replacing double switch (with two circuits) with single pole on/off and dimmer switch


Hello. Please see included photos as you read my details.

I currently have a double switch in a bathroom. Two different circuits power this switch- one for the lights and one for the fan. In the switch box, excluding the ground wires, there are two white wires feeding one side of the double switch. On the other side of the switch, two black wires are feeding that side. The bottom white wire is the incoming hot wire for the light and the top white wire is the incoming hot wire for the fan. The bottom black wire is powered when the bottom (light) switch is activated. When the top switch (fan) was activated, neither top wire (black or white) showed voltage on my meter. I do not know if that is normal or not but I got no read from either top screw when the fan was running.

Anyway, I am replacing that double switch with a single pole on/off switch (for the fan) and dimmer switch (for the lights). This new switch has three screws- a black screw for live, a switch screw, and a dimmer screw. It also has a ground wire attached.

On the new switch, the “dimmer” and “switch” screws are outgoing to the actual light and fan, right?

The live white wires coming in from the breaker box should connect to the black “live” screw, right? If I twisted both white wires together, wouldn’t that give power to both circuits, even when one is off?

Any help would be appreciated with this project. I have replaced light switches and outlets before but I am really questioning this one.

Thank you,





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teammazza 6 months 3 Answers 303 views 0

Answers ( 3 )

  1. See without being there this is really hard to figure out. One wrong move and bamm. Knowing what’s what is very important.

    Know this power in the switch, and power out to whatever. Sounds simple because it is. So just bring power to the switch and the other switched side to what is being powered.

    So in other words if I just walked in that’s how I would handle it. I would test what’s what and start wiring it up.

  2. Hi Dominick.

    As a person who attempts to learn things as they need done, and build a base of knowledge from those experiences, bear with me as I attempt to explain.

    I rechecked the wires. I used the ACV 200 setting on my multimeter and used the metal box as a ground. To recap, the left side of the current switch has two white wires (I will call them top-white and bottom-white). The right side of the switch has two black wires (top-black and bottom-black).

    When all circuits were on and switches were off, I had 123.6 reading on both white wires and 0 reading on both black wires. That tells me that, since the actual switches were off, that the whites are live. Right?

    When I turned the bottom switch (the light switch) on and kept the top switch (fan) off, I had the same readings as before in the white wires. Top-black still also had zero, but bottom-black read 123. That tells me that, since the bottom light switch was on, that the bottom-black is going from the switch to the light fixture. Right?

    This next part, I did a bit more carefully because of my results in my original post. When I turned the top switch (the fan switch) on and kept the bottom (light) switch off, the bottom-white and bottom-black did what I expected. Bottom-left still had 123 and bottom-right had 0. These were what I thought were the lights so should not be affected by turning the fan switch on. Now, the top-white read 0.9 and the top-black read 0.5. I do not know what that means and would appreciate any type of explanation.

    I did turn both circuits off at different times and it all matched above. When fan circuit was off, no power to top wires. When light circuit was off, no power to bottom wires.

    So is it confirmed that top wires are fan (and also the top switch) and bottom wires are light (and also bottom switch). And that the two whites (separate feeds) are coming straight from the circuit box and the two black are going to the fixtures?



  3. The question I have; how sure are you that those white wires are power: normally the white wires are the neutrals. Test them against ground and see what you get please.

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