Power Supplies and Secondary Power
Primary power
Commercial fire alarm systems are powered by a dedicated 120 VAC branch circuit with a dedicated overcurrent device, marked and locked on (breaker lock) so it cannot be casually switched off, with no GFCI protection and no other loads on the circuit. The circuit must be accessible only to authorized personnel, and the panel must annunciate loss of primary power as a trouble.
Secondary power
The standard default for protected-premises systems is 24 hours of standby followed by 5 minutes of alarm operation on secondary (battery) power; certain system types and occupancies require longer standby. Batteries must be sized to carry the calculated standby load plus the alarm load, and the calculation must include a derating factor for battery aging (commonly 20 percent, i.e., multiply by 1.2). The charger must be capable of restoring the battery after a discharge and must be listed for fire alarm use. Sealed lead-acid (VRLA) batteries are typical; a battery that has been deeply discharged or is more than a few years old may pass a voltage check and still fail a load test.
Doing the math
List every load in two columns: standby current (panel quiescent, communicator, annunciators) and alarm current (panel plus all NAC appliances plus relays). Multiply standby amps by 24 hours, alarm amps by 5 minutes expressed in hours (5/60 = 0.0833), add them, then apply the 1.2 derating factor and round up to the next available battery size.
Worked example
Standby 0.50 A × 24 h = 12.0 Ah. Alarm 2.0 A × 0.0833 h = 0.167 Ah. Total 12.17 Ah × 1.2 = 14.6 Ah required - a 12 Ah battery is too small; select the next size up (e.g., 18 Ah).
Traps
Forgetting to add notification appliance current; omitting the derating factor; sizing to the battery's nominal rating rather than its rated 24-hour capacity.