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I've pulled a MWBC into the crawlspace of my house, with one phase supplying outdoor GFCI receptacles and the other phase supplying nothing so far. The conductors are enclosed in a combination of all-metal conduit, MC cable, and metal junction boxes throughout.

I am thinking of using the unused phase to supply would-be new receptacles in bedrooms. This would be MC cable to metal boxes. For reasons of geometry and convenience, I'd like to split the unused phase at the junction box shown in the drawing below, with one leg running to Bedroom 1 and one leg running to Bedroom 2. This is a grossly simplified drawing; the bedrooms are not next to each other.

The question is whether installing a listed AFCI receptacle at the first outlet of each split in the branch as drawn is sufficient to meet 210.12(A)(5). Where I get tripped up is in the reading of "first outlet", as I have multiple first outlets in my plan.

210.12(A)(5) If [...] Type MC cable [...] , with metal boxes, metal conduit bodies, and metal enclosures are installed for the portion of the branch circuit between the branch-circuit overcurrent device and the first outlet, it shall be permitted to install a listed outlet branch-circuit-type AFCI at the first outlet to provide protection for the remaining portion of the branch circuit

I anticipate the community recommending the installation of an AFCI breaker, and in my panel this is possible as GE AFCI breakers can support MWBC's. However I'm nevertheless interested in an answer/solution in the context of 210.12(A)(5) specifically.

Thank you!

New picture of MWBC

Ground wire isn't shown above, but it's there.

edit: misspelled a word edit2: updated picture to show junction box inside crawlspace opposite exterior GFCI receptacle

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  • I have all but quit using multiwire branch circuits because of GFCI requirements having an additional device with a test may trip the GFCI if the load terminals are used. I don’t see a problem with multiple receptacles after the first protected circuit.
    – Ed Beal
    Dec 14, 2020 at 16:49
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    @EdBeal I think the question centers on whether having the GFCI (in a location that needs GFCI but doesn't need AFCI) in the chain prior to the AFCI would violate the "AFCI at the first location" rule.
    – manassehkatz-Moving 2 Codidact
    Dec 14, 2020 at 17:45
  • BTW: all currently made panels can support AFCI protecting MWBCs, its just that GE makes it easier
    – ThreePhaseEel
    Dec 15, 2020 at 0:26

1 Answer 1

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"First Outlet" is pretty clear.

You would need to blow that out to a 2-gang junction box, and put a GFCI+AFCI+receptacle combo receptacle on the black wire, and an AFCI on the red wire.

It could be an AFCI receptacle if GFCI protection is not needed there, otherwise it would need to be another GFCI+AFCI+receptacle combo device, or an AFCI deadfront.

... and this is why MWBCs aren't worth it anymore.

MWBCs used to be a nifty way to save some pennies. But we sure aren't saving pennies here, are we?

AFCI and GFCI requirements basically ruined MWBCs. It's time to stop using them.

I only use MWBCs when I need 5-8 circuits in a 3/4" conduit that I can't afford to lay another pipe alongside of.

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  • It just occurred to me that I did not show that there is a junction box in the crawlspace opposite the exterior GFCI receptacle. The red wire does not enter the exterior weatherproof box that houses the GFCI. So wouldn't the red wire's first outlet be in the bedrooms?
    – aerospark
    Dec 15, 2020 at 5:05
  • @aerospark -- that's a really good question, especially if you replace that 12/3 cable with a 12/2/2....
    – ThreePhaseEel
    Dec 15, 2020 at 5:18
  • I don't see how you can peel off one wire and say this isn't its first box. You have to splice the cable to go to the next box right? Splices are just the kind of thing you need AFCI for. Like I said, MWBC is a bad idea.
    – Harper - Reinstate Monica
    Dec 16, 2020 at 0:34

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