Power Supplies, Battery Calculations, and Secondary Power
Primary power
The fire alarm control unit requires a dedicated, unswitched 120 VAC branch circuit with a lockable or restricted-access overcurrent device marked for fire alarm service (commonly red). GFCI protection is not permitted on that circuit, and the circuit must not supply other loads. The breaker must be able to be locked in the "on" position.
Secondary power sizing
Stored-energy secondary power must carry the system through 24 hours of standby followed by 5 minutes of alarm (longer alarm time, commonly 15 minutes, for voice evacuation systems), and the charger must restore the batteries within 48 hours. Batteries are derated: plan on about 80% of the rated capacity being available at end of life, and add a 20% (or 1.25) safety margin to the calculated load.
Worked example: standby load 0.125 A; alarm load 3.5 A.
- Standby: 0.125 A × 24 h = 3.00 Ah
- Alarm: 3.5 A × (5/60) h = 0.29 Ah
- Total = 3.29 Ah → × 1.2 margin = 3.95 Ah required
- A 7 Ah battery derated to 80% gives 5.6 Ah - acceptable. A 4.5 Ah battery gives 3.6 Ah - too small.
Traps: using alarm current for the whole 24 hours or standby current for the alarm period; forgetting the safety margin and the 80% derating; forgetting that notification appliance current dominates and must be totaled from the appliance schedule; and assuming any receptacle circuit will do for primary power. Sealed lead-acid batteries must also be replaced when they can no longer deliver the calculated capacity.