I have an electrical box in the ceiling, currently with all the wires capped and a blank faceplate over it. Two cables coming in, both with four conductors (black, white, red, and bare).

I don't know whether the red is meant to be a traveler for a three-way switch, or an interconnect for smoke alarms. The weird part is that this particular breaker already has both: all the smoke alarms in the house plus a hallway light controlled by two switches. (Other three-way switches in the house use a red wire as the traveler.)

Is there a good way to tell which it is? I'd like to put a smoke alarm there, but obviously I can only do that if this isn't controlled by a switch and interconnects with the other alarms.

The house was built in 2002, if that helps.

  • Do you have a voltmeter/multimeter? (Or a 9V battery snap/ability to get one, for that matter) Sep 3, 2017 at 22:59
  • Yes, I have a multimeter. Sep 3, 2017 at 23:28
  • Do your smoke detectors have a backup battery in addition to being hardwired? Sep 3, 2017 at 23:37
  • Yeah, I have multiple hardwired / backup battery detectors. Sep 3, 2017 at 23:43

2 Answers 2


I was able to come up with two approaches

  1. This requires a 9V battery, a wire-leads 9V battery snap (like one you'd find in an old transistor radio or such), and your smoke alarms to have battery backup.

First, turn off the breaker. Then, take a 9V battery snap and wire it red on snap to red wire and black on snap to white wire, then snap a 9V battery in. If it's the interconnect wire, your smoke detectors should start going off like crazy as soon as you snap the battery in. If it's a traveller, or your smoke alarms don't have a battery backup, nothing will happen. Just remember to disconnect the battery snap before turning the breaker back on!

  1. This requires a multimeter and that your smoke alarms have a TEST button that sets off the whole shebang, or some of that "canned smoke" used for smoke alarm testing.

Have a helper trigger the detectors with either the TEST button or the canned smoke while you measure the DC voltage from red to white -- if it's the interconnect wire, you should see 9V DC there while the alarms are going off.

  • I tried the multimeter solution on a known smoke alarm box, and it worked. Turns out the mystery ceiling box is not for smoke alarms. Oh well. Thanks! Sep 4, 2017 at 20:32

If it's a traveller for a 3-way you can meter it to ground. 1 lead on red wire and the other to ground. Now go turn "on" switch #1 check meter, nothing? Now turn on switch#2 check meter...still nothing now go back to switch #1 and turn it "off" ....check meter. Still nothing good now go turn off switch #2 and finally check meter. This should cycle thru all switch positions of the 3 way and if you never got any voltage on the 3-way then you would be safe to say that it's a smoke detector box.

Another method would be turn off all power in the box and open both the hallway switchs and expose the red wires. Put your meter to continuity so it beeps when the two leads are touched, now meter between red and ground on both switchs, should be NO beep. Now go to the ceiling box and tie red wire to the bare ground. Now go and meter both the hallway switchs. Should still be open or no beep. Now open another smoke detector in the house and meter between red and ground and it should beep.

Good luck!

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