Last Updated on May 14, 2023 by Dave Farquhar
Sometimes you need to wire a bathroom fan and light on one switch. Code often requires a fan in bathrooms these days, but you may only have one switch. Or maybe you have two switches but just want one switch to run both. Fortunately it’s easy to do. Here’s how.
Wiring a bathroom fan and light to one switch
This kind of arrangement with a bathroom fan and light wired to one switch is a good idea anyway. It’s safe to assume if someone is in the bathroom, the light is on. And running the fan is the best way to keep mold from growing in the bathroom. Condensation from hot water causes mold, and can also reduce light fixture and bulb life expectancy. But the water won’t condense if the fan pulls the vapor out of the bathroom before it gets a chance.
Running the fan and lights on the same switch can solve a number of problems and prevent future issues, so you may want to do it even if you have two switches available in the bathroom. No one likes changing light bulbs prematurely, and mold can cause serious health issues. This simple trick is a good way to prevent both. And it also lets you add a fan to a bathroom that otherwise couldn’t have one, which is good because code requires them now.
Wire a bathroom fan and light on one switch when you have two switches
If you already have lights and a fan on two switches, simply label the wires off each switch, remove them from the switches, and remove the light switches. Tie the equivalent wires together with wire nuts while adding a new length of wire long enough to reach the switch you want active. This new wire is called a pigtail. Get a new 20-amp light switch, unless one of the existing switches is already 20 amps. The load of wiring a bathroom light and fan on one switch needs more amperage than the standard 15-amp switch. It’s not worth risking an electrical fire to save five bucks.
Run the pigtails to the new 20-amp switch, then remove the old switch for the fan. You might want to put an outlet there in its place. Otherwise, put a blanking plate where the old switch was.
Wire a bathroom fan and light on one switch when you have one
What if you don’t have a fan in the bathroom yet?
Your light fixture has black and white wires on it. So does your fan. Mount the fan close to the light fixture, then run the black and white wires from the fan over to the electrical box that feeds the light fixture. Shut off the power to the bathroom from the electrical panel. Then remove the wire nut on the bundle of black wires and tie the fan’s black wire into that. Replace the wire nut. Do the same on the white wires.
For safety’s sake, replace the light switch as well. Get a 20-amp switch, which shouldn’t cost much more than $5, instead of the 75-cent 15 amp switch you probably have. You don’t want the light switch to overheat under the load and potentially cause an electrical fire. Using energy-efficient bulbs in the bathroom will also reduce the load on the switch. Replacing the switch can solve other problems too.
The benefit of running the fan when the light is on
In one of the great ironies of landlording, I once had a tenant complain about a small amount of water in the basement after a record-setting rain. To my knowledge the basement had never flooded before. She convinced herself the basement would instantly become moldy. When I went over to show her how to run the dehumidifier to dry out the basement, I found mold in the upstairs bathroom.
Mold in upstairs bathrooms typically happens from taking prolonged hot showers without running the fan. That’s easy enough to prevent. Either take shorter showers, or run the fan while doing so. Another benefit is the mirror doesn’t fog up as badly.
This tenant had an irrational fear of mold where there was none, and didn’t recognize mold where she did. Running the fan all the time would have fixed the problem she didn’t recognize, even if she wouldn’t have appreciated it. I ended up rewiring the light and fan to a single switch to prevent a recurrence.
Further reading
While you’re renovating the bathroom, consider putting in a new fixture and LED bulbs. You can increase the life expectancy of your bulbs that way, while saving energy.
David Farquhar is a computer security professional, entrepreneur, and author. He started his career as a part-time computer technician in 1994, worked his way up to system administrator by 1997, and has specialized in vulnerability management since 2013. He invests in real estate on the side and his hobbies include O gauge trains, baseball cards, and retro computers and video games. A University of Missouri graduate, he holds CISSP and Security+ certifications. He lives in St. Louis with his family.
Dave, I have and existing continuously running motion sensor fan with I believe dedicated power, and a separate ceiling light with a double switch to also run the vanity light. My question is I bought a fan that I’d like to run with the regular bathroom light, can I tap into the light in the ceiling and just cap off the other power supply?
Sure, I’ve seen people do things like that before. Maybe when you cap off the other supply, write what it was for on a piece of tape and put it on the wire in case you ever want to set it back up the way it was.