What standby and alarm durations does NFPA 72 require?

For a protected premises fire alarm system, NFPA 72 typically requires 24 hours of standby followed by 5 minutes of alarm. For voice evacuation and mass notification, 15 minutes of alarm. For a supervising station, 60 hours of standby plus alarm. Confirm the exact requirement for your project against the current NFPA 72 edition.

What aging derate should I apply?

Most engineers use a 20 percent aging derate, meaning the calculated Ah requirement is divided by 0.80 to give the specified battery capacity. Some panel manufacturers require 25 percent.

Do I include auxiliary powered devices in the battery sizing?

Yes for anything that draws from the panel's aux 24 VDC output during standby or alarm. External NAC power supplies get their own separate battery sizing.

Can I use one battery for standby and alarm?

Yes. The battery is sized for the total energy: standby current times standby duration, plus alarm current times alarm duration, all expressed as amp-hours.

Fire alarm battery standby vs alarm load: full engineer's guide

Standby current and alarm current are two different current draws over two different durations. Both drive the battery amp-hour rating. This guide covers the formula, NFPA 72 durations, derating for aging, and worked examples for common panels.

By Umar Dar · Published 2026-07-27 · Last updated 2026-09-19 · ~8 min read

Short answer

Standby load is the total 24 VDC current the fire alarm system pulls while it is normal but running on battery, typically after utility power fails. Alarm load is the higher current pulled while notification appliances, relays, and evacuation signalling are active. Both periods matter because the battery has to serve the full standby time first, and still have enough charge left to drive the alarm devices for the required alarm duration.

NFPA 72 sets the durations: 24 hours of standby plus 5 minutes of alarm for most protected premises systems, 60 hours for supervising stations, and 15 minutes of alarm for voice evacuation and mass notification. The calculated amp-hour result is then divided by an aging factor (usually 0.80) to size the actual battery.

The core formula

Battery capacity, expressed in amp-hours, is:

Ah = (I_standby × t_standby + I_alarm × t_alarm) ÷ K_age

Where:

I_standby = total supervisory current in amps
t_standby = standby duration in hours (typically 24 or 60)
I_alarm = total alarm current in amps
t_alarm = alarm duration in hours (typically 5/60 for 5 min, 15/60 for 15 min)
K_age = aging factor, usually 0.80 (some panels 0.75)

The result is the minimum specified battery Ah. Round up to the next available battery size — 7 Ah, 12 Ah, 17 Ah, 26 Ah, 40 Ah, 55 Ah, 65 Ah, 100 Ah are common sealed lead-acid formats. Confirm that the physical battery cabinet or enclosure can hold the selected size.

What goes into standby current

Every device that draws from the panel or its aux 24 VDC output during normal supervision has to be listed. Typical items:

Panel base current from the manufacturer datasheet.
Loop drivers, addressable loop cards, or signalling line circuits (SLC). Each loop has a supervisory current for the driver plus a per-device standby draw.
Detectors, modules, and manual call points on the loop. Addressable detectors typically 300 to 500 μA each in standby.
Notification appliance circuits (NAC) in supervised state. Small drain per circuit for line supervision.
Auxiliary powered devices. Aspirating detection controllers, sounder beacons pulled from panel aux, magnetic door holders (if unlisted power), interfaces.
Optional cards for graphical annunciators, network cards, LCD annunciators, printer interface.

Sum the milliamp draws, convert to amps, and record the total as I_standby.

What goes into alarm current

Alarm current is dominated by notification appliances. Typical contributors:

Sounders and horn/strobe combinations on each NAC. Manufacturer alarm current at the design candela and dB setting. Alarm current is much higher than standby.
Speakers if voice evacuation, at the amplifier tap wattage.
Bells or motorised bells.
Panel and loop drivers under alarm. Slightly higher than standby because relays operate, LEDs light, LCD backlight goes on.
Relay modules, aux relays, and interfaces that energise on alarm.
Auxiliary loads released or activated on alarm: magnetic door holders that drop out reduce load, magnetic door releases on evacuation routes may be zero draw after activation.

The worst-case alarm scenario is the "general evacuation" state where every NAC is active. Some designs use phased or staged evacuation, where alarm current is capped by the maximum simultaneous zone count. Either way, use the largest realistic alarm current for the battery calculation.

Worked example: 3-loop addressable panel

A commercial office building has a 3-loop addressable panel supporting 240 devices, 8 NAC circuits with horn/strobes, one graphic annunciator, and a network card to a secondary panel.

Standby current.
- Panel base: 0.180 A
- 3 loop drivers @ 0.060 A: 0.180 A
- 240 addressable devices @ 0.0004 A: 0.096 A
- Graphic annunciator: 0.050 A
- Network card: 0.040 A
- 8 NAC supervisory @ 0.005 A: 0.040 A
- Total I_standby ≈ 0.586 A
Alarm current.
- Panel base under alarm: 0.240 A
- 3 loop drivers under alarm: 0.240 A
- Graphic annunciator: 0.080 A
- Network card: 0.050 A
- 8 NAC circuits, 48 horn/strobes total @ 0.140 A per device: 6.72 A
- Total I_alarm ≈ 7.33 A
Amp-hours before aging.
(0.586 × 24) + (7.33 × 5/60) = 14.06 + 0.611 = 14.67 Ah
Apply aging derate (0.80).
14.67 / 0.80 = 18.34 Ah
Select battery. Round up to 26 Ah sealed lead-acid batteries in the required series/parallel configuration to meet 24 VDC.

If the same building adds voice evacuation with a 15-minute alarm duration, the alarm contribution grows from 0.611 Ah to 1.83 Ah, and the specified battery would move to 26 Ah still, but any increase in speaker load could push into the next size.

Panel-specific notes

FireLite / Honeywell

FireLite panels (MS-9200UDLS, MS-9600LS, ES-200x/ES-1000x, MS-5UD) publish detailed current tables. Their standard aging derate is 20%. The manufacturer battery worksheet is authoritative — use the calculator here as a planning aid.

Notifier

Notifier NFS2-640, NFS2-3030, and NFS-320 have similar worksheet structures. Confirm loop count, expander cards, and NAC current before finalising.

Simplex / Johnson Controls

Simplex 4100ES and TrueAlarm systems use their own current spreadsheet. Watch the audio amplifier draw for voice systems, which can dominate alarm current.

Zeta, Kentec, and Morley-IAS

Widely used in the GCC and UK. Follow BS 5839-1 durations (24 hours standby, 30 min alarm for typical premises) rather than NFPA 72 for these markets, unless the project specification says otherwise.

Common mistakes to avoid

Forgetting auxiliary loads. Aspirating detectors, remote annunciators, and door holders on aux 24 V can quietly double the standby figure.
Using NAC power supply loads on the panel battery. External NAC boosters have their own batteries; do not double-count.
Skipping the aging derate. A battery that just meets the calculated Ah on day one will fail its 4th- or 5th-year duration test.
Using standby current for alarm. Alarm current is often 10 to 30 times standby.
Wrong alarm duration. 5 minutes is the standard NFPA 72 minimum for horn/strobe; voice evacuation is 15 minutes. BS 5839-1 uses 30 minutes. Confirm the applicable standard.
Missing the graphic annunciator or network card draw. Small individually, large in aggregate on big systems.

How this connects to the calculators on this site

The Fire Alarm Battery Backup Calculator automates the standby-plus-alarm sum and applies the aging derate. The Fire Alarm Loop Load Calculator checks the loop current budget before you finalise device counts. For intruder alarm systems, use the Intruder Alarm Battery Calculator.

Frequently asked questions

Is a 24-hour standby always required?

Not always. NFPA 72 defaults to 24 hours for protected premises fire alarm systems. Supervising station systems need 60 hours. UK BS 5839-1 uses 24 hours for Category L (life safety) and 72 hours for Category P (property protection with unattended manned watch). Local fire codes may vary.

Do I have to include the utility charger current?

No. The battery calculation covers the time when the charger is off. The charger has to be sized separately for a 24-hour recharge target after full discharge.

Do I need matched-pair batteries?

Two 12 V batteries in series to produce 24 VDC should be identical model, identical capacity, from the same manufacturing lot where possible. Mixing ages or capacities causes unequal loading and shortens life.

What about lithium batteries?

Some listed panels now accept lithium iron phosphate batteries with a matched BMS. The Ah calculation is unchanged, but the aging derate may be lower per the manufacturer. Confirm listing.

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Verification note

This guide is planning support only. Final fire alarm battery calculations must be verified using the panel manufacturer's own worksheet, cross-checked against NFPA 72 or BS 5839-1 depending on jurisdiction, and reviewed by a licensed fire protection engineer. Authority Having Jurisdiction approval is required before commissioning.