Electrical Theory and Wiring Practices for Fire Alarm
Core concepts
Fire alarm circuits are power-limited and must be installed per Article 760 using PLFA/FPL-rated cable where required, separated from power conductors, and protected against physical damage. Candidates must understand Ohm's law (V = IR), voltage drop on long runs, and how to compute battery standby and alarm capacity using the panel's standby load, alarm load, and required duration. Know the difference between primary (AC) power and secondary (battery) power, and why a dedicated unswitched circuit is required. Understand ground fault detection, surge protection, and proper end-of-line resistor placement for supervised circuits.
Traps in questions
- Forgetting derating for continuous vs. alarm loads when sizing batteries.
- Mixing voltage drop (affects device operation) with current capacity (affects wire ampacity).
- Placing EOL resistors at the panel instead of the end of the circuit.
- Assuming any cable can share a raceway with line-voltage conductors.
Rule of thumb
Battery size = (standby current × standby hours) + (alarm current × alarm minutes/60), then add a 20% aging/safety margin. Always compare to the panel's listed capacity.
Worked example
Standby load 0.20 A for 24 h = 4.8 Ah. Alarm load 1.5 A for 5 min = 0.125 Ah. Total = 4.925 Ah; with 20% margin ≈ 5.91 Ah. A 7 Ah battery is adequate; a 4.5 Ah battery is not.