How to install a GFCI?

Question

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!

Answer 1

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.

Answer 2

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.

Answer 3

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.