Power Supplies, Batteries, and Secondary Power
Primary and secondary power
NFPA 72 requires two independent power sources: primary (commercial AC branch circuit, dedicated and labeled) and secondary (battery, generator, or other standby). The secondary source must carry the system for a defined duration - commonly 24 hours of standby followed by 5 minutes of alarm for commercial occupancies, with longer alarm times where required by the occupancy or AHJ. The battery must be sized to that load, including a 20% aging/derating factor.
Battery math
Battery capacity is calculated as: standby current × standby hours + alarm current × alarm minutes/60, then multiplied by 1.2. Use the manufacturer's published current draws for the panel, devices, and NAC appliances in both standby and alarm states. Sealed lead-acid batteries are typical; charging must be supervised, and low-battery trouble must annunciate before the battery is depleted (often at 20% remaining capacity).
Remote power supplies and NAC boosters
Remote supplies extend NAC capacity and must be supervised and synchronized with the main panel. They require their own power and battery backup, and their trouble conditions must report to the FACU.
Traps
- Using only alarm current and forgetting standby hours.
- Forgetting the 20% derating factor.
- Mixing up which source is primary vs. secondary.
- Assuming a generator removes the need for battery backup - it does not unless the code permits it.
- Ignoring that the dedicated AC circuit must be labeled and accessible.
Worked example
Standby load 0.50 A for 24 h = 12 Ah. Alarm load 2.0 A for 5 min = 0.167 Ah. Total = 12.167 Ah. Apply 1.2 = 14.6 Ah. Select the next standard battery size above 14.6 Ah, and verify charger capacity.