I'm trying to install a bathroom fan on it's own switch.

There is a hot cable with a black, white, and green from a previous light fixture in the spot that I want to install a fan. This hot cable is separate from the existing light fixture and switch combination. I understand that is somewhat concerning but that is a project for another day.

I have also ran a 12 AWG Romex cable (Black/White/Copper) to the existing single gang box for a switch for this new bathroom fan.

Two questions:

  • How do I marry the new bathroom fan, existing hot cable and new romex cable + switch?

  • What is the usage of the bare copper wire connected to the metal gang box of the existing switch? My house was built in the 1950s and none of the outlets are grounded. Is this a fake ground?

Diagram of my current situation below.

Wiring diagram

  • What does the circuit including the "Metal Gang Box" have to do with your project? Oct 23, 2020 at 1:32
  • Do you already have a junction box at the location "Wanted: Switch to control fan"? Oct 23, 2020 at 1:33
  • I'm afraid your new cable should be 12/3-AWG instead of 12/2-AWG. New switch installations require constant-hot and neutral to be present in the junction box, in addition to any wires being switched. Oct 23, 2020 at 1:36
  • @A.I.Breveleri sorry for the confusion. The circuit with the "Metal Gang Box" is where the second switch is going to live. I'm going to expand the electrical box to be a 2 gang box. Since it had ground attached to the box I assumed it was relevant. I would also like to understand what that ground attached to that box is for since all of the outlets in my house are ungrounded
    – byte
    Oct 23, 2020 at 1:39

1 Answer 1


You need a 12/3 (black/red/white/ground) cable between the fan and the switch location.

You can no longer use a "switch loop" which is how that "used to be done" with a 12/2 cable, because current codes require a neutral at the switch location.

Black (hot) to the switch, red (switched hot) from the switch to the fan.

enter image description here

  • ...And white (neutral) connected to neutral at the fan just capped off with a wire nut at the switch for now. Oct 23, 2020 at 1:43
  • The other note to answer the question is that the ground, fake or otherwise is not meant to be fake, it is supposed to tie back to the panel and even to the main ground of the home. The ground is essentially "another 'ideal' neutral" with high potential that current will flow from the hot to the GROUND directly instead of 100% through you (should the real intended neutral become ineffective)
    – noybman
    Oct 23, 2020 at 2:29
  • Did I understand your answer? I made a diagram: i.imgur.com/2uiRHmJ.png
    – byte
    Oct 23, 2020 at 2:31
  • That looks like you understand correctly. I'll just make this community wiki and paste that in.
    – Ecnerwal
    Oct 23, 2020 at 2:50

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.