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Voltage Drop Calculator
Quick answer
How do I calculate voltage drop on fire alarm circuit wiring?
Multiply twice the one-way run distance in feet by the wire's resistance per 1000 feet and by the current in amps, then divide by 1000. That gives the voltage drop; subtract it from your supply voltage to find the voltage actually reaching the farthest notification appliance on the circuit. Smaller gauge numbers (thicker wire) and shorter runs drop less voltage.
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Planning tool only, not a substitute for a stamped design or the appliance manufacturer's own listing. Notification appliances are listed to operate down to a manufacturer-specified minimum voltage, commonly stated as no more than a 10 percent drop from panel voltage to the farthest device on general low-voltage wiring-practice guidance. This is a planning default the technician can edit, not an NFPA 72-mandated percentage; always confirm the actual minimum operating voltage on the specific appliance's listing label and data sheet.
How this calculator works
The formula is Voltage drop (V) = 2 x one-way distance (ft) x conductor resistance (ohms per 1000 ft) x current (A) / 1000. The standard two-conductor-circuit Ohm's-law voltage-drop calculation (out-and-back conductor length, hence the factor of 2), universal electrical-engineering arithmetic, not specific to fire alarm systems or NFPA 72.
Conductor resistance table
| AWG | Ohms per 1000 ft |
|---|---|
| 12 | 1.588 |
| 14 | 2.525 |
| 16 | 4.016 |
| 18 | 6.385 |
| 20 | 10.15 |
| 22 | 16.14 |
Standard published solid-copper-conductor DC resistance values (the same physical wire-gauge constants published in the NEC Chapter 9 Table 8 conductor-properties table and in countless free wire-gauge references), a physical property of copper conductors, not copyrighted expression. Stranded conductors of the same AWG size run slightly higher resistance; use a manufacturer's cable data sheet for a stranded cable's exact figure.
Sourcing: Ohm's law (universal physics) applied to the standard published copper-conductor resistance table (NEC Chapter 9 Table 8 conductor properties, cross-checked against independent public wire-gauge references), not sourced from NFPA 72 text. Verified 2026-07-30. See
src/data/PROVENANCE.md for the full sourcing note.
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