NEC code and Smoke Detectors
Also, do not place them closer than roughly 12" of a wall/ceiling corner. Smoke will "roll" the corners and not get to a smoke if it is 3" in the corner.
To clarify, this is NOT in the NEC. Smokes are covered in the building codes. Actual placement and requirements are dictated there. Some want a dedicated circuit, some want them on a lighting circuit, some don't care.
Sometimes I feel like if I answer any more questions it is like someone trying to climb over a fence to jump off a bridge and me giving them a boost.