Running and starting are different cases
Our voltage-drop calculator takes a single current input at a time. When you evaluate a motor circuit, run two separate checks: one using full-load running amps (FLA) and a second using locked-rotor or inrush starting current from the motor nameplate.
Because across-the-line starting can pull five to six times running amps for several seconds, a wire gauge that loses only 1.1% while running can dip more than 6% during startup, causing contactors to chatter or high-pressure compressors to stall.
How do you calculate voltage drop?
Every metallic conductor has internal electrical resistance. As current flows through a run of copper or aluminum wire, part of the source voltage is lost as heat along the cable path. Using Ohm's law (V = I × R) and circular-mil resistivity at 75°C, the voltage lost across a two-wire circuit is:
where:
- VD – Total voltage drop between the supply panel and the load terminals, measured in volts (need a target gauge instead? Use the wire size calculator);
- K – Conductor resistance constant at 75°C (K = 12.9 for copper and K = 21.2 for aluminum in Ω·CM/ft);
- I – Load current flowing through the circuit in amps (estimate from wattage with our watts to amps calculator);
- L – One-way wire distance in feet (the multiplier of 2 already accounts for the return conductor); and
- CM – Conductor cross-sectional area in circular mils from NEC Chapter 9, Table 8.
Three-phase AC voltage drop equation
In a balanced three-phase system, the return currents cancel in phase opposition, so the round-trip factor of 2 is replaced by √3:
Subtract VD from the source voltage (V_load = V_source − VD) to find the actual voltage delivered to the equipment terminals. Explore all related tools in our electrical calculators hub.
Illustrative comparison
| Condition | Current | Estimated volts lost | Estimated percentage |
|---|---|---|---|
| Running | 30 A | 5.11 V | 1.11% |
| Illustrative start | 180 A | 30.65 V | 6.66% |
Data needed for a real motor check
- 01
Motor nameplate
Record voltage, current, phase and rated characteristics.
- 02
Starting method
Across-the-line, soft starter and variable-frequency drive behavior differ.
- 03
Source strength
Transformer and upstream conductor impedance affect the available voltage.
- 04
Driven load
Acceleration time and required torque matter.
- 05
Equipment limits
Verify controller, protection and manufacturer voltage requirements.
Symptoms that require investigation
- Slow or failed acceleration
- Lights dimming or contactors dropping out
- Repeated protective-device operation
- Excessive conductor or connection heating
- Large difference between calculated and measured operating voltage