How to Calculate Voltage Drop on a Fire Alarm NAC Circuit
Updated October 18, 2026 · 6 min read
Horn strobes at the end of a long notification appliance circuit (NAC) won't flash or sound right if too much voltage is lost in the wire. The math is Ohm's law. Here's the formula, the numbers to plug in, and a circuit worked through.
General guidance based on NFPA 72 (2022) and the NEC. Always check the edition your authority having jurisdiction (AHJ) enforces and the manufacturer's installation instructions.
The formula
Voltage drop = current x resistance. On a NAC the current flows out and back, so the wire length is twice the one-way run:
V drop = I (amps) x 2 x L (one-way feet) x R (ohms per 1,000 ft) / 1,000
Wire resistance
Use the DC resistance of the conductor from NEC Chapter 9, Table 8. For uncoated solid copper at 75°C:
| Wire size | Ohms per 1,000 ft |
|---|---|
| 18 AWG | 7.77 |
| 16 AWG | 4.89 |
| 14 AWG | 3.07 |
| 12 AWG | 1.93 |
Start and end voltage
Don't start the calculation at 24 V. Panels and power supplies must still operate appliances near the end of battery life, so the common design practice is to start at 20.4 V (85% of 24 V).
Appliances listed for a 24 V regulated range typically operate down to 16 V. Your end-of-line voltage must stay at or above the minimum on the appliance data sheet - check it, because some models need more.
Worked example
Twelve horn strobes drawing 0.10 A each (use the current from the data sheet at the candela setting you chose) = 1.2 A. One-way run 200 ft of 14 AWG.
V drop = 1.2 x 2 x 200 x 3.07 / 1,000 = 1.47 V. End voltage = 20.4 - 1.47 = 18.9 V, comfortably above 16 V.
Treating all the load as if it sat at the end of the run, like this, is conservative. If it fails, either calculate each segment, move up a wire size, or split the circuit.
| Wire | Drop on this circuit | End voltage | Result |
|---|---|---|---|
| 18 AWG | 3.73 V | 16.7 V | Marginal |
| 16 AWG | 2.35 V | 18.1 V | Pass |
| 14 AWG | 1.47 V | 18.9 V | Pass |
| 12 AWG | 0.93 V | 19.5 V | Pass |
Sentinel Nexus works out voltage drop and battery size as you design, from the devices you actually placed.
See it in the live demoFrequently asked questions
Why double the length?
Current flows out on one conductor and back on the other, so the circuit's resistance is the round trip.
Do I use stranded or solid resistance?
Use the value for the wire you're installing - stranded is slightly higher (14 AWG stranded is about 3.14 ohms per 1,000 ft).
What about the 10% voltage-drop rule?
That's a general power-wiring recommendation. For a NAC, the test is whether every appliance sees at least its listed minimum voltage at end-of-battery conditions.