dig if you will the picture of one of my smoke detectors and the curious (for me) wiring. Actually, I've taken many pictures, but each is just a mess and I think will make it harder. We have ~7 detectors hard-wired in the house, no battery backup that I'm aware of (15 years old and surely dead anyway).

Behind the first detector I'm replacing are two romex wires each with black,white,red,ground... and then one black,white,ground romex wire... so we have three hot black wires coming into this single smoke detector that were all twisted together by color as you'd expect.

So I separated the black wires to test voltage independently and:

If I turn off a certain 15A breaker, then the black/white wire loses power

If I turn off a certain 20A breaker, then both of the black/white/red wires lose power.

I have to turn off both breakers if I want to work safely, but I don't understand and fear some GFCI circuit issue (the 20A is poorly labeled Kitchen GFCI and indeed turning it off impacts both the wires I mentioned and a GFCI outlet in the kitchen). Most curious to me is that I have untwisted and separated all black wires and the 20A breaker is supplying power to both of the BW/Red wires independently.

Is it normal / OK? I think maybe this 2001 wiring job is no longer a recommended configuration, but do you think I should call an electrician to dig more?

Thank you

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    It seems your picture didn't make it into the post... – ThreePhaseEel Dec 14 '16 at 12:46
  • ... now I have Prince running through my head... – Daniel Griscom Dec 14 '16 at 12:58
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    The red wire is usually used as a signal wire, so that the smoke alarms can signal each other if one alarm activates. What were you using to test for power? A multimeter, or a non-contact tester? – Connor Bredin Dec 14 '16 at 14:44
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    How are you testing the voltage? non contact testers may indicate voltage present when there is really no potential there because the wires being run in parallel the live wire induces voltage onto the wire not energized but there is only a "phantom voltage" or field there enough to trick the tester. What size are the wires are they both 12awg or 1, 12 & 14 ? – Ed Beal Dec 14 '16 at 14:44
  • ^ What Ed said. From what you posted, it sounds like the two conductor cable is supplying power to the smoke alarms, and the 3 wire cables branch off to the alarms with power and signal. – Connor Bredin Dec 14 '16 at 14:48

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