What the standard expects
NFPA 72 is the National Fire Alarm and Signaling Code used by many jurisdictions, often with local amendments and an adopted edition. For protected premises fire alarm systems, a common design basis is enough secondary power for a standby period followed by an alarm period; many systems use 24 hours of standby plus 5 minutes of alarm, while emergency voice/alarm or other systems may require a different alarm duration.
The calculation applies to designers, installers, plan reviewers, service companies, and facility owners responsible for fire alarm documentation. The authority having jurisdiction, project specification, equipment listing, and adopted code edition control the final requirement.
- Use the adopted code and local amendments.
- Use manufacturer current draws for the installed equipment.
- Check both required amp-hours and installed battery capacity.
How to calculate
The basic formula is required Ah = (standby current x standby hours) + (alarm current x alarm hours), then apply any required safety factor or derating used by the design standard, manufacturer, or jurisdiction. Convert minutes to hours before multiplying.
For example, a panel has 0.42 A standby current and 3.8 A alarm current. With 24 hours standby and 5 minutes alarm, required Ah before margin = 0.42 x 24 + 3.8 x (5/60) = 10.08 + 0.317 = 10.397 Ah. With a 20 percent margin, required capacity is 12.48 Ah, so a 12 Ah pair would be short and the next listed battery size should be selected.
Getting accurate current values
Use the device current in the correct state. A notification appliance circuit has little or no standby current for many appliances but high alarm current. Addressable modules, relays, annunciators, communicators, detectors, and releasing modules may draw current in standby, alarm, or both.
Do not use the panel's maximum power supply rating as the alarm current unless that is truly what the system draws. Add the listed current for each installed load, account for auxiliary outputs, and confirm the charger supports the selected battery size.
Common mistakes
A frequent mistake is entering alarm minutes as hours. Five minutes is 0.0833 hours, not 5 hours. Another is forgetting remote annunciators, dialers, network cards, door holder relays, or powered modules that remain active during standby.
Battery age and temperature also matter. The calculation may show adequate capacity on paper, while field batteries fail because they are old, damaged, undersized for the charger, or installed in an environment outside the listing.
- Missing auxiliary loads.
- Using nominal instead of listed current draw.
- Ignoring voltage-drop and NAC limits.
- Selecting a battery size the control unit cannot charge.
Documentation workflow
A clean submittal lists every load, standby current, alarm current, required standby time, required alarm time, total amp-hours, margin, selected batteries, and charger compatibility. Keep the battery calculation with voltage-drop calculations and product data.
For existing systems, update the calculation when devices are added, communicators are replaced, NAC extenders are installed, or sequence of operation changes. A small tenant improvement can push an old battery set below the required capacity.
Frequently asked questions
Is 24 hours standby and 5 minutes alarm always required?
No. It is a common basis for many fire alarm systems, but the adopted code, system type, local amendments, and project requirements can require different durations.
Why do battery calculations use amp-hours?
Amp-hours combine current draw and time. A device drawing 1 amp for 10 hours requires 10 amp-hours before margins and battery limitations.
Can I install a much larger fire alarm battery?
Only if the control unit or power supply is listed to charge that battery size and the installation instructions allow it.
Do notification appliances count during standby?
Usually their major current draw is during alarm, but always use the manufacturer's listed standby and alarm current values for the actual appliances and circuits.