How distance changes voltage drop
Distance drives voltage drop linearly. Double the one-way wire length at the same current and gauge, and you double the volts lost along the run. Because percentage drop divides volts lost by source voltage, a 120 V circuit loses twice the percentage of a 240 V circuit for the same voltage drop.
The reference table below holds voltage (240 V), load current (30 A), material (copper), and phase (single-phase) constant so you can see exactly how 50, 100, 150, and 200 ft runs behave across common gauges.
Switching to 120 V, aluminum conductors, three-phase power, or another amperage changes every number in the table, so always run your exact circuit values in the calculator.
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.
240 V, 30 A copper voltage-drop comparison
At fixed current and conductor area, voltage loss rises in direct proportion to distance in this simplified model. At fixed distance and current, increasing conductor area lowers resistance and estimated loss.
| Copper size | 50 ft | 100 ft | 150 ft | 200 ft |
|---|---|---|---|---|
| 10 AWG | 1.55% | 3.11% | 4.66% | 6.21% |
| 8 AWG | 0.98% | 1.95% | 2.93% | 3.91% |
| 6 AWG | 0.61% | 1.23% | 1.84% | 2.46% |
| 4 AWG | 0.39% | 0.77% | 1.16% | 1.55% |
How to adapt the chart to your circuit
- 01
Replace the example current
Use the actual design load determined for the circuit.
- 02
Use the real system voltage
The same volts lost become a larger percentage on a lower-voltage system.
- 03
Measure one-way distance
Record source to load once; the calculator applies the circuit multiplier.
- 04
Select material and phase
Copper, aluminum, single-phase and balanced three-phase use different inputs.
- 05
Check the selected AWG
Run the candidate through the voltage-drop calculator, then complete ampacity and installation review.
Worked example: why 150 ft changes the comparison
At 100 ft in the example, 10 AWG is estimated at about 3.11%, while 8 AWG is about 1.95%. At 150 ft, those estimates become about 4.66% and 2.93%. The additional 50 ft changes which sizes sit above or below a 3% planning line.
That does not make 3% a universal legal limit or automatically select 8 AWG. It shows why a distance-specific calculation is more useful than a statement such as '30 A always uses one wire size.'
Common chart mistakes
- Using total loop length in a field that expects one-way distance.
- Reading a 240 V table for a 120 V circuit.
- Using breaker rating when the relevant design current is different.
- Comparing aluminum to a copper table without changing the material constant.
- Selecting three-phase only to obtain a lower result.
- Treating voltage-drop performance as proof of ampacity or code compliance.