Control Equipment, Circuits, and Wiring Methods
Two circuit families under Article 760
NPLFA (non-power-limited fire alarm) circuits are treated like ordinary power wiring: Chapter 3 wiring methods, minimum conductor sizing, junction boxes, and grounding rules apply. PLFA (power-limited fire alarm) circuits rely on a listed power source that limits energy, which allows lighter cable - FPL, FPLR (riser), and FPLP (plenum) - but imposes separation from power conductors and requires that the circuit not be re-identified or substituted with general-purpose cable.
Conductors must be distinguishable (typically red), mechanically protected where exposed, and kept electrically separate from Class 1 and power circuits. Only listed equipment may be interconnected; T-tapping an addressable signaling line circuit is generally prohibited unless the listed equipment and topology permit it. Ground-fault detection, transient protection, and a single dedicated power circuit all live here as well.
The control unit itself provides alarm, supervisory, and trouble annunciation, alarm silence, reset, and the programmed sequence of operation. Its outputs must fail in a safe direction: a loss of primary power, a ground fault, or a single open must produce a trouble, not a silent failure.
Worked example (voltage drop): a 24 VDC NAC draws 2.0 A and runs 250 ft one-way in 14 AWG (500 ft of conductor, ≈3.07 Ω/1000 ft ≈ 1.54 Ω). Drop = 2.0 A × 1.54 Ω ≈ 3.1 V, about 13% - over the 10% design target. Moving to 12 AWG (≈1.93 Ω/1000 ft) halves the resistance, giving about 1.9 V (8%), which passes.
Traps: using FPL cable where a Chapter 3 method is required, mixing NPLFA and PLFA in one raceway without meeting separation rules, and forgetting class topology when a break occurs.