Fire Alarm Battery & NAC Voltage Drop Calculator

Fire Alarm Power Budgets: Standby vs. Alarm Load

Every fire alarm system has two distinct power states that must be calculated separately. Standby is the normal operating condition: the panel is monitoring initiating device circuits, polling addressable devices, and powering supervision relays. Standby current is typically low: addressable smoke detectors draw 50–375 µA each, while magnetic door holders draw a constant 20 mA per unit. The critical detail is duration: NFPA 72 requires 24 hours of standby for standard systems. That means even small per-device currents accumulate significantly.

Ten addressable smoke detectors drawing 0.3 mA each contribute only 3 mA, but over 24 hours that equals 72 mAh. A panel with 20 door holders drawing 20 mA each adds 400 mA of continuous standby load, which is 9,600 mAh over 24 hours. Alarm load is the opposite pattern: high current for a short duration. When the system activates, NAC circuits power horn/strobes that draw 60–300 mA each depending on candela rating and technology (LED vs. xenon).

NFPA 72 requires only 5 minutes of alarm operation for standard systems, but with 20 horn/strobes at 176 mA each, that is 3,520 mA for 5 minutes, totaling 293 mAh. The total battery capacity must cover both states plus a safety correction factor.

NAC Voltage Drop: The Point-to-Point Method

NAC voltage drop is the most common reason fire alarm systems fail final inspection. The requirement is straightforward: every notification appliance on the circuit must receive at least 16 VDC to operate reliably per UL 464 and UL 1638 listings. The starting voltage on battery power is 20.4 VDC, the minimum output voltage per UL 864 (85% of nominal 24V). That gives you only 4.4 volts of drop budget across the entire NAC circuit. The point-to-point method calculates voltage at each device location sequentially.

At the first device, the full circuit current flows through the wire from the panel. At the second device, only the remaining downstream current flows through the wire between devices 1 and 2. This cumulative approach is more accurate than the conservative end-of-line method, which assumes full circuit current flows the entire distance. Wire gauge has a dramatic effect: 18 AWG stranded copper has 7.95 Ω per 1,000 feet, while 14 AWG has only 3.14 Ω, less than half the resistance. Upgrading from 18 to 14 AWG can save a circuit that would otherwise fail.

Remember that voltage drop calculations use round-trip distance (panel to device and back), so a device 200 feet from the panel has 400 feet of wire in the calculation.

NFPA 72 2022 Changes: New 1.25 Correction Factor

The 2022 edition of NFPA 72 introduced a significant change to battery calculations that affects every new fire alarm installation. Section 10.6.7.2.14 increased the battery correction factor from 1.2 to 1.25. This 4% increase accounts for battery aging, temperature effects, and the reality that SLA batteries lose capacity over their service life. A battery rated at 12 AH when new may deliver only 9.6 AH after 3–4 years of float charging. The correction factor ensures the system meets standby and alarm requirements even as the battery degrades.

For existing systems, the applicable code edition depends on when the system was permitted. Systems designed under the 2019 or earlier editions use the 1.2 factor. New installations and major modifications permitted after the jurisdiction adopts the 2022 edition must use 1.25. This calculator supports both factors so you can match the code edition your AHJ enforces. Another 2022 change requires batteries to be UL 1989 or UL 2054 listed; generic SLA batteries without these listings are no longer code-compliant for new installations.

UL 864 Compliance and AHJ Submittal Requirements

Authority Having Jurisdiction (AHJ) submittals for fire alarm systems require documented battery and voltage drop calculations. UL 864, the standard for fire alarm control panels, establishes the 20.4 VDC minimum output voltage that forms the basis for NAC voltage drop calculations. This is not an arbitrary number; it represents the panel's guaranteed minimum output at end-of-battery-life conditions (85% of 24V nominal). Your calculations must demonstrate that every device on every NAC circuit receives at least 16 VDC at this worst-case starting voltage.

The submittal package typically includes a battery calculation worksheet showing all standby loads, all alarm loads, the required AH with correction factor, and the selected battery model. NAC voltage drop worksheets must show wire gauge, distance to each device, current draw per device at the selected candela setting, and calculated voltage at each point. Many AHJs require these calculations to use UL maximum current draw values from manufacturer data sheets, not "typical" or average values. Using typical values will result in rejection.

This calculator uses UL max values by default for exactly this reason. Common rejection reasons include: using 24V instead of 20.4V as starting voltage, using the old 1.2 correction factor in a 2022-code jurisdiction, and failing to account for door holder standby current in the battery calculation.