NAC Voltage Drop Calculator (Fire Alarm Circuits)
Notification appliances must operate at the worst-case voltage at the end of the circuit, which is the panel's battery-low output minus the drop in the wire. Enter the load, the run length and the wire gauge to see if it passes, and what gauge you need if it does not.
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Read the how-to guides
How to Calculate Fire Alarm Battery Size (NFPA 72)
24 hours standby plus 5 minutes alarm, the 1.2 derating factor, standard battery sizes and a worked example that lands on a 12 Ah set.
5 min readHow to Calculate NAC Voltage Drop: End-of-Line vs Point-to-Point
Panel minimum minus wire drop must leave the last appliance above its minimum. Lump-sum and point-to-point methods, with a worked example.
The math
Round-trip resistance R = 2 × length × (ohms per 1,000 ft) ÷ 1,000, using the 75°C stranded copper values from NEC Chapter 9, Table 8 and the manufacturer data (18 AWG 7.77, 16 AWG 4.89, 14 AWG 3.07, 12 AWG 1.93, 10 AWG 1.21 ohms per 1,000 ft). Drop V = I × R, and the end-of-line voltage = source − drop. If the load is spread evenly along the run the effective drop is about half.
Use the panel's published minimum NAC voltage and the device's minimum operating voltage. Appliances using a regulated supply, or a voltage-drop calculation supplied by the manufacturer, govern.
Worked example
1.5 A over 400 ft of 14 AWG: R = 2 × 400 × 3.07 ÷ 1000 = 2.456 ohm, drop = 3.68 V. From 20.4 V, the end of line sees 16.72 V, above a 16 V minimum, so 14 AWG passes.
Frequently asked questions
What source voltage should I use?
The worst-case NAC voltage: usually 20.4 V, which is a 24 V system with batteries at end of discharge, or the figure in the panel installation manual.
Does the manufacturer's calculator override this?
Yes. Panel and device manufacturers publish their own software and tables, and the AHJ will usually want those numbers on the submittal.
Is solid or stranded wire used?
Resistance is slightly lower for solid wire. This tool uses the stranded 75°C values, which are the conservative choice.
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This calculator gives an estimate for planning and is not tax, legal or engineering advice. Confirm code-related results against the code edition adopted in your jurisdiction and with your authority having jurisdiction, and follow the manufacturer's instructions.