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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.

  • Does this switch control only the fan right now, or is it controlling other things as well? – ThreePhaseEel Dec 16 '18 at 16:42
  • It only controls the fan. – MinnesotaDIY Dec 17 '18 at 23:29
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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.

  • If you downgraded the circuit to 15A so you can use #14 wire, you would then have to revisit your compliance with Code, which requires at least one 20A receptacle circuit in a bathroom. – Harper - Reinstate Monica Dec 16 '18 at 17:15
  • @Harper I was thinking of that, but hopefully (even if this is an old bathroom prior to the code requirement) it has at least one receptacle somewhere already. But agreed - that is yet another reason to stick with #12. – manassehkatz-Reinstate Monica Dec 16 '18 at 17:20
  • This is a great explanation. Thank you! As to side the conversation about 20A receptacles, the existing receptacle is 15A. It's a bummer but I'm not tearing apart the house to run new wire. I'll double check when I trace the circuits. @manassehkatz, appreciate the very specific fool-proof instructions. A final question: will this be a switched outlet or always-on? I assume always on because hot and neutral continue through the pigtails/wire nuts to the receptacle. – MinnesotaDIY Dec 16 '18 at 17:42
  • Just to clarify: Your NEW receptacle should be a dual 15A w/GFCI. That satisfies the 20A circuit requirement even though each receptacle is only 15A. So #12 cable for the new cable, together with all existing # 12 cable and you should be good for a 20A circuit. If it turns out the existing breaker for the fan (to be expanded to GFCI/receptacle) circuit is only 15A then you can (and should) still install everything as described, but only upgrade the breaker to 20 A if you are absolutely 100% certain that all wiring connected to the circuit is # 12. – manassehkatz-Reinstate Monica Dec 16 '18 at 17:48
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    @Harper - I suspect (but no idea if this is written into code anywhere) that part of the rationale for dual 15A receptacle on "required 20A" circuit is so you are guaranteed to be able to run one near-15-A-limit device (e.g., hair dyer) at the same time as some smaller devices (e.g., plug-in shaver, toothbrush charger, etc.) without worrying about going over 15A since the little stuff will normally be under 5A and therefore 20A for circuit + 15A max. per device works well. – manassehkatz-Reinstate Monica Dec 16 '18 at 18:05
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In the old days, there was a wiring scheme called a switch loop. Google that and you'll find a whole bunch of diagrams that are outlawed today. In an old switch loop, there's no neutral. Typicaly it's done with /2 cable, but for bathrooms and ceiling fans, it's normally done with /3 cable for separate control of fan and light. So the expected wiring was white=always-hot, black=switched-hot 1, and red=switched-hot 2. The white wire was to be marked with tape to mark it as a hot, but most people didn't bother, thinking the usage was obvious.

Today, bringing a neutral to a switch is required. You just found why. So a lot of builders are just continuing to wire with /3 (they should now be using /4) and installing it with neutral capped and without separate fan/light control, and expecting the homeowner to change it to the old way once the inspector is gone.

But you can absolutely stay with the new way and use that white wire for a neutral. Convention says use black for always-hot and red for switched hot.

You can achieve separate fan/light control with smart switches and modules.

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