Free calculator · NFPA 72
Fire Alarm Standby Battery Calculator
Size the secondary battery for a fire alarm control unit or power supply from its standby and alarm current. Built by Joseph Montuori, NICET Level IV, for US systems under NFPA 72.
Required battery capacity
- Standby
- Alarm
- Subtotal
- Safety margin
Planning aid only. The manufacturer's battery calculation for the installed panel and batteries governs. Derate for cold locations per the battery datasheet.
How the battery calculation works
The secondary power supply has to carry the system through the standby period and then the alarm period. Convert each load to amps, multiply by its time in hours, add them, then apply the safety margin:
Ah = [(Istandby × hstandby) + (Ialarm × halarm)] × safety margin
Example: 300 mA standby for 24 hours is 7.2 Ah. 3,000 mA in alarm for 5 minutes (0.083 h) is 0.25 Ah. The subtotal is 7.45 Ah, and with the 25% safety margin the requirement is 9.31 Ah, so a 12 Ah battery set covers it.
Durations under NFPA 72
| System | Standby | Alarm |
|---|---|---|
| Protected premises (typical) | 24 hours | 5 minutes |
| In-building voice evacuation | 24 hours | 15 minutes |
| Backed by an automatic-start generator | 4 hours | 5 minutes |
These come from NFPA 72 section 10.6.7. Other system types and jurisdictions can require different times, so confirm with the edition your AHJ enforces.
Read more: Fire Alarm Battery Calculations: 24 Hours, 5 Minutes and the Safety Margin.
Battery Calculation Questions
How long must fire alarm batteries last under NFPA 72?
For a typical protected premises system, the secondary power supply must run the system for 24 hours in standby and then 5 minutes in alarm. In-building voice evacuation systems need 15 minutes of alarm. Where an automatic-start generator backs up the system, NFPA 72 allows 4 hours of standby. Always confirm against the edition your AHJ has adopted.
What safety margin should I add?
NFPA 72 requires a safety margin on the calculated battery capacity: 20 percent in the 2019 edition and earlier, raised to 25 percent in the 2022 edition. Pick the edition your jurisdiction has adopted.
Why does the result not match the panel manufacturer’s calculation?
Manufacturer worksheets include panel-specific currents and sometimes their own derating. The manufacturer's battery calculation for the installed panel and batteries governs; use this calculator for planning and checking.
Do cold temperatures affect battery capacity?
Yes. Sealed lead-acid capacity drops in cold ambient temperatures. If the batteries will see cold conditions, derate using the battery manufacturer's datasheet.
More free calculators
This calculator is a planning aid and does not replace a full design review, the manufacturer's calculation, or the requirements of your authority having jurisdiction. Not affiliated with NFPA.