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My kitchen has a single duplex outlet (so two devices can be plugged in) on a multiwire branch circuit. I'd like to tear that out, replace it with a larger box and install two GFCI duplex outlets.

Right now there's no GFCI protection in the kitchen, which I guess was fine when the place was built, but I'd much rather have it. GFCI outlets won't let you split the plugs into separate circuits, but two of them should be fine since they will only see their own part of the shared neutral wire, right?

I'd also love to have 2 more plugs there so we can just leave the coffee maker plugged in and not have to fiddle with a pile of cords under the plugs. So that would be a big plus and part of the reason I don't want to just install a GFCI breaker (the other reasons being the electrical panel is old and I don't want to touch it, and it's inconveniently located if it does trip).

Is doing that allowed? I'm not quite sure exactly what the requirements are for kitchen outlets. This is in Ontario, Canada.

If it is allowed, any advice on the easiest way to replace the current electrical box with a larger one without too much damage/repairs to the wall?

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  • GFCIs do not like multi wire branch circuits. Sharing neutral makes them go weird. I think that a breaker GFCI is the only choice for MWBC, but will let the experts help answer.
    – crip659
    Jun 8, 2022 at 21:09
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    @crip659 There really should be no problem if you are willing to have two duplex receptacles, as stated in the question. Jun 8, 2022 at 21:17
  • On an MWBC there is no requirement to split receptacles. That is just a style some people prefer. I'm not a fan. Jun 9, 2022 at 5:48

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Assuming that this is truly "MWBC to a single duplex receptacle", including "hot side split", do the following. Note that this assumes that the wires are in a standard black/red/white cable.

  • Black to LINE - HOT of one GFCI
  • Red to LINE - HOT of the other GFCI
  • White neutral - connect to two 12 AWG white wires. One goes to the LINE - NEUTRAL of each GFCI

At this point you have effectively split the circuit. Each half is GFCI protected.

If there are additional wires that were connected previously - i.e., extending the MWBC then:

  • Connect the blacks together and add a pigtail to go to the GFCI
  • Connect the reds together and add a pigtail (could be black if that is all you have handy, but mark it red with tape for consistency) to go to the GFCI
  • Connect all the neutrals together (already using pigtails anyway)

That is important because what you can't do is to run an MWBC from the LOAD side of two GFCIs - that just won't work.

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    Giving you an UV for your last sentence! And for the complete answer. Jun 8, 2022 at 21:58
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    That was probably the problem I read on here with GFCIs and MWBCs. People were using LOAD side.
    – crip659
    Jun 8, 2022 at 22:09
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    There's no problem with powering downstream outlets from the load side of a (as in ONE) GFCI. But like manassehkkatz said, you can't keep going with a MWBC past the GFCI. It becomes a conventional load side circuit. Jun 9, 2022 at 9:40
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You'll have to check with your local authority having jurisdiction to be sure. I have a couple of MWBCs in my kitchen on GFCI outlets and haven't had a problem since the house was built 15 years ago. I'm no longer a fan of MWBC bc the 12/3 wire now is about double the price of 12/2, so why bother?

Next: Are you sure you have a MWBC feeding ONE DUPLEX OUTLET? That would be pretty unusual. The tab between the 2 hots would have had to be removed. Maybe the other half of the MWBC is feeding something else? With the power off, remove the outlet (don't disconnect any wires!) Take a couple of pics and UPDATE your question with the pics.

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    100% sure. It is, or at least was, a very standard setup here. That double breaker feeds only that outlet, with the tab on the hot side removed and each outlet connected to a different hot wire.
    – Grant
    Jun 8, 2022 at 21:44
  • OK! Sounds like you got it! Still, that's pretty unusual. So like you are proposing to remove the box and replacing it was a double gang box with 2 gfci outlets and a shared neutral should work just fine. All the GFCI cares about is "Is the current going out on the hot the exact same as what is returning on the neutral?" Like I said in my answer, I've never had any issues with GFCIs on MWBCs. If others have, please chime in. Jun 8, 2022 at 21:52

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