Power Supplies, Batteries, and Secondary Power
This domain tests primary and secondary power for fire alarm systems under NFPA 72 Chapter 10. You must know the dedicated-circuit requirement, the standby and alarm durations, and the battery sizing formula with its aging margin.
Core rules
- The primary supply is a dedicated 120 VAC branch circuit, marked “Fire Alarm,” with no other loads and protected against accidental disconnection.
- Secondary power is typically sealed lead-acid batteries. Commercial systems require 24 hours of standby followed by 5 minutes of alarm; voice evacuation systems commonly require 15 minutes of alarm.
- Battery capacity is calculated as standby current × 24 hours plus alarm current × alarm minutes ÷ 60, then multiplied by a 1.25 aging/derating factor.
- The charger must restore the battery within the code time and be listed for fire alarm use.
- Voltage drop must be checked at the end of battery discharge, not only at nominal voltage.
Worked example
A panel draws 0.25 A standby and 1.5 A alarm. Standby energy is 0.25 × 24 = 6.0 Ah. Alarm energy is 1.5 × 5 ÷ 60 = 0.125 Ah. The subtotal is 6.125 Ah; multiplied by 1.25 gives 7.66 Ah. Select the next standard battery size, 8 Ah, and verify it fits the cabinet and charger rating.
Traps
- Using 5 minutes for a voice system that requires 15 minutes.
- Omitting the 1.25 derating factor.
- Sizing from nameplate current instead of measured standby and alarm loads.
- Sharing the fire alarm circuit with other equipment.
- Ignoring end-of-discharge voltage drop on long NAC runs.