Worked problem

Fire Alarm NAC Voltage Drop with Distributed Devices

Calculate the voltage drop segment by segment for eight notification appliances spaced along a circuit and compare it with the common end-of-line method.

Fire Protection · Last reviewed October 17, 2026

Given

A notification appliance circuit (NAC) is fed at 24 V DC. Eight horn/strobe appliances draw 0.15 A each. The first is 60 ft from the panel, and the others are 60 ft apart along a single cable run of 480 ft. Wire: 14 AWG solid copper, 2.57 Ω per 1,000 ft, so a round trip (two conductors) is 0.00514 Ω per foot of run. Exercise assumptions: the panel output at the end of battery standby is 20.4 V, and each appliance needs at least 16.0 V (check the listed range of the real product).

Find

The voltage at the last appliance by a segment-by-segment calculation, the result of treating all devices as one lump at the end of the line, and the result with 16 AWG wire.

Solution

Each 60 ft segment has a resistance of 60 × 0.00514 = 0.3084 Ω.
The first segment carries all 8 devices (1.20 A), the second 7 devices (1.05 A), and so on down to 1 device (0.15 A) in the last segment.
Total drop = 0.3084 × 0.15 × (8 + 7 + 6 + 5 + 4 + 3 + 2 + 1) = 0.3084 × 0.15 × 36 = 1.67 V.
Voltage at the last appliance = 20.4 − 1.67 = 18.7 V, above the 16.0 V minimum.
End-of-line method (all 1.20 A flowing the whole 480 ft): 0.00514 × 480 × 1.20 = 2.96 V, so 17.4 V at the end.

Answer

distributed: 1.67 V drop, 18.7 V at the last device  |  end-of-line method: 2.96 V drop, 17.4 V

Wire	Distributed drop (end voltage)	End-of-line drop (end voltage)
14 AWG	1.67 V (18.7 V)	2.96 V (17.4 V)
16 AWG (4.02 Ω/1,000 ft)	2.60 V (17.8 V)	4.63 V (15.8 V, fails)

What it shows

The end-of-line method puts the whole load at the far end, so it always overstates the drop. Here it gives 78% more drop than the segment-by-segment calculation.
It is a conservative shortcut. It is also easy to apply, and many designers use it for that reason.
With 16 AWG wire the shortcut rejects a design that the detailed calculation accepts (15.8 V against 17.8 V). Whether the more detailed method is acceptable is for the authority having jurisdiction and the panel manufacturer, who may supply their own calculator.

Common mistakes

Using the one-way length. The current flows out and back, so the circuit resistance counts both conductors.
Using the panel nominal 24 V, not the lowest voltage at the end of standby, as the starting voltage.
Using the device nominal current when the listing gives a different current at the minimum operating voltage.
Using stranded wire resistance for solid wire or the reverse, and forgetting that resistance rises with temperature.

Related tools and labs

Calculators, labs and simulators are part of the All-Access Pass.

Questions

Where do the 20.4 V and 16.0 V come from?

They are exercise values. The actual panel output and the appliance operating range come from the manufacturer listing.

Does a longer cable always fail?

Not necessarily. Heavier wire, fewer devices per circuit, or an extra power supply near the devices all reduce the voltage drop.

Does this cover the circuit current limit?

No. The total current must also stay within the output rating of the circuit, and the panel battery calculation includes the alarm load.

Educational material. Examples use simplified, hypothetical inputs and are not a design, a construction document or professional engineering advice. Confirm requirements with the codes and standards adopted where the work is built and with a licensed engineer.

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