On the first floor, there is the main breaker box. There are two 100 Amp breakers in it. One feeds the downstairs heat pump system and the two outside units.
The second 100 Amp breaker feeds a sub-panel upstairs in the attic. This sub-panel feeds the lighting in the attic, a few receptacles, and the propane furnace in the attic.
I had someone watch the on light on the smoke detector. I turned off each breaker on the main box, and the smoke detector never lost power. I left it off for a couple of minutes, and checked to see if there was a backup battery. No backup battery at all. The only time the light went off was when the main breaker was turned off.
I have not pulled the front cover to see if the smoke detector was pulled off the main circuit. I have removed it because I'm redoing the ceiling drywall. The wires are either #12 or #14, and they measure 120 volts AC.