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?