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I am replacing a bathroom vent, light, heater combo.
The wiring from the house has three switches, but has only two lines with three wires (black, white and ground). The new vent, light and heater has 2 wires (colored and white) each function and ground.

How do you wire this configuration?

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    Can yo provide pictures of the existing box and the fixture wiring? – bib Aug 31 '14 at 2:51
  • The model number or a picture of the wiring diagram of the new unit may be helpful as well. – TomG Sep 1 '14 at 11:51
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Not counting the ground wires, you have four insulated conductors. One could be the supply hot, each of the others could be switched hot leads. The presumption would be that the neutral is in the ceiling box and not present in the wall box. Totally possible. If this is true, please mark the switched hot leads appropriately.

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One switch originally controlled the light and vent, and the other originally controlled the heater? Or did only one switch control everything? Clearly you don't have three separate switched wires and a neutral and a ground. It's barely possible that the ground was used as a neutral before, and the white was used as a switch leg, but that's a seriously bad wiring job.

You'd better open up the switch boxes and decide what's inside there. with the wiring you have, you can only safely run all three (light, vent, heater) controlled by a single switch. That means you need to find that same white wire back at the switch box and make sure that it's connected to a neutral instead of being a switched hot.

After -- and ONLY after you get the switch box wiring straightened out, you'll go back to the new light/vent/heater and connect the ground to ground, all the whites together, and all the blacks together.

  • Not counting the ground wires, you have four insulated conductors. – Jimmy Fix-it Aug 31 '14 at 13:40
  • @JimmyFix-it - I think I see what you're saying; you & I read "two lines with three wires" as meaning two different things. You're right - if that's two different pieces of NM-B with black/white/copper each, then there are four usable conductors. – TDHofstetter Aug 31 '14 at 13:55
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It is hard to figure out what you are saying, but I assume you mean there are two 14/2 or 12/2 cables coming to your existing vent fan.

The way it was probably wired before was one of the white wires is the neutral, and the rest of them were all switched hot wires.

If you have already taken the wiring loose you are going to have to figure out which cable is which.

If you haven't taken the wiring loose already, and you can't figure out what wire is what, call someone else to do it for you.

  1. First make sure the breaker is off by testing at the switches and at the vent fan. If you are using a non-contact tester make sure to test that it works both before and after you use it to ensure the breaker is off. Go ahead and put a piece of tape over the breaker handle to indicate it shouldn't be turned back on.
    If you do this part incorrectly you will have a dangerous situation.

  2. Short out the leads of one of the cables at the vent fan. If you didn't do the first part correctly you will have a spark.

  3. Use a multimeter with a continuity feature to find out which cable is which at the switch box with all of them off. ( If any of them are on you may get a false positive )

  4. Mark the neutral that goes to one of the switches with electrical tape at both ends. ( any color but green yellow or white, usually black or red ) Now it should be easy to figure out what wire goes to what switch, and which one is the neutral.


After you know which wire is what, go ahead and wire it up.

If you can't figure out which wire is which, get someone else to do it that knows what they are doing.

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