Notification appliance circuits (NACs) lose voltage along the wire. If the voltage at the last appliance falls below its rated minimum, horns and strobes may not work as listed. The conservative check assumes the whole load sits at the far end of the circuit.
The method (end-loaded)
- Drop (V) = 2 × one-way length (ft) × load (A) × wire resistance (ohms per 1,000 ft) ÷ 1,000. The 2 accounts for the out and back conductors.
- End voltage = starting voltage − drop. 20.4 V (85% of 24 V nominal) is a common end-of-battery-life starting value; use your panel's rated value if it differs.
- Pass if the end voltage is at least the appliance's minimum operating voltage (16.0 V is common for "regulated 24 V" appliances; check the data sheet).
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Results
Resistance in ohms per 1,000 ft, uncoated copper at 75 °C, from standard published conductor data.
Worked example
14 AWG stranded (3.14 Ω/1,000 ft), 300 ft, 1.5 A: drop = 2 × 300 × 1.5 × 3.14 / 1,000 = 2.83 V. End voltage = 20.4 − 2.83 = 17.57 V, which passes a 16.0 V minimum. Max length at that load is (20.4 − 16) × 1,000 / (2 × 1.5 × 3.14) = 467 ft.
Checking a whole riser? The NAC Voltage Drop Calculator workbook checks 30 circuits per sheet with pass/fail and max length for each.