Add a Humidity Switch for a Bathroom Exhaust Fan

What You'll Need
Electrical tape
Philips screwdriver
Straight screwdriver
Drywall saw
An upgrade/existing work junction box. Also called a cut-in box.
2 feet of 14-2 romex
3 wire nuts

Money’s tight right now and running that bathroom exhaust fan all the time every time you turn on the bathroom light is a waste. A sure way to save money on your electric bill is by installing some sort of secondary switch that allows that exhaust fan to stay off when it’s not really needed. This article will describe the steps required to hook up most humidity detecting switches for bathroom exhaust fans.


Follow these steps to install your new humidity switch.

Step 1 - Ceiling Opening

Cut an opening in the ceiling just big enough to fit the cut-in box. Average size is 1.75 inches across by 3.5 inches up and down. Leave room for the switch. The sides of the opening should be about 1 inch from the fan cover.

Step 2 - Mounting

Open 2 of the wire entry points on the back of the box. Mount the box in the ceiling right next to the exhaust fan. Climb into the ceiling and locate the fan you’re working on. Bring the romex wire with you.

Step 3 - Power Supply

Open the cover on the junction box supplying power to the fan. Disconnect the supply wire from the load/fan wires. Route supply wires into your newly mounted box. Route the romex you bought between the 2 boxes.

Step 4 - Reconnect

Reconnect the fan wires to the romex you just routed. Replace the cover.

Step 5 - Connection

Insert the other end of the romex into your new box. White goes to white. Black goes to black. Green goes to green. Exit the ceiling, bringing all tools with you. Place the ladder under the fan and the new box. Make connections between existing supply wires and supply side of humidity switch. White wire to white or silver terminal. Black wire to black or brass terminal. Green wire to green wire and green terminal, if present. Make output side of switch connections. White to white or silver. Black to black or brass.

    When making wiring connections, some switches will utilize screw terminals for making power and load connections, while others will use wire nuts. Still others will utilize a combination of the 2.

    Step 6 - Finishing Up

    Mounting the humidity switch right next to the exhaust fan isn’t as easy as mounting it next to the switch, but it will be more effective, since heat and humidity will rise in a bathroom. By the time the humidity and heat reaches the switch on the wall, the humidity level in the bathroom will be outside of the limits of the switch. And you won’t have to turn off the power to the circuit.