Circuit Design, Wiring Methods, and Power Supply Calculations
Circuit design
Fire alarm circuits include initiating device circuits, signaling line circuits, and notification appliance circuits. Class B circuits use an end-of-line resistor and lose the circuit if the path opens. Class A circuits provide a return path so a single open does not disable the circuit. Notification appliance circuits need voltage-drop checks: calculate the round-trip conductor resistance, multiply by load current, and confirm the appliances still receive their listed operating voltage. Excessive drop causes low sound output or strobe failure even when the panel is healthy.
Power supply calculations
Secondary supply capacity is normally 24 hours of standby followed by 5 minutes of alarm for a commercial system, unless the adopted code requires 15 minutes for voice evacuation. Use the formula: Ah = (standby current x 24 + alarm current x alarm hours) x 1.25. The 1.25 factor accounts for battery aging and efficiency. Include the FACU, annunciators, transponders, and any auxiliary loads that remain during standby. Then compare the result with the battery rating at the actual discharge rate, not just a nominal marketing number.
Worked example. Standby load is 0.20 A; alarm load is 1.5 A. Alarm time is 5 minutes = 0.0833 hour. Ah = (0.20 x 24 + 1.5 x 0.0833) x 1.25 = (4.8 + 0.125) x 1.25 = 6.16 Ah. Select the next standard size above 6.16 Ah, such as a 7 Ah battery, after verifying its rated capacity at the expected discharge rate. If the load includes a 0.10 A auxiliary device in standby, add it before multiplying.
Traps that cost marks
Using alarm current for the full 24 hours over-sizes the battery; forgetting the 1.25 factor under-sizes it. Do not omit idle panel or auxiliary loads. Do not confuse Class A redundancy with a battery backup. Do not ignore NAC voltage drop on long runs.