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Duplex outlet with 2 hot blacks, one hot white, a neutral and a ground. Oh yeah, connected to two breakers too. Can anyone help me unravel this mess?

I was planning to switch out this kitchen outlet to a GFCI when I found this mess. Two hot blacks, and two whites (one of them hot) spliced and then connected to the neutral side. I'm not sure what to do, but I'd like to have a GFCI on this outlet.

Upon testing with a multimeter, I got the following readings:
Ground to Black 1: 121.5
Ground to Black 2: 115
Ground to Hot White (which it was spliced to): 119.6
Ground to Neutral: 0

Black 1 to Black 2: 236
Black 1 to Hot White: 240
Black 2 to Hot White: 2.8

When I turn off breaker 2, and leave breaker 1 on, I get the following readings:
Black 1 to Ground: 0
Black 2 to Ground: 115
Hot White to Ground: 119

When I turn off breaker 1 and leave breaker 2 on, I get the following readings:
Black 1 to Ground: 121
Black 2 to Ground: 0
Hot White to Ground: 0

In addition to all this, when I had the wires disconnected from the duplex, all the other outlets in the kitchen (with the exception of one) had a reading of "HOT/GRD REV" on my outlet tester. When the mystery outlet is wired as it was before, all the same outlets read CORRECT.

I'd love to get a GFCI outlet installed, and to also know that my house is correctly and safely wired, especially as we have an infant.

Does anyone know what the heck is going on with this outlet?

Here are some photos:

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https://photos.google.com/u/1/share/AF1QipN3m5nqBURWY4oN9h7K5CfYLqIIR7ZYJ9MdddSmTmQtP23KN95ig_qnJNaj8L8nBA?pli=1&key=X2FTblhZbzJWRW5XR2JTenJEc09VUU0zMkEtTFB3

enter image description here

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  • Yeah, you've got a mess there! can you add a picture of the side of the outlet. When you do, edit your post, the use the "sun and mountains" icon to insert a link to to the image (or upload it directly from your computer/phone). That way the image can be embedded in the post instead of an external link - makes everyone's lives easier & SE takes care of the hosting for you.
    – FreeMan
    Jun 18, 2020 at 18:33
  • On the old receptacle.... look closely at the two brass screws the hot wires were on. You see the metal behind them. You see how they're sort of separated. Is there a little metal tab connecting them? (the silver side will have that tab intact, for comparison.)
    – Harper - Reinstate Monica
    Jun 18, 2020 at 18:34
  • Is the tab on the outlet between the two hot (brass) screws still intact? What about the one on the neutral side? (Just like Harper suggested.) It looks like they wanted one half serviced by one circuit and the other outlet by another circuit. But I can't explain the hot/ground reverse error.
    – Duston
    Jun 18, 2020 at 19:21
  • Sorry - I should have mentioned that before. Both sides still have the tabs intact, so the outlets are NOT wired independently.
    – Pato
    Jun 19, 2020 at 20:25

1 Answer 1

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It's a Multi-Wire Branch Circuit (MWBC)

Which is a thing that is new to you, and you'll probably need to do some learning about it. To cut to the chase: To hook up a GFCI outlet here,

  • note the warning tape on the GFCI that says "Do Not Use. For Wizards Only." Do not remove that tape. In fact it will be important that you follow that rule anywhere on this or any MWBC.
  • Most GFCI receps have a mechanism for accepting 2 wires on the LINE screws. Both black wires go on LINE brass. The white pigtail (after you put all that back together) goes on the LINE silver. Don't even think about using the "2 wires under a screw" feature to attach the 2 white wires and skip the pigtail. That's a safety issue!
  • Ground wire of course.
  • Button it up and you're done.

More on this "Multi-wire branch circuit"

A MWBC is a setup with 2 hots wires sharing a neutral. The neutral carries only differential current, which can never be larger than the max current on a hot wire. Thus the circuit breaker is able to protect the neutral. Certain rules must be followed for safety:

  • The neutral wire, where shared, must be pigtailed. That way a device can be removed without severing the neutral wire for the other half of the circuit. As you saw, when you do that, things got wackadoodle.
  • The two breakers MUST be phased opposite, so there is 240V between them. That means breakers must be very carefully placed, especially if double-stuff breakers are involved.
  • The two breakers need to be handle-tied so that turning them off together is compulsory. You can buy special handle-ties for those breakers, but a far better plan, especially for a novice, is to use a 2-pole breaker. This has a factory built-in handle-tie, and better yet, it mechanically enforces the correct opposite phasing.
  • Because neutral is shared, the LOAD terminals can never be used on GFCI receps. (and novices should not be using them anyway unless they know exactly what they do, and I don't mean "give me a place to land these other 2 wires" :)

You know how many GFCI receps you have to install, and how much they cost. If you use a GFCI 2-pole breaker, ka-zam! you GFCI protect the entire circuit (both sides) in one step. Not only is this safe, it can also be more economical if you have 4+ receps to replace, depending on the obsolescence of your panel.

Those measurements

The measurements you took were 100% consistent with a MWBC in which an appliance on the other circuit had been switched on. The red wire is 240V away from the black; imagine there's a resistor between red and "Hot White" (really downline white). With zero current flow (since you interrupted it), the "hot white" is being pulled up to the red wire's voltage.

I suspect this circuit also has a 240V load somewhere on it; that would act like a resistor connecting red and black. That explains why supply-side black was 240V away from downline black. Given that a 240V load is present, you cannot use a handle-tied breaker, and must use a 2-pole breaker. Although a GFCI 2-pole will be just fine.

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  • Got it, mostly... I'll add the GFCI per those instructions, and I will keep the pigtail with only one neutral going to the LINE side. For any downstream outlets, would they have GFCI protection under this setup? I was under the impression that downstream outlets were only protected if connected via the LOAD connectors.
    – Pato
    Jun 19, 2020 at 20:44
  • Also, regarding the breaker - I'm confused by your comments as they seem to contradict regarding tying the breakers. Should I or should I not tie them if I am using a GFCI at the outlet? They are two 20amp breakers.
    – Pato
    Jun 19, 2020 at 20:46
  • Lastly, is a 15amp GFCI adequate for this outlet, or should I use a 20amp GFCI? Same goes for another "washer" outlet (in the laundry room) that is wired to a 20amp circuit with a second outlet downstream that I also want to install a GFCI on.
    – Pato
    Jun 19, 2020 at 20:48
  • @Pato -- yes, you need to tie the breakers (get a Siemens/Murray handle-tie, part #ECQTH3), and a 15A GFCI will do (as it's a duplex receptacle, not a single)
    – ThreePhaseEel
    Jun 19, 2020 at 23:02
  • 1
    Got it. GFCI installed, will pick up the handle tie. All the other kitchen outlets already have GFCI duplexes installed, this was the only one that didn’t. If not, I probably would have opted for GFCI breakers for the kitchen circuit instead of a bunch of outlets. All is working well, thanks much!
    – Pato
    Jun 21, 2020 at 5:56

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