Circuit Design, Wiring, and Power Supplies
Circuits and pathways
- Class B circuits use a two-wire path with an end-of-line device; a single open conductor can prevent operation beyond the break. Class A circuits return to the panel, so a single open or short does not disable the circuit; they are used where survivability is needed.
- NFPA 70 Article 760 covers fire alarm wiring. Power-limited fire alarm (PLFA) circuits are separated from non-power-limited (NPLFA) and from power/lighting unless a permitted method is used.
- Cable markings matter: FPLR for risers, FPLP for plenums, FPL general use. Using the wrong listing is a common inspection finding.
- NACs must be supervised and must operate all appliances at their minimum rated voltage after voltage drop.
Power supplies and battery calculation
Secondary power for a protected-premises system is typically 24 hours standby plus 5 minutes of alarm. Calculate: (standby current × 24 h) + (alarm current × 5/60 h), then add about 20% for aging and derating, and compare with the battery's rated Ah at the manufacturer's derated capacity.
Worked example
Standby load 0.25 A, alarm load 2.0 A. Standby: 0.25 × 24 = 6.0 Ah. Alarm: 2.0 × (5/60) = 0.167 Ah. Subtotal 6.167 Ah; ×1.20 = 7.4 Ah. A 7 Ah battery is too small; select at least an 8 Ah battery (verify charger and listing).
Traps
- Using alarm current for 24 hours, which grossly oversizes the battery.
- Omitting the aging factor or the charger's ability to recharge.
- Reversing Class A and Class B descriptions.
- Forgetting voltage drop on long NAC runs, so the last horn is under voltage.