure that neutral is not severed for the other half of the circuit. (this can cause all sorts of bad).

This applies to GFCI on MWBC

You can absolutely install GFCI+receptacles at the beginning of the wall run. You just won't be able to use the LOAD terminals to carry the circuit onward - you will need to pigtail both hot and neutral. Because you can't use the LOAD terminals, only those receps will have GFCI protection - you can't pass it downward to the remainder of the MWBC. You'll need one GFCI per outlet that you want to have GFCI protection.

If you want to GFCI-protect the whole circuit in one shot, use a GFCI breaker at the start, then plain receps throughout. . Mind you, the circuit may require AFCI also, in which case you'd use a GFCI+AFCI breaker.

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  • Why can't the LOAD terminals be used to carry the circuit onward?
    – Mike
    Feb 2 '20 at 2:42
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    I think he meant you can't share the neutral after a gfci receptacle, but for instance if you pigtail off two separate runs at the first j-box then load terminals could be used. I had to read it twice too. Feb 2 '20 at 2:59
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    @Mike Because you wouldn't be able to bring the other hot past that point in a /3 cable. LOAD isn't for onward power (LINE is for that). LOAD provides a GFCI protected zone, but both hot and neutral must be in the protected zone. A load can't be half in half out. So what happens with an outlet put on the other MWBC leg? The neutral is in the protected zone and the hot is not. Doesn't work. Feb 2 '20 at 6:28
  • @Harper Okay, then as long as the run with the GFCI has only one hot and neutral I'm good? That is, I can split the MWBC into legs that only have a hot and neutral. Then I can put a GFCI at the beginning of that run. And this is because the legs are no longer MWBC, like NoSparks comment above.
    – Mike
    Feb 2 '20 at 18:35
  • Very good @Mike! It's one of my favorite tricks, Use the 12/3 for the long haul then fork out two 12/2s for the receptacle hop. Yes, the GFCI rcep is at the first place past the split. You can also stop pigtailing neutral past the split. Feb 2 '20 at 19:40

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