NAC Voltage Drop on a Long Horn/Strobe Run
A warehouse retrofit adds a notification appliance circuit run of 620 feet of 14 AWG stranded copper from the FACP to the farthest horn/strobe. The circuit serves eight horn/strobes drawing a total of 2.4 A in alarm at 24 VDC nominal. The installer used the panel's default NAC output and did not perform a voltage drop calculation, stating that '14 AWG is what we always use and the first device sounds fine.' At the far end of the run, the last two devices are noticeably dimmer and quieter than the first ones, and one strobe occasionally fails to flash. The panel does not report a trouble. The AHJ questions whether the circuit meets the appliance's listed operating voltage range.
1. Explain why the far-end devices may be underperforming and what calculation the installer should have performed.
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The symptoms are consistent with excessive voltage drop along the long, small-gauge run. Voltage drop depends on conductor resistance, length (out and back), and current. With 620 ft of 14 AWG and 2.4 A, the resistance of the run produces a drop that reduces voltage at the far devices below their listed minimum operating voltage, causing dimmer/quieter output and unreliable strobe flashing. The installer should have performed a voltage drop calculation using the conductor's resistance per unit length, the total loop length (both directions), and the alarm current, and compared the resulting voltage at the farthest device against the appliance's listed minimum operating voltage. The panel's default output and the fact that the first device works do not establish adequacy; the worst-case device is at the end of the run.
2. What corrective options and verification steps should the AHJ require?
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Corrective options include: increasing conductor size (e.g., 12 AWG or larger) to reduce resistance; reducing the load on the circuit by splitting devices across additional NACs; moving the panel or adding a power supply closer to the load; or using a listed booster/notification power supply with its own supervised circuit. After correction, require a voltage measurement at the farthest device under alarm load to confirm it is within the appliance's listed operating range, verification that all devices on the circuit operate within their listed voltage, a documented voltage drop calculation, and a retest of the entire NAC for proper activation. The AHJ should also confirm the panel's NAC output and synchronization are within listed limits.