Fire alarm voltage drop calculator
Estimate circuit loss before you reach the last device.
Planning estimate. Verify before applying to a project.
How the calculation works
Vdrop = 2 × one-way length × resistance per 1,000 ft × current ÷ 1,000
Worked example
A 300 ft run at 1 A with 2.525 Ω/1,000 ft resistance loses 1.515 V. A 24 V source gives an estimated 22.485 V at the end.
What this estimate covers
This lumped-load estimate places all current at the end of the run. Distributed loads, regulated outputs, temperature, appliance behavior, and circuit topology can change the result. Supply voltage must reflect the required operating condition, including secondary power.
Southwire voltage drop reference
Keep your work with the project
Review the related fire alarm calculators and NICET practice samples. Explore calculation tools, project notes, PDF reports, and plan markups in Fire Alarm Field Pro.
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Common questions
Why does a fire alarm voltage drop formula use twice the wire length?
A two-wire circuit has an outgoing conductor and a return conductor. When both have the same length and resistance, the current passes through twice the one-way length. Enter the one-way distance in this calculator; do not double it first.
Should I enter 24 V or the supply’s minimum operating voltage?
Enter the supply voltage for the condition you are checking. A nominal 24 V label is not necessarily the minimum voltage available on secondary power. Use the actual supply documentation and repeat the estimate for each required operating condition.
Does this calculate every device along a NAC circuit?
No. This tool puts the total entered current at the end of a two-wire run. A segmented calculation follows the device locations and current remaining in each wire segment. That is a different model, particularly when appliance current varies with voltage.
How do I know whether the end-device voltage is acceptable?
Compare it with the published operating requirements of the actual appliances and supply. The calculator shows margin above the minimum you enter; it does not establish compatibility or system approval.