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I'm remodeling a place built in the 80s and found that the dishwasher and disposal are on a multi wire branch circuit - i.e. live wires come from separate breakers but shared neutral.

AFAIK in California to bring this up to code i only need to gfci the dishwasher (although if they have already adopted newer nec's maybe its disposal now too + afci - ill need to talk to my inspector).

In any case, this is what it looks like right now: enter image description here

I was originally hoping I could just throw a GFCI in place of the existing outlet at the box above the counter there - but seeing as how the neutrals are shared I don't think this will work.

At the breaker, the two circuits are on separate breakers one right on top of the other but oddly enough there is no bar/linkage piece to ensure that they flip together - perhaps they forgot it?

So what are my options for doing a GFCI correctly for the dishwasher? So far all I can come up with is:

a) install a double pole gfci breaker (very expensive, prefer not)

b) run another wire from the counter box to the box below the sink to allow me to do something like the below (running another wire would be really difficult in this situation):

enter image description here

c) install another box below the sink to give seperate boxes for dishwasher & disposal and then install GFCI on the dishwasher one (maybe disposal too?) - I am not actually sure if this would work, but if it did it would be the easiest/cheapest of these options: enter image description here

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  • Is the connection between the two boxes made using a cable or individual wires in a conduit?
    – ThreePhaseEel
    Jul 17, 2021 at 4:20
  • @ThreePhaseEel it's a 3+gnd Romex cable (black/white/red + bare copper gnd)
    – user67081
    Jul 17, 2021 at 4:34
  • Is your existing undercounter receptacle a split duplex?
    – NoSparksPlease
    Jul 17, 2021 at 5:19
  • @NoSparksPlease correct, I was trying to convey that in the drawing, the disposal is switched and goes to the top socket in the outlet and the always on plug below it is for the dishwasher
    – user67081
    Jul 17, 2021 at 5:20
  • Is replacing the under-sink box with a two-gang box an option?
    – ThreePhaseEel
    Jul 17, 2021 at 11:48

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Option C would certainly work, there is no problem sharing the neutral on the line side of the GFCI devices.

It can be a bit tricky adding a box if the existing box is installed recessed in the wall. One easy option is an 4s extension box with a single device in the back of the box like a Raco 187. Then use a raised steel cover with two devices like a Raco 809C. enter image description here

About the missing handle ties on the MWBC, changes that require handle ties and identification of wires are a relatively recent change, in the 80's breakers weren't even required to be adjacent.

GFCI protection is not specified for the disposal in Section 422.5(A) in the 2020 NEC like the dishwasher is, but it is not uncommon for inspectors to call it out as within 6' of the sink.

You should check the amperage rating on the dishwasher, the NEC only allows fastened in place appliances to use a maximum of 50% of a branch circuit when shared with receptacles.

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  • The two breakers for the MWBC are 15A each. I haven't bought a new dishwasher yet but it seems they might use up to 1800W (15A). A disposal looks to be around ~7A. Are you saying it would be 50% of 15+15=30A so 15A max allowed for the dishwasher?
    – user67081
    Jul 17, 2021 at 15:01
  • Note that if all the appliances are fastened-in-place, the 50% limit doesn't matter even if some of the fastened-in-place appliances are plugged into receptacles instead of being hardwired. However, it's not clear whether a garbage disposal is "fastenend in place" or not for the NEC's purposes...
    – ThreePhaseEel
    Jul 17, 2021 at 16:53
  • No, the two 15A circuits are calculated separately, you never add breaker handles to get amperage. The limit would be 50% of 15, but you opened a new can of worms. I didn't save all my old Code books but the way the 210.52 has read for many cycles is that all receptacle circuits in the kitchen are required to be 20A except for dedicated circuits. So that receptacle above counter doesn't seem to be in compliance at all. Your best hope is there are two other required 20A circuits that are laid out to satisfy minimum spacing requirements and the 15A was scabbed in and can be eliminated.
    – NoSparksPlease
    Jul 18, 2021 at 14:43

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