Battery Sizing for a Fire Alarm Control Unit
A fire alarm control unit (FACU) is being installed in a small commercial building. The system includes 10 smoke detectors, 5 pull stations, and 8 notification appliances (horns/strobes). The FACU has a primary power supply rated at 24 VDC, 4 A. The standby battery must be sized to support the system for 24 hours of standby followed by 5 minutes of alarm. The contractor has proposed a 7 Ah battery. The installation instructions for the FACU specify a maximum battery size of 18 Ah and a minimum of 7 Ah. The contractor has provided the following current draw data: smoke detectors: 0.05 mA each in standby, 0.05 mA in alarm; pull stations: 0 mA; notification appliances: 0 mA in standby, 0.5 A each in alarm; FACU: 0.1 A in standby, 0.2 A in alarm. The plan reviewer must verify if the proposed battery is adequate.
1. Calculate the required standby battery capacity in ampere-hours for the system.
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First, calculate the total standby current: smoke detectors: 10 x 0.05 mA = 0.5 mA = 0.0005 A; pull stations: 0 A; notification appliances: 0 A; FACU: 0.1 A. Total standby current = 0.1 A + 0.0005 A = 0.1005 A. For 24 hours standby, capacity = 0.1005 A x 24 h = 2.412 Ah. Next, calculate the alarm current: smoke detectors: 10 x 0.05 mA = 0.0005 A; pull stations: 0 A; notification appliances: 8 x 0.5 A = 4 A; FACU: 0.2 A. Total alarm current = 0.2 A + 0.0005 A + 4 A = 4.2005 A. For 5 minutes (0.0833 h) alarm, capacity = 4.2005 A x 0.0833 h = 0.35 Ah. Total required capacity = 2.412 Ah + 0.35 Ah = 2.762 Ah. However, NFPA 72 requires a safety margin of 20% for battery capacity, so required capacity = 2.762 Ah x 1.2 = 3.314 Ah. The proposed 7 Ah battery is greater than 3.314 Ah, so it is adequate. But also check the maximum battery size: 7 Ah is within the 18 Ah limit. Also, the battery must be able to supply the alarm current for 5 minutes, which it can. Therefore, the 7 Ah battery is acceptable.
2. What other factors must be considered when sizing the battery, according to NFPA 72?
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NFPA 72 requires that the battery be sized to include a 20% safety margin. Also, the battery must be capable of providing the required standby and alarm currents at the end of its useful life, so derating factors may apply. The battery must be charged within the time specified by the manufacturer, typically 48 hours, and the charger must be capable of recharging the battery within that time. The battery must be tested periodically. The contractor should also consider the temperature effects on battery capacity; batteries are rated at 77°F (25°C), and capacity decreases at lower temperatures. The installation location should be within the temperature range specified by the battery manufacturer. Additionally, the battery must be listed for fire alarm use. The plan reviewer should verify that the battery calculations include all devices, including any auxiliary devices, and that the battery size does not exceed the maximum specified by the FACU manufacturer.