Frequently Asked Questions about Wiring a Bathroom Exhaust Fan and Light
Adding an Exhaust Fan to a Bathroom
I currently have a single pole switch in a bathroom that works a light fixture. The power source goes to the light first and then to the single pole switch so the white is actually the hot. This is the only wire to this box. I have added a bathroom exhaust fan and replaced the single gang box with a double gang box in order to add another single pole switch in the same box.
I would like to use the power from the original light switch to run the fan independently from the light. Both the light and the fan are running on romex with 2 wires plus the ground. I cannot seem to wire the box correctly to make both switches work independently. Can you tell me how to do this?
Lets Begin By Focusing on the Power Source
You have identified that the power source which is at the existing light fixture which will be extended to the new exhaust fan.
At the existing light fixture junction box, make sure to identify the existing cable that leads to the light switch and either wrap black electrical tape on the white wire or color part of the white wire with a black felt pen. This is to clearly show that the existing cable to the switch is being use for the light switch only so there will not be any confusion about the wires of that cable.
Power Source to the New Exhaust Fan
A new cable will need to be installed between the existing light and the new exhaust fan.
Switch for the Exhaust Fan
From the new exhaust fan a cable will need to be installed from the exhaust fan to the double switch box.
Make sure to identify the new cable that leads from the exhaust fan to the double switch box and either wrap black electrical tape on the white wire or color part of the white wire of the cable with a black felt pen.
Now that the white wires that are used for the switches of the light and the exhaust fan have been identified with black, lets concentrate on the white wires that are the actual neutral wires.
White Neutral Wire Connections
The incoming white / neutral wire from the power source will splice with the white / neutral wire of the light fixture and the white neutral wire of the new cable that extends the power source to the new exhaust fan. The white / neutral wire of the extended power cable at the exhaust fan splices with the white / neutral wire of the exhaust fan.
Exhaust Fan Switch Wiring Connections
Keep in mind that a switch is just a loop of the hot leg of the power source which is accomplished by using a 2-wire cable as you already have with the existing switch for the light fixture.
With this in mind, the black of the power source to the exhaust fan splices with the white wire (which has been colored black to show that it is being used for the switch).
The black of the cable being used for the new exhaust fan switch splices with the black lead wire for the new exhaust fan.
Connections to the New Exhaust Fan Switch
Single pole switches typically have two side screw terminals and or push in connections on the back of the switches, the side terminals are typically a better connection than the push in connections.
It does not matter which wire of the pair used for the switch is attached at the terminals because the switch is not polarity sensitive, the switch is simply a contact.
The ground wires of the cables should be bonded to the junction boxes and switch boxes if these boxes are metal and or have a provision for a ground wire attachment which is typically identified by a green screw. A ground wire should also be attached to the green ground screw which may be found on the frame of the switches.
A Basic Summary for Installing this Bathroom Exhaust Fan
Basically you will have the same wiring configuration as the existing light and switch, except there will be the added cable between the light fixture and the exhaust fan, so the key is to identify each cable and what the individual wires of each cable are used for.
The circuit should be positively identified and the circuit should be shut OFF before any wiring is performed.
The wire gauge or wire size of the additional cables must match the existing wire size.
The installing of additional electrical wiring may require a permit and inspections.
Please consult your local building department for further information.
New Bath Exhaust Fan with Timer Switch and a Light Switch
Becky, from Beloit, Wisconsin asks:
Hi Dave. I'm installing a new bath exhaust fan with 1 timer switch and a light switch. I have power coming to the switch box but nothing going to the fan/light. Do I use a 3 wire? I need a full explanation of this-my brain doesn't work well. How to hook up light switch and how to hook up timer; then how the wires hook up at fan.
You are absolutely correct about the 3-wire cable, lets review the process:
So, your source power source, (hot, neutral and ground) is located at the switch box where you will have two switches, one for the light and the timer for the fan.
Next, the 3-wire cable is installed from the switch box up to the exhaust fan.
The 3-wire cable needs to be the same wire gauge or size as the power source cable, which is either #12 or #14.
Note: The 3-wire cable is actually 3-insulated wires, black, red, white and a ground wire.
At the Switch Box
The power source white / neutral wire will splice with the white / neutral of the 3-wire cable leading to the fan.
If the timer switch that you have requires a neutral wire then it too will be spliced with the other neutral wires mentioned above.
The black / hot of the power source will be shared for both of the switches, so because you have two separate switches then two separate pieces of black wire will splice to the power source black wire, however if the time switch has lead wires then the LINE (typically the black) wire from the time switch will connect with one additional black wire that will share the black wire of the power source for the time switch and the light switch.
The ground wires all splice together, and an additional ground wire should be added to connect to the light switch, and the time switch unless the time switch has a green ground lead wire.
Lets make the Black wire of the 3-wire cable for the Light of the exhaust fan, so the black of the 3-wire cable wire will attach to the single light switch.
Lastly, we have the Red wire of the 3-wire cable that will be used for the Exhaust Fan, so the red wire of the 3-wire cable will attach to the timer switch or splice with the red wire of the time switch (or other color wire as described in the instruction sheet that came with the time switch).
* Refer to the instruction sheet that came with the time switch for specific wire color assignments if the switch has lead wires.
Connections at the Exhaust Fan Junction or Splice Box
There should be a green ground wire already provided for the ground wire of the 3-wire cable, or there will be a green ground screw where you will attach the ground wire of the 3-wire cable
There will be either one or two white wires that are used for the light and the fan, and these white wires splice to the white wire of the 3-wire cable.
The lead wire for the Light splices to the black wire of the 3-wire cable.
The lead wire for the Exhaust Fan splices with the red wire of the 3-wire cable.
* Refer to the instruction sheet that came with the exhaust fan for specific wire color assignments.
Make sure to positively identify the circuit and turn the circuit OFF before performing any work.
Basics of Installing a Bathroom Exhaust Fan
I just had a quick question about installing a bathroom fan. I currently have one light switch with a single outlet on it i want to install a bathroom fan. I just need to know how to go about running the electric for it and what kind of outlet I need.
The existing box is typically replaced by a two-gang box to accommodate a switch for the bathroom fan. A cable is installed from the switch box to the location of the new bathroom fan. The exact method and materials will depend on several factors of the home and the existing circuit cable.
Updated: October 17, 2019 By: Dave Rongey
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