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I have a single switch that turns on both the bathroom light and a separate exhaust fan at the same time. I want to change this to be able to power them on independently via a dual rocker switch or two switches. I have read that I may have run a new wire for the exhaust fan. I'm a bit confused on the wiring and need clarification on which wires are which. I have three wires crimped together with a wire nut which one wire seems to be going to one of the terminals. Would this be possible without running a separate wire for the exhaust fan?

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  • Can you provide a zoomed-in photo of the back of the box's interior, with the box in the same state as your prior photo? – ThreePhaseEel Nov 19 '15 at 4:22
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It is possible that the two black wires (other than that going to the switch) are going separately to the fan and light. If so, the change is easy.

To test this do the following:

  1. turn off the breaker
  2. separate the three black wires nutted together
  3. wirenut the black to the switch with one of the others and insulate the other in a separate wirenut
  4. turn the breaker back on test the switch. Hopefully, the switch will operate only one of the fan or light.
  5. turn the breaker back off
  6. switch the black wires so the other one is connected to the wire to the switch
  7. turn the breaker back on and test the switch. Hopefully, it will only control the other of the light or fan
  8. Turn the breaker back off and optionally restore the original wiring

If the test results followed what I marked "hopefully", you are in business. Remove the other wire to the switch and wirenut it to two wires to run to one side of the dual switch (if one side of the switch has a removable bridge, just connect the single wire to that side). Connect the two blacks originally wirenutted together to the other side of the switches (if that side has a removable bridge, remove it).

For the pigtail wires, you should use the same guage wire as the rest. Examine the wires for printing on the insulation (look for an AWG number).

I'd also recommend against using the back push-in connectors. It's better to use the screw terminals.

  • I have to say, it is highly, highly unlikely that the two separate cables go to the fan and light separately. The likely scenario is that the two blacks and both constant hots and are the feed. – Speedy Petey Nov 19 '15 at 11:54
  • it depends on the age of the house I have found this plus a ceiling heat element in 1 switch, the switch failed so the owner put them on 1 switch. years later wanted them seperated but forgot what he had done, crazy to have a heat element at your exhaust fan but that was a popular fixture in the late 50s early 60s – Ed Beal Nov 19 '15 at 14:05
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There's no way to know for sure what the wires are for, without actually testing to find out. The bundle of black wires could be power fed to the box (likely), or independent wires to the fan/light (not as likely). There are a few ways to figure out what's going on.

Using a non-contact voltage tester

This method is fairly safe, as you're not working with exposed live electrical parts.

  1. Put the twist-on wire connector back on, and separate the bundle of black wires from the switch as much as possible.
  2. Turn the breaker on.
  3. Hold the non-contact voltage tester near the bundle of black wires.
  4. Toggle the switch.

If the voltage detector senses voltage no matter which way the switch is flipped, you're out of luck and will have to run an additional wire.

Using a multimeter/voltmeter

This way requires working on exposed live electrical wires, so don't attempt it if you're not comfortable with that.

  1. Pull the grounding wires out of the box, enough so that you can get a probe on them.
  2. Put the twist-on wire connector back on the black bundle.
  3. Turn the breaker on.
  4. Stick one probe from the meter into the twist-on wire connector, so that it makes contact with the exposed wires and/or the interior metal cap.
  5. Touch the other probe to the grounding conductor.
  6. Toggle the switch.

If you measure line voltage no matter which way the switch is flipped, you're out of luck and will have to run an additional wire.

Eliminating wires

  1. Remove one of the wires from the black bundle (not the one going to the switch).
  2. Put the twist-on wire connector back on the bundle, and put one on the black wire that was removed from the bundle.
  3. Turn on the breaker.
  4. Toggle the switch

If nothing happens when the switch is in either position, the isolated wire is the feed to the box. If both the light and fan come on when the switch is toggled, the isolated wire is probably feeding through to another circuit. In both these cases, you'll have to run an additional wire.

If only the light or fan turns on when the switch is toggled, then you're in luck. Connect the wire that's on the switch (not the one leading to the bundle), to the common terminal of the new double switch. Connect each of the wires from the bundle (excluding the one to the switch), to separate load terminals on the new switch. Install the switch and coverplate, and enjoy.

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