I was trying to wire in a timer switch for a vent fan. When I went to replace the switch (which I knew was a 3 pole) I was baffled by the way it was wire. 4 wires run into the box, two of the hot wires go straight to a switch and the other are marred with 2 other black wires that run back to the neutrals on the switches. All 4 white wire are marred. Hopefully these pictures below help make more sense, the switch on the left is a light and the switch on the right if for the fan that I want to put a timer on but as it's a 3 way switch, I'm not really certain how to go about determine which wires are hot/common/traveler etc)

enter image description here


  • 1
    With the exception of illuminated switches, there is generally no neutral on a switch. Are both switches 3-way? (BTW, 3-way and 3-pole are not the same thing - a 3 pole switch would have 6 or 9 contacts in 3 separate circuits)
    – Ecnerwal
    Commented Dec 17, 2013 at 22:52
  • 3
    I don't see anything that looks like a 3 way switch, can you show the backs of the switches to show where the wires connect to them? I see what looks like a hot coming into the box and back out to the next outlet/switch in the circuit (so that's 2 of the black wires coming into the box). Then 2 switched hots that go to the light and fan (that's the other 2 black wires). Then all 4 neutrals are wire-nutted together. But it's hard to be certain without seeing the wires connected to the switces. Are you sure it's a 3 way switch? Can you turn the fan on/off from another switch?
    – Johnny
    Commented Dec 17, 2013 at 22:53
  • I can't see anything... Can you please add the images directly to the post? Using an external source for images is a pain in the neck, and requires the other source to last as long as this question does.
    – Tester101
    Commented Dec 18, 2013 at 13:07
  • Where is the other switch that controls the fan, and what would you like it to do? I actually just did this exact conversion in my house. The other switch is downstairs near the thermostat, and the fan is in an upstairs bathroom in the middle of the house (so I presume it's meant to act as a "whole-house air exchange"). I wired the switch in parallel so the timer works normally, but the switch downstairs overrides it and it stays on as long as the switch is on. Some timers do have the ability to be put in 3-way configurations with other timer switches, however.
    – gregmac
    Commented Dec 19, 2013 at 3:04

3 Answers 3


I've annotated your image...

Annotated image

It's not possible from the image to tell which of the wires marked with red is the feeder from the breaker, and which is the feeder to other devices on the circuit. However, since you're simply replacing a switch with a timer, you'll not have to touch these wires anyway. The neutrals can also be ignored, unless you're installing a "smart" switch that requires a neutral wire.

Without knowing the exact make and model of the timer being installed, it's not possible to provide exact installation instructions. Instead I'll provide basic "typical" installation steps.

  1. Start by turning off the power to this circuit at the breaker/fuse box, and verifying the power is off using a non-contact voltage tester.
  2. Remove the two black wires from the switch you'll be replacing (One marked orange, and one marked blue in the image above).
  3. Throw the now disconnected switch haphazardly over your left shoulder.
  4. Connect a bare or green wire from the grounding screw in the box to the green screw or wire labeled Ground on the device.
  5. Connect the black wire coming from the twist-on wire connector (labeled orange in the image above) to the black screw or wire labeled Common on the device.
  6. Connect the other black wire to the brass screw or wire labeled Load on the device.
  7. (Optionally) If a neutral is required, follow steps 7a - 7c
    a. Remove the twist-on wire connectors connecting the neutral wires.
    b. Include a short bit of white wire in the bundle, and reinstall the twist-on wire connector (make sure the connector is rated for the proper number of conductors, a new twist-on wire connector may be required).
    c. Connect the other end of the short bit of white wire to the silver screw or wire labeled Neutral on the device.
  8. Restore power at the breaker/fuse box.
  9. Turn the fan on/off a few times and marvel at how awesome you are. YOU have harnessed ELECTRICITY!!!

Here's some wiring diagrams from various timers on the market...

enter image description hereenter image description hereenter image description hereenter image description here

  • Lol I'll stop answering in DIY, you guys are too awesomely detailed. How do you find the time?
    – Roberto
    Commented Dec 18, 2013 at 20:48
  • And the guy doesn't even upvote you???
    – Roberto
    Commented Dec 19, 2013 at 11:12

As others commented, it's a bit hard to tell without looking at the back of the switches, but trying to picture what you describe:

"two of the hot wires go straight to a switch"

One per switch, right? In my understanding these would be the wires that actually go from the switch to either the lightbulb or the fan, so they should only become live when the switch is on.

"and the other are marred with 2 other black wires that run back to the neutrals
on the switches."

You have 4 black wires here:
* One of the two coming from the wall is the live wire that comes from the breaker and brings power to everything.
* Another of the four goes straight back to the wall, presumably to power other switches (connected to the same breaker of course).
* Each one of the remaining two black wires should go to the middle of one of the two switches, to power it.

So if I got you right, you will want to identify the two black wires connected to the fan switch - one of the 4 marred ones is the one that powers the switch, and the other goes from the switch straight to the wall to power the fan.

This seems a pretty simple setting, so please be extra wary in messing there if you found it confusing. I mean, know your abilities... No offense meant!


the blacks that are all tied together are your hots coming from the breaker. one goes to each switch, one from the breaker, then one going out to the next location on the circuit. Same with the whites, all the neutrals connected together. the blacks on each switch that are seperate, go out to the actual devices, light and fan. for the new switch, black to the hot (the group connected together), red to the black going to the fan, and white with the other whites (will replace the old white from old switch). then grounds all get connected together.

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