I have a ceiling fan connected to a two way switch. When I turn the ceiling fan/light on, the power to the hard wired smoke detector is cut off, whereas turning the fan/light off returns power to the smoke detector. I am stumped. Any ideas?

Thank you in advance for your responses!


  • Did you install the fan, or was it there when you moved in?
    – Tester101
    Aug 16, 2015 at 12:23
  • Have you removed the switch plate, or the fan's cover to see if there are any additional wires, pigtails, etc that might indicate how this is wired? If so, do you have photos? Sep 14, 2015 at 20:03

1 Answer 1


Updated solution per comment:

The fire alarm should not be connected to a switch at all. It's likely that it is connected to the same switch as the light but with the wires reversed. The solution would be to remove the fire alarm black & white wires from the switch and splice them in the box to the cable that feeds the switch, but before it "hits" the switch.

Old answer saved for posterity... i.e. it may help someone else:

There's so many ways to get into this situation and it's impossible to guess which one you're in but: Take a look at this site: https://www.homeimprovementweb.com/information/how-to/three-way-switch.htm It contains diagrams for all the different ways of wiring a light with two switches. Something you will notice about the diagrams is that there is no other load (device that consumes electricity) in the diagram.

In your situation a separate load, the fire alarm, was introduced into the two-switch system and it should not be. In each diagram you will see a section that says something like From power source. This is where the electricity is coming in and this is where your fire alarm should be spliced in. i.e. The fire alarm should be connecting to the power supply before the two-switch system.

Now, how to fix this? If you're lucky the electrical box that feeds the fire alarm will have two separate three-wire cables running through it and it just happens to be spliced into the wrong one. In this case you just need to re-splice. But it's also possible that there is no 2nd wire in which case you will need to splice into whichever light-switch box (or possibly the fan box) and run a three-wire cable to your fire alarm. Yes, this could be just as painful as it sounds. If you have drywall up you may need to get into the wall to thread the cable between the boxes and then have to repair and paint...

In any case, good luck.

P.S. It's also possible that the power supply is coming into this system from the fan's electrical box. If this is the case then your worst case (having to bust through the drywall) becomes a little easier.

Wiring diagram with fire alarm cable

Here's a sample diagram (modified from option #1 of the web site given above) of where a separate cable may be spliced in so it doesn't behave the way you're describing.

  • Hi Paul, thank you for your reply. My issue is that it is only a one-switch system with one single on/off light switch. Turn the light on, smoke detector turns off, turn the light off, smoke detector comes back on. I am working with a two wire system.
    – Alex
    Aug 16, 2015 at 15:10
  • D'oh! That's even a weirder problem actually! The fire alarm should not be connected to a switch at all. Have you taken the light switch apart to see what's going on in the box behind it? Your situation and resolution may actually be exactly what I posted with the diagram, if you ignore the second three-way switch.
    – Paul Sasik
    Aug 16, 2015 at 15:15
  • Also, see my revised answer added to the top of the OP above. It's basically an extension of my comment.
    – Paul Sasik
    Aug 16, 2015 at 15:20
  • Thank you for the revision. I can see that the switch interferes with the alarm in some way, so I will wire it before the switch.
    – Alex
    Aug 17, 2015 at 18:57

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.