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Some terminology, because I don't know any better:

light-fixture  - An existing, separate, light fixture...currently with dedicated 14/2
vent-unit      - A new vent just installed, has three functions:
    vent-fan        - The fan component of the vent-unit
    vent-light      - The main light component of the vent-unit
    vent-nightlight - The nightlight component of the vent-unit

I just replaced an old bathroom vent with a new one that has a light, nightlight, and fan. The bathroom also has a separate light fixture. There are two switches to control them (one for the light-fixture, one for the vent-unit), and this means there are two 14/2 cables going up into the attic (one to the light-fixture, one to the vent-unit).

The electrical diagram for the vent-unit I bought indicates that the vent-light and the vent-nightlight are tied to the same white, internally. There is a separate white for the vent-fan. The easiest way to get the vent-unit hooked up would be to tie all the blacks (one each for vent-light, vent-nightlight, and vent-fan) to the black of the incoming 14/2, and both whites (one goes to the vent-fan, and one shared between the vent-light and vent-nightlight) to the incoming white. This means that the vent-unit switch would control all three functions of the vent.

What I WANT is for one switch to control the light-fixture, vent-light, and vent-fan...and for the other switch to control the vent-nightlight. I specifically want the vent-fan to come on anytime a real light is turned on, and I specifically want the vent-nightlight to be controlled separately (since I will usually want it on regardless).

This means one switch would control lights/fan, and one switch would control vent-nightlight.

What this boils down to

My first reaction is to run the "nightlight" switch black to the vent-nightlight black, but the "lights/fan" switch white to the vent-nightlight white (since the vent-nightlight white is shared, internally, with the vent-light). Would this work? Would the fire department become unwittingly involved several hours later?

EDIT

Another idea, is it safe to run the whites from BOTH switches to one termination? Could I run the "nightlight" switch black to the vent-nightlight black, the "nightlight" switch white to the vent-nightlight, AND the "lights/fan" switch white to the vent-nightlight white?

EDIT 2

Here's an ugly picture of the situation. What I WANT is for one switch to control the vent-nightlight, and the other switch to control everything else.

The weird box thing at the bottom of the pictures is supposed to be a two-gang light switch.

Wires and Switches

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How many wires of each color are on the new fixture, is it only 1 black and 1 white? –  Tester101 May 21 '12 at 17:09
    
The old light fixture is staying where it is (it is separate from the vent). The new vent unit has two whites (one is shared between the light and nightlight, one goes to the vent. The new vent unit has three blacks (one each for the light, nightlight, and vent). –  dolphy May 21 '12 at 18:20
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1 Answer

up vote 4 down vote accepted

You either have to replace the Vent wire with 14-3 AWG wire, or run a second 14-2 wire from the existing light to the vent. Assuming both switch circuits are from the same feed, you only need 3 wires to the vent. All the whites get tied to the white neutral of the feed. You could use the black (or Red) to go to the light - fan, and the Red (or black) to go to the night light.

This quick schem shows the connection with adding one 14-2 cable from the existing light to the Vent.

M-L-N can be switched around, depending on which functions go with the shared switch. I assumed the night light gets its own switch.

enter image description here

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I'm fine with running a second 14/2 from the light fixture to the vent. I think that implies that the whites from both switches will be tied to the shared white going to the ventlight/nightlight, while the ventfan white is only tied to the light fixture white (via this new 14/2 you suggest). Is this OK? –  dolphy May 21 '12 at 20:31
    
I added to my answer. The whites all get connected together as long as there is only one power source in the switch box (probably is). You will be bringing the white from the existing light over in the 14-2 cable anyway. So yes you can use that white to go to the fan-light circuit. –  SteveR May 21 '12 at 20:37
1  
+1 for low-tech drawing. –  Tester101 May 22 '12 at 12:32
    
Excellent, thanks! –  dolphy May 22 '12 at 14:47
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