Power Cable or Signal Cable

Er, does it matter?

Signal cable, usually run as power limited. Some of the older systems used power AND signal to the detectors. Most modern systems use a "signaling line circuit" whereby all detectors are on the same pair of wires, derive their operating power from them, and communicate their status using bi-directional bitstream data packets on them.

Depends on the type of detector. Many units are 24VDC and power limited so the wire is considered signal cable. AC powered detectors may be 24V, 110V or 220V. Anything above 30V must be considered power cable.

That may have been true in the past. The only AC powered detectors sold today are for "single station" use, and several of them can be connected together with some models, or Duct Detectors, which can be powered by AC or DC. All spot type smoke detectors (Photoelectric and Ionization) sold today for connecting to a control panel are of the 24 VDC standard.

Are you talking about residential or commercial? I just installed 120VAC networked smoke detectors in my residence and bought them at Home Depot a month before. They utilize a third wire which is used to network the detectors, in lieu of a fire control panel.

At my facility, however, everything is run from 24VAC and run through control panels.

Yes, the residential detectors are 120 VAC, are considered "single station" devices, and can be interconnected with an additional wire. They almost always come with a 9 volt back-up battery. They actually operate on 9 volts DC all the time (even though it seems to the homeowner like they are running on 120VAC).

The commercial and industrial smoke detectors (as seen in every office building in the US) are all low voltage DC. If you have a fire control panel at your facility that feeds 24 volts AC to the detection circuits, you have a dinosaur and a system which will not meet any fire code in the USA. It is mandatory that all fire systems are battery backed. Batteries put out DC voltage, not AC. Your AC system will not have batteries. If it DOES have batteries, put a meter on the detector wires and you will find that they are DC.

Yes, very dinosaur, but that's how the government does things. That's why they don't write the code.

I double checked the drawings. I was thinking of our emergency power-off system, that's 24VAC for the shunt trips. The detectors are DC (and battery backed). Thanks for keeping me fresh on that.

It's always good to get into the drawings. (They're still paper, too...gov't style).