Bathroom exhaust fans with built-in light fixtures are fairly common. If you have one of these, you may have wired the fan and light to separate switches. Now, you'd like to operate both of them from the same switch. The wires are already where you need them to be, so you don't have to run new ones. That detail greatly simplifies the electrical procedure you're about to undertake. If the light and fan are separate fixtures, you're going to have to run a short length of wire through the attic to connect them.
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A Word of Caution
Shut off the breaker controlling the light and fan circuit. This is the single most important part of any home wiring procedure. Even after switching off the breaker, it's still a good idea to use a voltage tester to test the wire connections before you work on them.
You'll Need to Make Pigtails
In electrical work, a pigtail is a collection of wires spliced together with a short length of wire. The pigtail allows you to connect all the wires to a single terminal. The secret to making an effective one is to twist the wires together clockwise – not counterclockwise – before screwing on the cap. That way, the clockwise threads of the cap make the connection tighter when you screw it on.
Converting From Two Switches in the Same Box to One Switch
Disconnect the two black wires from both switches. If you don't know which one supplies power to each switch, turn the power back on and test the wires by connecting each to ground with your voltage tester. The live circuit wires will give you a reading and the others won't. Turn the breaker back off when you know which is which.
Make a pigtail with the two live wires and a short length of black wire of the same gauge. Connect the short wire to the top terminal of the switch you want to use. Do the same with the two other wires, and connect them to the bottom terminal of the switch.
Unscrew the cap from the two white wires serving the switch you're no longer using, add a short length of white wire of the same gauge and screw the cap back on to make a pigtail. You may need a larger cap than the one that was on there. Splice the other end of the short wire to the two white wires serving the new switch. If you want, you can also simply splice all four white wires together under the same cap, but you may find that difficult to do with four wires.
Remove the ground from the out-of-service switch and the one from the switch you're using. Connect them by twisting them together or using a crimp ring, then attach them to the green ground terminal on the switch you're using.
Connecting a Light and Fan to a New Switch
Locate the switch cable, which should be sticking through the top or side of the box serving the fan or light. Using wire strippers, strip an inch of insulation from the end of the black and white wires. Splice the black wire from the fan together with the black wire from the light and the black wire from the switch, and cap them. Connect the white wires together in the same way, then repeat with the ground wires. When you splice ground wires, you don't have to cap them, but it's still a good idea to do so because the cap helps keep them together. Mount the fan and light and move to the switch box.
Connect the black wire from the fan to the bottom terminal of the switch and the live circuit wire to the top terminal. Splice the white wires together and cap them. Twist the ground wires together and connect them to the green ground screw on the switch.
Chris Deziel is a contractor, builder and general fix-it pro who has been active in the construction trades for 40 years. He has degrees in science and humanities and years of teaching experience. An avid craftsman and musician, Deziel began writing on home improvement topics in 2010. He worked as an expert consultant with eHow Now and Pro Referral -- a Home Depot site. A DIYer by nature, Deziel regularly shares tips and tricks for a better home and garden at Hunker and Family Handyman.