I'm going through my house (built 1940s) and replacing a bunch of receptacles with GFCIs.

Most of the receptacles that I've opened up have one black and one white: I hook them up to the hot and neutral sides, pack it back in to the box, mark it ungrounded, done.

We have a square metal junction box in the bathroom next to the sink (mounted on the wall, no less) that I want to throw a GFCI in, but I can't figure it out. Flipping each breaker on the main panel and a subpanel, I couldn't even stop power to it (?): I had to flip the main breaker to do so. When I open it up, it has:

  • two whites pigtailed to one white, connected to the upper terminal on the receptacle (neutral)

  • one red connected to the upper terminal (hot)

  • two blacks pigtailed to a stranded white, connected to the lower terminal

  • two grounds pigtailed to two more grounds, one connected to the box, one just hanging free

  • the receptacle says 15A on it, but the wires are all solid 12 gauge (except for that stranded white one going from the two blacks to the lower hot terminal)

  • the tab between the upper and lower hot terminals is intact, so both plugs are always hot (neither seem to be controlled by a switch)

I tried hooking up a 20A GFCI in the same configuration, but with the loose ground attached, but no dice. Any ideas what's going on here and how I can safely attach the GFCI? (Bonus points for actually being grounded!)

My next steps are going to be to crawl under the house to see if I can figure out where the wires are coming from/going to, I guess. Any input is much appreciated

  • You know you can protect all the outlets on a circuit with one GFCI circuit breaker. You can also get GFCI+AFCI+breaker for additional protectikn against fires started by arcing. Jul 22, 2018 at 20:08
  • I thought about pigtailing the two blacks and red together, to the stranded white, leading into the hot terminal on the GFCI, but I have no idea if that's a good idea, and I don't want to electrocute anybody.
    – ATD
    Jul 22, 2018 at 20:08
  • @Harper I've read that, but don't know how to do it. It seemed like a lower investment to just replace the receptacles
    – ATD
    Jul 22, 2018 at 20:10
  • As long as you never, never, never remove the warning tape from the LOAD terminals, fitting them to LINE only is safe, simplest, easiest and not wrong. In this particular instance, it's absolutely vital not to use the LOAD terminals, as what's going on in the box is fairly complex. It may also be miswired and dangerous. Jul 22, 2018 at 20:24
  • What is all this talk about warning tape on the load terminals? I just bought an EATON SGF20 GFCI self-test receptacle and there was no warning tape on the load terminals. It would be easy to get distracted and connect to the wrong terminals, but I suppose if one failed to connect to the line, there would be no power from the receptacle, right? Jul 22, 2018 at 23:02

1 Answer 1


This is a multi-wire branch circuit (MWBC) that has been wrecked by idiots, and made rather dangerous by their bungling.

North American houses use split-phase power. A normal MWBC takes advantage of this to bring two full circuits' worth of power on only 3 wires. Two hots share a neutral: the hots must be on opposite poles (240V between them) so the neutral only carries differential current, and can't be overloaded.

The neutral being pigtailed is a normal safety requirement of MWBCs. That's so you are able to remove a device (receptacle) without interrupting the neutral for the other half-circuit. That is still required, but it's a bit obsolete because now, both breakers must have a common shut-off.

Yes, this circuit is powered by two breakers. They should have been next to each other. Modern code requires they have common maintenance shutoff to avoid the problem you had finding the right breakers, so either a handle-tie or a 2-pole breaker. It's possible someone changed the panel and put both of them on a double-stuff breaker - that's a common blunder, and very dangerous on a MWBC.

The black supply wire is one half-circuit, the red supply wire is the other. The receptacle must have its tab broken off, because red and black are supposed to be 240V apart! Why isn't it? I don't know what happened. I suspect either

  • somebody moved the breakers so both sides are on the same pole (0V apart) and then someone changed the receptacle, and forgot to break off the tab. (Probably didn't even realize tabs exist).
  • somebody replaced the receptacle, didn't break off the tab, and got a big BLAM as they shorted the entire service. Chasing that problem, they moved the breakers around.

Either way, all the current from both breakers is being returned on the same neutral wire. The currents are not canceling each other out - they are stacking, with the 20A rated wire carrying as much as 40A. Neutrals do not have fuses. Nothing protects them except care not to do this!

What was originally intended

The original idea was to provide dual 20A circuits in the bathroom so you could run a heater and a hair dryer at the same time. They served them on the split receptacle. One pole, the black, also went off to serve other bathroom loads e.g. the light or fan.

This is a very deluxe setup, because it means not worrying about overloads. I highly recommend it for every bathroom or kitchen receptacle if you can afford the 2-pole GFCI breakers.

15A receptacles are allowed on 20A circuits **if there are two or more sockets*. If no tabs are broken, a duplex receptacle has 2 sockets. On the original setup here, a 20A receptacle would be mandatory because each half-circuit had only 1 socket.

How to fix it: Quick and dirty.

Cost: $18

Cap off the red wire, and never use it again.

Somewhere in your panel will now be a breaker that does nothing. Next time someone has the panel open, eliminate that.

Get a GFCI+receptacle combo device, or "GFCI". Attach its LINE terminals to the black and white pigtail. As always, leave the warning tape on the LOAD terminals. They are not extra general purpose terminals, they are for wizards only.

How to fix it: As the original architect intended

Cost: $93+++

Swap the current 15A receptacle for a 20A receptacle (required to meet Code as discussed). Break off the hot-side tab.

Plug a radio and a vacuum cleaner into them. Flip breakers until the radio OR vacuum cleaner are silenced, then keep flipping to find the other. Mark those breakers, on the breaker not the cover. You'll be taking the cover off and playing with live busbars.

Pull the red and black off those breakers and sanity-check to make sure they go to the same cable. Also find the accompanying neutral and pull it off the neutral bar.

Now, figure out how to rearrange space in the panel to give you two full spaces. Be careful: this house may be full of MWBCs, others may be wrong too, fix them and don't break any. This is the problem with this approach: you may keep stumbling across more problems.

The two spaces need to be adjacent. They need to be full spaces, you cannot use double-stuffs here because GFCIs are not made in double-stuff.

Now buy a 2-pole GFCI+breaker combo device, or even better a dual-mode GFCI, combo AFCI and 2-pole breaker. Expect to pay $90. On this super-breaker, land the two hots and neutral from this circuit. Take the breaker's pigtail and put it on the neutral bar.

Now you have full dual 20A capacity in the bathroom, and will never trip a breaker (except for GFCI/AFCI of course).

  • Thanks for the detailed response! I think I'm going to go with the quick and dirty for now. If I ever get up the gumption to mess with the main panel, I'll try to replace the breakers with the 2-pole. I learned a lot from your write-up: thanks again. (And of course, now that I know what it's called, I'm finding descriptions and diagrams everywhere)
    – ATD
    Jul 23, 2018 at 1:30
  • one last question (I hope): any idea why there are two white and two black coming into the pigtails? Is that common? Didn't see it in your answer or in any example diagrams elsewhere
    – ATD
    Jul 23, 2018 at 4:22
  • The second white and black carry current onward to another point-of-use. The pigtails are #1 on neutral because Code requires this for MWBCs, and #2 on hot because in the original configuration, there was only 1 screw available. Jul 23, 2018 at 4:35
  • Based on how the house is arranged, I don't know what outlet would be downstream of this one, but I'll keep an eye out on my campaign. Thanks again
    – ATD
    Jul 23, 2018 at 4:39

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