Electrical – add a receptacle from a bath fan switch box

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I hope this is simple. I want to add a GFCI receptacle on the wall of my bathroom behind a medicine cabinet. The closest electrical is a switch for my bathroom fan. Even though we have just one switch, it seems it was wired for separate control, because we have 12/3 wire coming into the switch box. Neutral is capped and red/black are attached to the switch.

Can I tap into this for the GFCI receptacle? I had trouble finding a matching diagram even though I know this is a common configuration (12/2 into fan then 12/3 to switch). How does the circuit work right now before I make changes? I will likely retrofit the bath fan soon and would like to have separate control.

Best Answer

Assuming your fan switch is wired normally, it has the following wires:

  • Hot (black) - this is coming in and should be connected to power in the fan box (but not connected to the fan)
  • White (neutral) - this is coming in and is not currently used - but you will use it shortly.
  • Red (switched hot) - this is going out to the fan.

Trust, but Verify

You should first verify that black and red are wired correctly. We can be fairly certain that white is indeed neutral because it is capped off, which is normal for a modern basic switch installation.

  • Turn off the breaker.
  • Open up the switch and disconnect (and pull apart for safety) the wires
  • Turn on the breaker and check that you have 120V (nominal) on the black wire. You can check with a non-contact tester (should light up next to the black wire but not next to the red or white wire) and with a multimeter (should show 120V between black and white and 0V between red and white). If black and red are swapped then you can still do this installation but it will be more confusing.

Install the new Cable

The new wire should be 12/2. You may only need 15A but since you (a) are installing a dual receptacle so you CAN install it on a 20A circuit and (b) if you changed to 14/2 you would have to replace the 20A breaker that is likely in place (since all the existing wiring is 12), stick with 12 so you don't have to change the breaker.

These instructions assume the black wire is hot.

In the switch box:

  • Turn off the breaker.
  • Wire nut together the original black wire, the new black wire, and a short piece of black wire (aka pigtail).
  • Wire nut together the original white wire and the new white wire.
  • Wire nut together the original ground (maybe green, probably bare) wire, the new ground and a pigtail (green or bare)
  • Connect the switch to the black pigtail, the ground pigtail and the red wire.

Run the new cable to the new box. In the new box:

  • Connect black to the Line hot screw
  • Connect white to the Line neutral screw
  • Connect ground to the ground screw

Do not use the Load screws on the GFCI receptacle at this time.

Then turn on the breaker and make sure the switch/fan and new receptacles all work.

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