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Electrical Wiring for a Bath Light Fan Combo

How to Identify Electrical Wiring for Installing a Bath Fan Combo with a Light: A Common Wiring Problem When Installing a Bathroom Exhaust Fan and How Fix It.


Identifying Electrical Wiring for a Bath Fan Combo

Electrical Question: I have a junction box in an old house that was used to power a bathroom light.

  • I would like to swap the light for a light/fan combo.
  • The junction box of the original light has two Romex cables coming into each side.
  • The wiring is very weird. Three of the Romex black wires are connected in a wire nut, the forth goes to the light. The four whites are wired together and go to the light. The grounds are also wired together.
  • I copied this to the rest of the circuit and it works fine, but the fan and light doesn’t work.

This electrical question came from: John, a Homeowner from Denver, CO.

Dave’s Reply:
Thanks for your electrical question John.

How to Identify Electrical Wiring for a Bath Fan Combo with a Light

Application: Wiring a Bathroom Exhaust Fan
Skill Level: Beginner to Intermediate, best installed by a licensed electrical contractor.
Tools Required: Basic Electricians Pouch Hand Tools, sheet rock saw, stud locating sensor, a non-conductive ladder, drop cloth and a Voltage Tester.
Estimated Time: Depends on personal level experience, ability to work with tools and the access to the attic area above the bathroom for the new exhaust fan.
Precaution: If there is an existing circuit then it should be identified, turned OFF and Tagged with a Note before working with the wiring.
Notice: Modifying existing wiring or installing additional bathroom electrical wiring should be done according to local and national electrical codes with a permit and be inspected.

A Common Wiring Problem When Installing a Bathroom Exhaust Fan and How Fix It

  • John, you may have to use a voltage tester to identify the wiring.
    • What you are describing tells me that:
      1. You may have wired into a switched leg that leads down to the wall switch and returns back to the ceiling junction box location.
      2. A Switched leg wiring configuration does not have a neutral wire, but actually uses the white wire to switch the power  leg only.
      3. This can be fixed by making the proper connections at the junction box with the new cable for the combo exhaust fan.
      4. At the junction box, locate the neutral wires and connect to that group the new neutral wire, and then locate the the switched leg that returns from the wall switch for the power and connect the black wire for the power, then connect this cable to the bath combo exhaust fan.

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4 Responses to “Electrical Wiring for a Bath Light Fan Combo”
  1. Randy says:

    I am wiring a bathroom exhaust fan combo. There is a power line that goes to the light fixture, and there are 2 sets of wires from the wall switch. I think its a switch loop where the white wires connect to the black wire of old light fixture. There isn’t a 3-wire cable from the switch, just 2-wire cables each with black, white and a ground wire for each. How would I wire this?

    • Dave Rongey says:

      Hi Randy,
      There are two ways to wire a bath exhaust fan combo with the wiring layout you have described. You either extend the power from the light fixture to the exhaust fan location, or extend the power to the wall switch. Either way you will ultimately need to run at least one cable to the wall switch for the combo exhaust fan functions which will be controlled by new switches. The power source will need to be at the combo bath fan, or the wall switch. My personal preference is to run the power to the wall switch location, and enlarge the switch box because you will have to do this anyway for the new switches for the exhaust fan unit. Also keep in mind that the junction box in the bath fan combo unit is typically small with just enough space for making up the lead wire connections with the wires from the switches, so if the power was been extended up into the ceiling then you will most likely have to install an accessible junction box just for the power source, then from there a 3-wire cable to the wall switch, and another 3-wire cable to the combo unit. The actual number of cables and wires for the combo unit switches will depend on the actual combo unit you have because some will require only 2 new switches, and some will require 3 or more switches.
      Access into the ceiling area or attic will be a factor with all of this of course – it just depends on the extent of the project.
      I hope this helps,
      Dave

  2. DBallard says:

    WHY WON’T MY GU24 LIGHT BULBS WORK? I AM USING THE EXISTING ON OFF SWITCH FROM THE BATH ROOMS OLD EXHAUST FAN THAT HAD NO LIGHTS. I CONNECTED THE WHITE WIRE FROM THE SWITCH TO ALL THE WHITE WIRES ON THE NEW FAN UNIT. I HOOKED THE BLACK WIRE FROM THE SWITCH TO THE BLUE,RED AND BLACK WIRES ON THE NEW FAN UNIT. THE FAN WORKS AND THE NIGHT LITE WORKS BUT NOT THE 2 18 WATT GU24′S. THE NEW EXHAUST FAN/LIGHT/NIGHT LIGHT UNIT IS A NUTONE QTN 110 LE. HELP PLEASE.

    • Dave Rongey says:

      Hi D Ballard,
      Based on what you have stated, I’m going to suspect that the problem is not with the GU24 Lamps, but possibly the wiring.
      I would double check all the wiring and connections for the light circuit. The wiring may need to be positively identified with a voltage tester. Understand that if a “white wire” is attached to a switch, it is most likely NOT a White Neutral wire. It is common practice to use a “white wire” as a switched leg of power, and the “white wire” may not have been identified as such, by using a black marker or black piece of tape.
      Dave

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