NFPA 72 secondary power supply requirements
Under NFPA 72 (National Fire Alarm and Signaling Code Chapter 10), secondary power supplies (standby batteries) must operate the system under non-alarm supervisory conditions for 24 hours, followed by operating all notification appliances in alarm.
Alarm duration requirements: 5 minutes of alarm at the end of 24 hours for standard systems, or 15 minutes of emergency voice/alarm communications (EVACS) for high-rise buildings and assembly occupancies.
Formula: C = 1.20 × [ (I_standby × 24 h) + (I_alarm × t_alarm) ], incorporating the mandatory 20% safety margin factor (1.20 multiplier).
Worked example: NFPA 72 secondary battery sizing
A fire alarm control unit with 0.18 A standby current and 2.4 A alarm current requiring 24 hours standby and 5 minutes (0.0833 h) alarm:
Standby load = 0.18 A × 24 h = 4.32 Ah.
Alarm load = 2.4 A × 0.0833 h = 0.20 Ah.
Minimum capacity C = 1.20 × (4.32 + 0.20) = 5.42 Ah.
Recommended commercial size is a standard 7.0 Ah or 8.0 Ah battery set.
Inputs for the NFPA 72 fire alarm battery calculator
Standby and alarm currents
Enter supervisory current and maximum alarm current in milliamperes (mA).
Alarm duration
5 minutes for standard evacuation or 15 minutes for emergency voice alarm (EVACS).
CALCULATOR QUESTIONS
NFPA 72 secondary power battery calculator questions
What is the NFPA 72 safety margin for fire alarm batteries?
NFPA 72 requires a 20% safety factor (1.20 multiplier) applied to the sum of supervisory and alarm loads.
How long must US fire alarm batteries last during a power outage?
NFPA 72 mandates a minimum of 24 hours of standby operation followed by 5 minutes of full alarm (or 15 minutes for voice evacuation systems).