Electrical – Red wire in ceiling box: traveler or smoke alarm interconnect

electricalsmoke-detectors

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.

Best Answer

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.

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