When my buddy (alarm tech) wired my house, all the hardwired smokes & heats were run with a (red) fire-rated cable of some sort - no bigger than 18AWG max - for both 24vdc power and signal loop connection. (He stated it was fire-resistant, which automatically made me think of plenum rated cable... don't know if it's the same or not) According to him, it's more of a UL requirement for the individual installation than an NEC item.
