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Trying to replace an old outlet in my kitchen with a GFCI outlet. The outlet has two lines in the outlet box. the first line has black, red, white, and ground while the other line has white, black, and ground. The red from the first line is spliced to the black from the other line. See pictures:

outlet box

outlet box

How do I actually wire this setup to the back of the new GFCI outlet? I first connected the black and the white to the line in and connected the solitary white to load and it kept tripping the breaker on the plug.

So I disconnected the solitary white and the plug works but a different plug in the kitchen then does not work. I'm not sure what the red/black splice is doing. I know a red can mean a switch is involved but there is only one switch nearby and it controls the garbage disposal and is on a completely different breaker. Any help would be greatly appreciated.

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I've joked that if I were king, GFCI receptacles would have 4 screws on them, all "LINE", and the "LOAD" terminals would be concealed so novices did not use them by mistake. Things would work so much better! Someone replied "well they're almost that good now, GFCIs have warning tape on the Load terminals!" I got a chuckle out of that, because - well, you know. The warning tape deters no one.

Anyway, I just answered your question.

But yes, as crip659 says, this is a MWBC, and the black and red splice should be pushed into the back of the box and ignored.

However, use of the receptacle to splice neutral is not allowed with MWBCs, because if the other half of the circuit is still on, that neutral has power moving on it! You must be able to remove devices without interrupting nuetral. The neutral must be pigtailed with another wire nut or WAGO, so it is 1 wire to the receptacle. As an aside, this will wrap up the "GFCI problem".

Also, back at the panel, the 2 breakers are required to be handle-tied so a maintainer is forced to turn them off together. The requirement for an approved handle-tie helps with another MWBC rule: 2 hot wires must be on different poles/phases (so neutral is not overloaded). You need to see 208V or 240V between the breaker outputs or it's wrong.

Oh, and I'm mentioning this at the end for reasons which are now obvious, but FYI, GFCI terminals can support 2 wires each. How do do that is a closely held secret, hidden in the GFCI's instructions :) :)

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  • Legit question: will activity downstream (let's call it breaker 2) create an imbalance on the neutral that the gfci on breaker 1 will recognize and trip? Or does having the ground in place take care of that?
    – Aloysius Defenestrate
    Apr 15 at 22:26
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    @Aloy Ground is not a player in GFCIs. They don't attach to ground at all. (GFCI receptacles do, but only to connect ground to the socket ground pins, the GFCI mechanism doesn't use it.) But yes, the GFCI is examing Hot1 and Neutral current to assure they are equal. Current on the Hot2-Neutral loop will imbalance that and trip it. That's why that was never going to work. They do make 3-pole hot-neutral-hot GFCIs but only as circuit breakers.
    – Harper - Reinstate Monica
    Apr 15 at 22:29
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That is a multi wire branch circuit(MWBC). It uses two breakers with two hots and a shared neutral(white). The red will go to the next outlet. Hopefully the two breakers will together and have a handle tie if they not a dual breaker. Some of the early circuits used two separated breakers and was a shocking experience.

GFCI outlets/receptacles do not like MWBC circuits.

I do think you can use a GFCI double breaker for those circuits, but not the outlets.

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  • Thanks for the answer and it totally makes sense! The next outlet over is definitely connected to what is going on here. The two breakers do not have a handle tie unfortunately. So this means I will just use a common outlet for these two boxes?
    – Justin H
    Apr 15 at 22:20
  • @JustinH It will depend on the age of that circuit, since must have GFCIs is only a few decades old(90s), and if closer than 6 feet to water(sink). A GFCI double breaker will cure the two problems, having protection and no handle tie.
    – crip659
    Apr 15 at 22:31
  • Definitely the original breaker and outlets from 1974. I guess a call to an electrician is in my future. Thanks for your help.
    – Justin H
    Apr 16 at 1:10

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