Installation and Wiring
Two rule sets in parallel
NEC Article 760 divides fire alarm circuits into power-limited (PLFA) and non-power-limited (NPLFA). Cable markings matter: FPL for general use, FPLR for risers, FPLP for plenums. Power-limited fire alarm conductors must be separated from non-power-limited conductors and from light and power by at least 2 in. or a listed barrier, and must not share a raceway, box, or enclosure with them without that barrier. NFPA 72 Chapter 12 then governs pathway performance: supervision, class, and survivability.
Pathway classes and survivability
Class A provides a redundant, fully supervised path; Class B is a single supervised path; Class X and Class N address data and network pathways. NFPA 72 establishes survivability Levels 0 through 3 for systems with partial evacuation or emergency communications, with the highest levels requiring 2-hour circuit integrity. Long-standing traps: running notification appliance circuits beyond the manufacturer's voltage-drop limits, mixing power-limited and non-power-limited conductors, and assuming a Class B SLC tolerates any number of T-taps regardless of loading.
Power and batteries
NAC voltage at the last appliance must remain within the listed operating range, conventionally 85 percent of nominal (20.4 V on a 24 V system). Standby battery calculations use 24 hours of standby followed by 5 minutes of alarm, extended to 15 minutes where the system provides voice evacuation, plus a derating or aging factor per the manufacturer's method. Document the calculation; exam items often ask which load is included in standby versus alarm.
Worked example
A 24 V NAC drawing 1.5 A over 300 ft of 14 AWG: verify with the manufacturer's voltage-drop table that the end-of-line appliance sees at least 20.4 V, then confirm the panel's NAC rating and battery capacity cover the connected load at that voltage.
Trap
Wiring method lives in Article 760, not NFPA 72; a question about cable type or raceway separation is a code question, not a design question.