Old capped wires that weren’t connected to anything.

We moved in to a new home and the prior homeowner capped the old smoke alarm wires and replaced the old wired alarm with a battery unit that wasn’t wired up at all.

The nest smoke alarm has a 120V connection with a black and white wire that I can splice on to this. Before I do anything like that I wanted to ask.

I notice the old wires have two extra snipped wires (black and red) that are disconnected and not capped. It has a green wire spliced into a red wire that’s capped and goes nowhere. And it has a yellow wire similarly spliced.

This may have been a ceiling fan connection or it may have been a smoke detector hookup but I have no idea.

Advice appreciated!

  • 2
    That's "telephone" wire not electrical. It isn't robust enough for powering a fan.
    – gnicko
    Feb 18, 2022 at 0:34
  • Why is telephone wire in the ceiling like this by the smoke alarm? I guess is that enough for 120v? I'm definitely confused about this wiring and how the nest is supposed to hook up to it if at all.
    – M2tM
    Feb 18, 2022 at 0:46
  • 3
    No. Not good for 120V. The previous smoke alarm must have been a low voltage (12V DC) or similar. You can't use those wires for a Nest which, I believe, needs 120 VAC.
    – gnicko
    Feb 18, 2022 at 1:06
  • Can you trace those existing wires to where they go? Feb 18, 2022 at 5:03
  • 1
    Those may have been for an interconnect between alarms, too. If one goes off it signals the rest and they all go off, waking up the whole house.
    – FreeMan
    Feb 18, 2022 at 12:59

1 Answer 1


You can't use those wires. They're likely left over from a 12V DC alarm system. The Nest smoke detectors require 120V AC which those wires are not capable of supplying. You'll need to figure out a different power source.

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