NAC Voltage Drop on a Clinic Circuit
A 24 VDC addressable panel feeds NAC circuit 1 with 18 wall-mounted horn/strobes in a new 2-story clinic. Each appliance draws 75 mA at 24 VDC, so the connected load is 1.35 A. The panel NAC is rated 3.0 A and has a low-battery voltage of 20.4 VDC. The contractor proposes a single 16 AWG copper pair with a one-way run of 650 ft. The appliance minimum operating voltage is 16.0 VDC. The panel is not equipped with a NAC extender. The contractor’s voltage-drop sheet uses 24.0 VDC as the source and ignores low-battery conditions and wire resistance temperature effects. The AHJ is expected to require a calculation at worst-case voltage. No other loads are on the circuit. Inspection is in two weeks.
1. Calculate the end-of-line voltage at worst-case panel voltage using 16 AWG copper at 4.016 ohms per 1000 ft. Is the circuit acceptable?
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Use the loop resistance for both conductors: R = 2 × 650 ft × 4.016 Ω/1000 ft = 5.22 Ω. Voltage drop = 1.35 A × 5.22 Ω = 7.05 V. At normal 24 VDC, the end voltage is about 16.95 V, which appears to pass. At low-battery voltage of 20.4 VDC, the end voltage is 20.4 − 7.05 = 13.35 V, below the 16.0 V minimum. The circuit is not acceptable at worst-case voltage. The normal-voltage check alone is misleading. The calculation should also consider that conductor resistance rises with temperature and that appliance current can vary with voltage. The panel load of 1.35 A is within the 3.0 A rating, so the failure is voltage drop, not NAC capacity.
2. What wiring or circuit change would you recommend, and why not simply accept 14 AWG?
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With 14 AWG at 2.525 Ω/1000 ft, loop resistance is 3.28 Ω and drop is about 4.43 V. At 20.4 VDC the end voltage is about 15.97 V, still marginally below 16.0 V, so 14 AWG is not a comfortable fix. With 12 AWG at 1.588 Ω/1000 ft, loop resistance is 2.06 Ω and drop is about 2.79 V, giving about 17.61 V at low battery. That passes with margin. Another acceptable option is to split the 18 appliances into two NAC circuits or add a listed NAC extender close to the load. The recommendation should preserve circuit supervision and not exceed panel ratings. Do not simply increase the panel source setting, because the low-battery condition remains the design case.
3. The contractor says the calculation is unnecessary because the panel is rated 3.0 A. How do you respond?
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NAC rating is only a current limit; it does not prove that every appliance receives its listed minimum voltage. NFPA 72 and the manufacturer require voltage-drop calculations for notification appliance circuits, especially where long runs and multiple appliances are involved. A 3.0 A panel can still fail a 1.35 A circuit if the wire is too small or the run is too long. The correct response is to require a calculation at the lowest stated panel voltage, with the appliance minimum voltage and actual current draw, and to include the calculation in the submittal. The contractor should also confirm that the appliances are listed for the panel’s coded signal and that synchronization is maintained.