NAC Voltage Drop Chart
End-of-line voltage on a 24 V notification appliance circuit by wire gauge and run length, computed for a 1 A load from a 24 V source. The last appliance must still see at least 16 V under full alarm load — cells below that are flagged. Scale the drop linearly for other loads, or read the companion wire-size chart for maximum run lengths.
End-of-line voltage at 1 A load
| Run length | 18 AWG | 16 AWG | 14 AWG | 12 AWG |
|---|---|---|---|---|
| 100 ft | 22.4 V | 23.0 V | 23.4 V | 23.6 V |
| 200 ft | 20.9 V | 22.0 V | 22.8 V | 23.2 V |
| 300 ft | 19.3 V | 21.1 V | 22.2 V | 22.8 V |
| 400 ft | 17.8 V | 20.1 V | 21.5 V | 22.5 V |
| 500 ft | 16.2 V | 19.1 V | 20.9 V | 22.1 V |
The formula, and why gauge and length trade off
Vdrop = 2 × I × L × (R ÷ 1000) → EOL voltage = 24 − Vdrop ≥ 16 V
Three things drive the drop: the current (total alarm load on the circuit), the length (one-way, but doubled for the round trip), and the conductor resistance (higher for thinner wire). To reach farther, you either drop to a heavier gauge — 14 or 12 AWG carries much lower resistance than 18 — or reduce the load per circuit by splitting appliances across more circuits. The wire size chart works the same physics the other way: the maximum run length for a given gauge and load.
Get the load from the NAC current draw chart (typical horn/strobe current by candela) for early planning, and confirm against the device data sheets. The voltage drop calculator runs a specific circuit for you.
Common questions
How do you calculate NAC voltage drop?
Voltage drop on a two-wire notification circuit is Vdrop = 2 × I × L × (R ÷ 1000), where I is the total alarm current in amps, L is the one-way run length in feet, and R is the conductor's resistance in ohms per 1000 feet. The ×2 accounts for the current traveling out and back. The end-of-line voltage — supply voltage minus the drop — must stay at or above the appliance minimum, 16 V for a 24 V system.
What is the minimum voltage for a fire alarm strobe?
A 24 V notification appliance is listed to operate over a regulated 16 to 33 V range, so the voltage at the last device on the circuit must not fall below 16.0 V under full alarm load. That 16 V floor is the appliance's UL-listing spec, not an NFPA 72 number — always use the minimum on the actual device data sheet, and the panel's minimum regulated output as the source voltage for a worst-case design.
Why does the last appliance matter most?
Voltage drops continuously along the circuit, so the appliance farthest from the panel sees the lowest voltage. If that last device stays above its minimum, every device before it does too. The whole calculation is really about the end of the line.
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