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Fire alarm voltage drop calculator

Voltage drop on a notification appliance circuit = 2 × one-way length in feet × alarm current in amps × wire resistance in ohms per 1,000 ft ÷ 1,000. Subtract it from the panel’s output voltage to get what the last device sees, and compare that with the lowest voltage the device is listed to run on. Start from 20.4 V, a 24 V system at the end of battery life, unless the panel’s manual gives another figure.

Method: Ohm’s law on a two-wire run, as in AlarmAmps. NFPA 72 requires notification appliances to operate within their listed voltage range; the panel and device figures come from their datasheets. Not affiliated with or endorsed by NFPA. NFPA 72 is a trademark of the National Fire Protection Association. Last checked 15 October 2026.

Circuit

Total alarm current of every device on this circuit, in amps (61 mA = 0.061).

Panel to the last device.

One conductor, from the cable datasheet.

Voltages

Result

Enter the circuit current, run length and wire resistance.

AlarmAmps runs this offline on iPhone and saves the job.

Worked example

19.48 V at the last device: passes

Computed by the same code as the calculator above.

One notification appliance circuit with ten horn/strobes at 61 mA each in alarm, 300 ft from the panel to the last device, on wire whose datasheet gives 2.525 Ω per 1,000 ft. The calculation starts at 20.40 V and the devices are listed down to 16.00 V. The currents are example figures, not values for any real model.

  1. Alarm current on the circuit: 10 × 61 mA = 0.61 A.
  2. Voltage drop: 2 × 300 ft × 0.61 A × 2.525 Ω per 1,000 ft ÷ 1,000 = 0.92 V
  3. End-of-line voltage: 20.40 V − 0.92 V = 19.48 V
  4. Percent drop: 0.92 V ÷ 20.40 V × 100 = 4.5%
  5. Check: 19.48 V against the 16.00 V device minimum = passes

Solved the other way, the same load on the same wire reaches 1,428 ft before the last device drops below 16.00 V.

Limits, and when to ask your AHJ

  • Not a substitute for engineered design. The designer of record is responsible for the final calculation. Verify against the code edition adopted by your AHJ and the equipment’s installation manuals.
  • Wire resistance comes from your cable’s datasheet. There is no built-in wire gauge table, because resistance varies with conductor, stranding and temperature.
  • It treats the whole load as sitting at the far end of the run, the worst case. A point-to-point calculation that spaces the devices out will show less drop.
  • It checks one circuit at a time. It does not check the panel’s NAC rating, the battery, or what a booster power supply does to the starting voltage.
  • Ask your AHJ or the panel manufacturer if the manual specifies its own voltage drop method, starting voltage or maximum percentage.

Check every circuit and print the submittal in AlarmAmps

Battery sizing, voltage drop per circuit, saved projects and a PDF submittal sheet for the AHJ, offline on your iPhone. Free download with a one-time unlock.

Download AlarmAmps on the App StoreAlarmAmps details

Questions

Why multiply by 2?

Current flows out on one conductor and back on the other, so a 300 ft run is 600 ft of wire. The formula uses the one-way length and doubles it.

Why start at 20.4 V and not 24 V?

A 24 V fire alarm system has to keep working on battery, and a battery near the end of its standby period sits around 20.4 V, which is 85% of 24 V. Starting there checks the worst case. Use the panel manufacturer’s figure if its manual gives a different one.

What is the minimum voltage for a horn/strobe?

It is on the device’s datasheet as its listed operating range. Many 24 V notification appliances are listed down to 16 V, which is the default here, but check the exact model you are installing.

Where do I find the wire resistance?

On the cable manufacturer’s datasheet, in ohms per 1,000 ft for one conductor. Enter that figure, not a value for the pair.

What does the AlarmAmps app add?

The app runs this check free for one circuit, plus the standby battery calculation. Submittal Pro, a one-time $4.99 purchase, adds multiple circuits and booster panels, multiple projects, a saved device library and the PDF submittal sheet.

More free tools

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