Size the standby battery for any fire alarm system per NFPA 72 §10.6.7 — 24-hour standby load plus 5-minute full alarm load, with derating factor applied.
This calculator sizes standby batteries for fire alarm systems per NFPA 72 §10.6.7, computing the required ampere-hour capacity from 24-hour standby load plus full-alarm load with a derating factor. Fire alarm engineers use it during system design to select a compliant sealed lead-acid or lithium battery for the FACP.
NFPA 72 §10.6.7 requires that fire alarm system batteries support the full standby load for 24 hours, followed by the full alarm load for a minimum of 5 minutes. The raw ampere-hour requirement is calculated as: C = (I_standby × T_standby) + (I_alarm × T_alarm), where currents are in amperes and time is in hours (5 min = 0.0833 hr).
Because battery capacity degrades with age and temperature, NFPA 72 §10.6.7.4 requires a derating factor — typically 20% — so the actual battery selected must be at least C ÷ (1 − D), where D is the derating fraction. Standby and alarm currents must be taken directly from the FACP manufacturer's Installation and Maintenance manual, as they vary by panel model and number of modules installed.
The primary governing document is NFPA 72 National Fire Alarm and Signaling Code, specifically Chapter 10 (Fundamentals) and Annex B (Battery Calculations). Section 10.6.7.1 mandates that secondary power (battery backup) be capable of operating the system through a complete 24-hour standby period followed by an alarm period appropriate to the occupancy type. High-rise buildings under NFPA 72 §24.4 may require 60-hour standby. NFPA 70 (NEC) Article 760 governs wiring methods, and UL 864 lists the FACP and battery together as a listed system.
Always use the worst-case alarm current — this is typically all notification appliance circuits (NACs) energized simultaneously plus FACP CPU and relay loads. At 24 VDC nominal, battery capacity is specified in ampere-hours (Ah), and commercially available batteries come in standard sizes (7 Ah, 12 Ah, 17 Ah, 18 Ah, 33 Ah). Round up to the next available standard size above the calculated derated requirement.
Battery chemistry matters: sealed lead-acid (SLA/VRLA) is the most common for FACPs, but some panels now support lithium iron phosphate (LiFePO4), which has a longer service life and flatter discharge curve. Confirm the battery is listed for use with the specific FACP model. Battery replacement intervals under NFPA 72 §10.6.7.6 are typically every 4 years for SLA or per manufacturer direction.
Enter each device type with its standby current (from the datasheet, mA), alarm current (mA), and quantity. The FACP CPU / power supply current is especially significant and must not be omitted. Set the standby duration (24 hr standard, 60 hr for high-rise), alarm duration (5 min standard), and derating factor (20% per NFPA 72). The results panel shows the raw ampere-hour demand, the derated capacity, and the minimum battery size rounded up to the nearest 0.5 Ah. Use that value to select a standard SLA battery size.
Battery capacity decreases over time due to sulfation in SLA cells and temperature effects — a battery near end-of-life at 20°C may only deliver 80% of its rated capacity. The 20% derating ensures the system remains compliant throughout the battery's rated service life, not just when it is new.
While a 12 VDC system draws the same wattage, the ampere-hour (Ah) capacity required is twice as high for the same energy content. Most modern FACPs operate at 24 VDC, and the majority of listed NAC devices (horn/strobes) are rated for 24 VDC, so 12 V is uncommon in commercial systems.
Use the RMS alarm current from the device's listed data sheet, not the peak or inrush current. For horn/strobe combinations the typical value ranges from 80 to 175 mA at 24 VDC. Multiply by the number of devices on each NAC circuit and sum all circuits to get total alarm current.
NFPA 72 allows the battery to be remote from the FACP if it is within the same room or secured area, provided wiring is sized per NFPA 70 Article 760 and is in conduit or cable assembly. Many larger systems use a dedicated battery cabinet adjacent to the FACP.
NFPA 72 Chapter 14 requires annual battery testing (load test or conductance test) and replacement every 4 years for sealed lead-acid batteries, or whenever a battery fails a load test. The FACP installer should document battery installation date on a label inside the panel.
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