Adding Horn/Strobes to an Existing Fire Alarm NAC
A fire alarm technician is adding 10 wall-mounted horn/strobes to one notification appliance circuit (NAC) in a 4-story hotel in Albany. The existing NAC is 16 AWG FPLR, 520 ft from the panel to the last device. Each new horn/strobe draws 90 mA at 24 Vdc in alarm. The panel's NAC is rated 2.0 A continuous and is currently loaded to 0.7 A before the new devices. The panel is 24 Vdc nominal, but the measured terminal voltage under alarm is 26.4 Vdc. The technician wants to use the existing 16 AWG cable and add the devices at the end. The owner also wants a 24-hour standby battery calculation for the new load. The panel has a 7 Ah battery and a standby load of 120 mA; alarm load with the new devices will be 1.6 A. The fire marshal requires synchronized strobes and audibility per NFPA 72.
1. Calculate the NAC current and end-of-line voltage. Is the existing 16 AWG cable acceptable for the new load? Show your reasoning.
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The new device load is 10 x 0.090 A = 0.90 A. Adding the existing 0.70 A gives 1.60 A, which is below the panel NAC rating of 2.0 A, so the panel output capacity appears adequate on a steady-state basis. For voltage drop, 16 AWG copper has about 4.02 ohms per 1,000 ft per conductor. The circuit length is 520 ft one way, so the loop is 1,040 ft, giving about 4.18 ohms. At 1.60 A, the drop is 1.60 x 4.18 = 6.69 V. Starting at 26.4 V at the panel, the last device sees about 19.7 V. That may be within the device's listed operating range if the range is 16-33 V, but it is close enough to the low end that the technician must check the manufacturer's data for inrush current and synchronization. Horn/strobes draw higher current briefly at strobe flash, so the steady-state calculation alone is not enough. If the device data show a minimum of 20.4 V or if the inrush drop is excessive, the cable should be upsized to 14 AWG, which would reduce the drop to roughly 4.3 V and leave about 22.1 V at the last device.
2. Perform the battery calculation and state whether the 7 Ah battery is adequate.
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The standby load is 120 mA for 24 hours: 0.120 A x 24 h = 2.88 Ah. The alarm load is 1.60 A for 5 minutes: 1.60 A x 5/60 h = 0.133 Ah. The raw total is 3.013 Ah. Applying a 20 percent aging or derating factor gives about 3.62 Ah. On the face of it, a 7 Ah battery has enough capacity, but the calculation must use the panel manufacturer's required standby and alarm durations, the actual battery derating at the expected temperature, and the panel's charger capability. The technician should also confirm that the 1.60 A alarm load includes the panel's own alarm current, not just the NAC devices, and that the battery is listed for fire alarm use. If the panel requires 24 hours standby plus 5 minutes alarm and the battery is in a warm mechanical room, the 7 Ah battery may still pass, but the results should be documented. If the calculated requirement exceeds about 80 percent of the battery rating, the next standard size should be used.
3. What fire alarm code and listing issues must be checked before the devices are connected?
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The technician must confirm that the new horn/strobes are listed for fire alarm use and are compatible with the panel's synchronization protocol. NFPA 72 requires visible and audible notification to meet the design criteria, and the fire marshal has specifically required synchronized strobes. The existing FPLR cable is suitable only if the run does not enter a plenum; if the cable passes through a return-air ceiling, it must be replaced with FPLP or installed in a plenum-rated raceway. The NAC must remain supervised and power-limited as designed; adding devices must not create an unsupported T-tap or an unlisted circuit extension. The technician should also check the panel's NAC rating and the battery calculation, verify the device candela rating and mounting height, and confirm audibility in the sleeping rooms. Finally, the work should be coordinated with the fire alarm permit and inspection process, and the new devices should be tested and documented per NFPA 72 after installation.