I would like to replace a regular plug with a GFCI plug. The current plug has:

  • Ground (green in picture)
  • White
  • Black
  • Red (connected on the same side than the Black with broken tap)

The breaker is "double" (see picture)

To the best of my knowledge, there is no switch linked to half of the outlet.

Thank you!

Original plug and breaker controlling the plug

  • Hello, and welcome to Home Improvement. Would you add a picture of the wiring on the old plug (without disconnecting the wires)? And, you should probably take our tour so you'll know you'll know the details of contributing here. Aug 8, 2020 at 19:37
  • 1
    Read our summary on the issue... #1 Is there a “tab” broken off between red and black? #2 Did you have to turn off 2 breakers to de-energize this outlet (you did turn it off, right?) #3 Were the breakers handle-tied or a double breaker? #4 Is half of the outlet controlled by a light switch? Aug 8, 2020 at 19:39
  • Thank you very much for your suggestions. I have added a picture and further detals explaining all questions by Harper - Reinstate Moninca
    – collifu
    Aug 8, 2020 at 22:42
  • Where in the house is this outlet located? Aug 9, 2020 at 0:55
  • The outlet is located in the garage
    – collifu
    Aug 9, 2020 at 2:19

3 Answers 3


Multi Wire Branch Circuit

You have an MWBC. It provides 2x the power with 1.5x the wires (3 wires instead of 4). The catch is:

  • If wired to a typical double-receptacle, you need to remove the tab for it to work properly
  • Neutral (white) must be wired in a way that removing a device from the circuit won't affect other devices
  • Must have a double-breaker or handle-tied breaker

Your old setup has all of that. What it doesn't have is GFCI protection. While with an ordinary single circuit, you can install GFCI at the point of use (a combination GFCI/receptacle) or in the panel, with an MWBC the usual solution is in the panel. You do actually have an alternative, but it is more work and only makes sense if you also want to have more receptacles:

  • Replace this receptacle with a combination GFCI/receptacle. Do not remove the tab. Connect black to hot, white to a pigtail to neutral.
  • Add a new combination GFCI/receptacle in another location (could be another box right next to this one or could be on the other side of the garage, whatever you want). Do not remove the tab. Run a new cable 14/2 (or 12/2 is OK) and connect the existing red wire to the black wire of the new cable and connect the white wire of the new cable to the old white/pigtail (i.e., don't chain it through the receptacle). Then wire up the new receptacle standard black/hot, white/neutral.

That will essentially split the MWBC into two separate circuits, each of which can handle a full 15A load. If you want to use two large tools in one place and nothing else, then the existing receptacles (plus GFCI in the panel for safety) makes sense. But having 4 receptacles (2 on each of 2 circuits) can be very useful.

  • Thanks! Very clear!
    – collifu
    Aug 10, 2020 at 11:22

If the tab is broken is one outlet switched and the other hot all the time ? This is the most common connection but we need to know to help. Also changing this if only one is switched may provide unwanted switched down stream outlets as gfci’s don’t have the switched non switched capabilities.

The best way for this circuit to be protected with ground fault protection is to add a GFCI breaker not a GFCI receptacle in any case.

  • Hi Ed, thanks. Absolutely, adding a GFCI breaker would be ideal. To the best of my knowledge, there is no switch controlling half of the outlet, but... I can be wrong (I bought the house some years ago and this is the first time that I would like to plug something into this plug!) - I think that I will remove the red wire and if in the future I notice a switch that it does not do anything... I will know where the issue was!
    – collifu
    Aug 8, 2020 at 22:45
  • Someone may have replaced the receptacle wire for wire and not have broken the tab on the hot side so the receptacle was always hot. Your photo shows the tab gone between the black and red your update shows a handle tied so a multi wire branch circuit, 2 seperate 15 amp circuits.
    – Ed Beal
    Aug 9, 2020 at 1:11
  • What can be the purpose of having 2 circuits linked to the same outlet? I assume that the previous owners of the house thought of breaking that outlet into 2 independent plugs, one controlled with a switch, which would be independent of the other plug? Perhaps there is a good logical reason to do that, but I cannot see it.
    – collifu
    Aug 9, 2020 at 2:23
  • It can be a multi wire branch circuit this allows 2 circuits at 1 duplex receptacle, before GFCI requirements I used to wire kitchens this way. That way with the toaster running on the top plug the waffle iron could be going on the bottom receptacle, if they were on the same circuit “trip goes the breaker”.
    – Ed Beal
    Aug 9, 2020 at 2:49
  • Followup data confirms this is an MWBC. Aug 9, 2020 at 18:03

As mana' discusses, this is an MWBC, and the red and black wires are 240V apart. Now...

Do you anticipate needing to power any 240V tools?

If so, the best option is a GFCI breaker back at the main panel.

At this point you're free to wire any combination of common NEMA 5-15 or 240V NEMA 6-15 receps off this circuit (it is ONE circuit).

Otherwise the simplest way is to split the MWBC so each half of it serves a separate 2-socket receptacle, as mana' discusses above. Then you can put a normal GFCI recep on the place of each receptacle.

Sorry to suddenly force you to deep-dive into MWBC arcana when all you wanted to do is slap up a GFCI... but GFCI issues notwithstanding, MWBCs are awesome in garages and workshops. They bring a lot of power (saw AND dust collector) and they can support 240V.

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