Fire Alarm Battery Calculations: 24 Hours, 5 Minutes and the Safety Margin
The battery calculation is one of the shortest sheets in a fire alarm submittal, and one of the easiest to get wrong. A missed module, the wrong alarm duration or an outdated safety margin can leave a panel with batteries that look fine on paper and come up short in a real outage.
This is how I approach a fire alarm battery calculation as a NICET Level IV designer, written for techs and designers who need a calc that holds up at plan review and in the field. Always confirm the NFPA 72 edition your AHJ has adopted, and use the manufacturer’s data for every device on the system.
What NFPA 72 Requires for Secondary Power
Secondary power is what keeps the system running when primary AC is lost. The required duration depends on the type of system and what backs it up.
Protected Premises Systems
For a typical protected premises fire alarm system, NFPA 72 (10.6.7.2.1) requires secondary power capacity for 24 hours of standby followed by 5 minutes of alarm.
Voice and Emergency Communications Systems
For in-building emergency voice/alarm communications systems (EVACS), the alarm portion increases to 15 minutes of alarm operation at the end of the standby period. Voice amplifiers draw serious current in alarm, so this one change can move the battery size a lot.
Systems Backed by a Generator
Where an automatic-start generator backs up the system, NFPA 72 (10.6.7.3) allows the battery to be sized for 4 hours of standby plus 5 minutes of alarm. The generator has to meet the code’s requirements for that reduction to apply, so confirm the generator arrangement before you take credit for it.
The Battery Calculation Formula
The calculation itself is straightforward:
Required Ah = [(standby current in A x standby hours) + (alarm current in A x alarm hours)] x safety margin
A few notes on the inputs:
- Standby current is the total draw of the panel and every connected device in normal, non-alarm condition. That includes the panel itself, SLC devices, modules, annunciators, communicators and anything else powered from the panel’s secondary supply.
- Alarm current is the total draw with the system in full alarm: notification appliances operating, relays energized and every device in its alarm state.
- Convert minutes to hours. 5 minutes is 5/60 = 0.0833 hours. 15 minutes is 0.25 hours.
- Convert mA to A. Datasheets list most device currents in mA. Divide by 1,000 before you multiply by hours.
The Safety Margin: 20% or 25%
After you total standby and alarm capacity, you multiply by a safety margin. Which one depends on the edition:
- 2019 edition and earlier: 20% safety margin (multiply by 1.20)
- 2022 edition: 25% safety margin (multiply by 1.25)
This is where older templates cause trouble. If your calc sheet was built years ago and still uses 1.20, it may not match the edition your AHJ enforces now. Check the adopted edition on every project and set the safety margin to match.
Worked Example
The numbers below are round illustrative values for a simple protected premises system, not data from any real product. Your inputs come from the manufacturer’s datasheets for the actual equipment.
Example inputs:
- Total standby current: 300 mA (0.300 A)
- Total alarm current: 3,000 mA (3.0 A)
- Standby duration: 24 hours
- Alarm duration: 5 minutes (0.0833 hours)
Step 1: Standby capacity
0.300 A x 24 h = 7.20 Ah
Step 2: Alarm capacity
3.0 A x 0.0833 h = 0.25 Ah
Step 3: Subtotal
7.20 Ah + 0.25 Ah = 7.45 Ah
Step 4: Apply the safety margin
- 2022 edition (x1.25): 7.45 x 1.25 = 9.31 Ah
- 2019 edition and earlier (x1.20): 7.45 x 1.20 = 8.94 Ah
Step 5: Pick the battery
Select the next standard battery size at or above the required capacity, within the range the panel’s charger is listed to charge. In this example, a common 12 Ah battery would cover either result.
Notice how the standby side dominates. In most conventional systems, 24 hours of standby uses far more capacity than 5 minutes of alarm. That is why a few extra milliamps per device in standby, multiplied across a large SLC, can push you to the next battery size.
Mistakes I See on Submittals
- Missing devices. Annunciators, communicators, relay modules and addressable modules all draw standby current. Every one needs to be on the sheet.
- Wrong alarm duration. Using 5 minutes on a voice system that needs 15 minutes.
- Old safety margin. Using 1.20 on a project under the 2022 edition.
- Ignoring the charger limit. A calculated battery size that exceeds what the panel can charge is not a valid result. Larger systems may need an external charger or a different power arrangement.
- Mixing units. Adding mA to A without converting. It happens more than you would think.
Cold Weather and Temperature
Battery capacity drops in cold conditions. If batteries are installed in an unheated space, the derating comes from the battery manufacturer’s datasheet, which shows available capacity at different temperatures. Apply that derating so the installed battery delivers the required capacity at the expected temperature.
The Manufacturer’s Calc Still Governs
Every panel manufacturer publishes its own battery calculation method or worksheet for its equipment, with current values for its panels, modules and appliances. Use it. A generic calculation is a good check, but the manufacturer’s calc for that specific panel, along with its installation instructions, is what governs.
For a quick cross-check, our free fire alarm battery calculator applies the standby, alarm and safety margin math for you.
Battery capacity is only half the power story. Once you know the battery holds up, check that your notification circuits deliver enough voltage at the end of the line. See NAC voltage drop: there is no 10% rule, plus our guides on smoke detector spacing and fire alarm design in AutoCAD.
FAQ
How many hours of battery backup does NFPA 72 require for a fire alarm?
For protected premises systems, 24 hours of standby plus 5 minutes of alarm (10.6.7.2.1). Voice systems need 15 minutes of alarm. With a qualifying automatic-start generator, 4 hours of standby plus 5 minutes of alarm (10.6.7.3). Confirm your adopted edition.
What safety margin should I use in a fire alarm battery calculation?
The 2019 edition and earlier use a 20% safety margin (x1.20). The 2022 edition raised it to 25% (x1.25). Use the margin from the edition your AHJ adopted.
Should I use the manufacturer’s battery calculation?
Yes. The manufacturer’s calculation for the specific panel, with its published current values, governs. A generic calc is a useful check but does not replace it.
Battery Calcs Done Right, as Part of a Complete Package
Titan Fire Protection prepares NICET-prepared fire alarm drawings for projects nationwide, including battery standby and voltage drop calculations, designed and prepared by Joseph Montuori, NICET Level IV, SET. Need a second set of eyes on a package you already have? Our flat-fee drawing review covers plans, specs and calcs. See how our design process works.
Request a quote or call 860-322-9028.