Replacing old smoke detectors - yellow interconnect is connected to a white wire

Question

Replacing smoke detectors in our home and found that on the old Firex G-18 detectors that were connected, the yellow interconnect wire was hooked up to a separate white wire coming out of the electrical box. Can someone please explain what is going on here? I have to replace the harness as it's not compatible with the alarms I purchased so I have to figure out what the purpose of this single white wire is. Is it potentially the interconnect? Thank you.

Answer 1

Nearly all hardwire detectors will have the same three wire interface. There should be a hot wire (black), a neutral (white), and a third wire (red or yellow) that is the interconnect. Interconnecting, btw, makes all of them alarm when one detects fire/smoke. The interconnect is a single wire, and relies on all of the interconnected units being on the same electrical circuit (breaker). Interconnecting detectors is required by code in many places, so you should definitely re-connect that when replacing.

From Firex install instructions:

Firex is a brand made by Kidde, so if your replacements are not all identical it's worth checking to see if they are electrically compatible. If that detector failed or is expired, I would consider replacing all of them since the others are likely to expire soon.

A two-conductor romex or BX cable may have been originally used out of convenience, even though only one conductor was needed. (It should be three-conductor romex with black/white/red, technically.) In that case, the black wire is likely unused and the black/white color coding has no real meaning.