Fire alarm systems also make extensive use of conventional 2-wire input circuits (IDCs) and output circuits (NACs). These are parallel circuits that work by detecting voltage changes. Inputs are configured as normally open devices. Output devices use a blocking diode so that the wire can be supervised by reversing the polarity in standby (non-alarm) mode. A single end-of-line resistor causes a medium voltage drop that the control unit interprets as nominal.
When a conventional fire alarm device activates, it pulls down the circuit voltage. The panel interprets low voltage on an IDC as an activation. On a NAC, low voltage is treated as a fault condition – i.e., a short. In the reverse, if a wire breaks, the voltage jumps up, which the panel recognizes as an open circuit.
Hard-wired door contact, glass break, PIR, and REX circuits operate in a similar fashion, using voltage to determine state. One key difference is that intrusion wiring typically fails secure, with an open circuit generating an alarm and shorts being ignored.
Another key difference is that the supervisory resistor for fire alarm circuits must be installed at the last device instead of at the panel.
Fire alarm wiring is regulated by NFPA 72 - National Fire Alarm and Signaling Code and NFPA 70 - National Electrical Code. These codes contain numerous rules that may be unfamiliar to folks coming from a security background. If you’re new to the fire alarm industry, be sure to read up on these essential rules. Access is free on NFPA’s website.
Fire Code-Compliant Wire
With code-minimum fire alarm wiring, an open circuit causes all downstream devices to become inoperable. In the July 2025 issue, I introduced the concept of Class A return wiring. To improve resiliency, some cities require fire alarm circuits to be installed with an extra wire that runs from the last device back to the source. In the event of a wire break, this redundant loop keeps devices operational.
Class A wiring can be used with SLCs, IDCs, and NACs. In each case, the panel will report the open circuit fault while maintaining full functionality.
It would be a tragic irony if the fire alarm wire spread flames and generated toxic smoke. To address this possibility, fire alarm wire is rated for the environment in which it may be installed. Areas where flame and smoke spread would be more catastrophic will require better jackets.
Fire Wire Runs
When running fire alarm wire between floors, you need to upgrade from plain FPL to FPLR (riser wire) or better. When running wire in an area with open-air HVAC returns, you need to use the next grade up, FPLP (plenum wire). In some circumstances, you may even need to use tough circuit integrity (CI) cable.
If you thought you could just run fire alarm wire above the ceiling and leave it resting on the tiles, you would be sorely mistaken. Fire alarm wire must be secured, and you cannot use ceiling grid wires. Mount it to the deck, run it in a cable tray or raceway, or install your own supports.
Any time fire alarm wire runs through the floor or ceiling, it must not be exposed below 7 ft. (2.13 m). Be sure to use a knockout bushing any time wire passes through metal other than conduit or flex. This applies to panels, boxes, and studs/top plates.
These are best practices in security; they are hard and fast rules in fire alarm.
Power Transformers
Burglar alarm systems frequently use plug-in transformers for power. These are not permitted for fire alarm systems – even if the panel is listed as a combo fire/burg panel. Aside from the fact that most outlets are below 7 ft (2.13 m), code requires fire alarm systems to be hardwired to a dedicated circuit. Securing a transformer in place with the ground screw doesn’t count.
