Wiring a switch to control a bathroom exhaust fan


Old 03-19-05, 02:14 PM
Visiting Guest
Posts: n/a
Wiring a switch to control a bathroom exhaust fan


I'm adding a bathroom exhaust fan, and am looking to steal power from the existing bathroom vanity light switch. I understand that I need to cut out the single size box and install a double size box.

My question comes from the wiring, as I was not expecting to find what I saw when I removed the cover plate.

In the existing box are three romex cables coming in, 1 cable through the left top tab, two coming through the right top tab.

All 3 whites are pigtailed together.
The black wire from the left romex is pushed in to the bottom of the existing switch.
The other two black wires go to the screw and the push-in location respectively on the top side of the existing switch.
Does this mean that this switch is in a middle of a run? In other words, are the two romex cables in the right hand tab 'in' and 'out', while the romex on the left hand side runs up to the vanity light?

Clearly, I'll need to run new romex up to the exhaust fan, but how do I tap into the existing light switch?

Same deal? pig tail in the 4th white wire? I'm not clear on how to bridge power over to the top side of the new switch. Just run a little black wire? Doesn't seem quite right.

All advice welcomed.

Thanks in advance.
Sponsored Links
Old 03-19-05, 02:47 PM
Join Date: Sep 2000
Location: United States
Posts: 18,497
Yes, this is all very standard. The switch is indeed in the middle of a run as you surmised. And your guess as to the purpose of all three cables is correct.

Here's what you need to do. I will go with your plan to change the single-gang box to a double-gang box, although another alternative would be to use a duplex switch in the single-gang box.
  • Shut off the breaker.
  • Mark all three cables so that you know where they went.
  • Remove the old box (using a hack saw or saws-all).
  • Enlarge the hole for a double-gang box.
  • Feed the original three cables into an old-work double-gang box.
  • Run a new cable from the hole to the location of your new fan.
  • Feed this fourth cable into the box too.
  • Mount the double-gang box to the wall.
  • Connect all four white wires with a wire nut and push it to the back of the box.
  • Use a big wire nut to connect all four bare wires to two bare pigtails (short segments of bare wire). Connect the other end of each pigtail to the green screw on each switch.
  • Use a wire nut to connect the two black wires that were connected to the top of the old switch to two black pigtails. Run one to a screw on the two switches.
  • Connect the other two black wires, one to the remaining screw on each switch.
  • Repack the box carefully.
  • Wire the new fan according to manufacturer's instructions.
  • Turn on the breaker and test.
Old 03-19-05, 03:57 PM
Visiting Guest
Posts: n/a
Excellent, thank you for the detailed description. Fully understood.

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Thread Tools
Search this Thread
Display Modes