Voltage Drop Calculator
Wire Has Resistance Too
Copper conductors aren't perfect conductors — every foot of wire has a small amount of resistance, and over a long run that resistance quietly eats into the voltage that actually reaches the load. A circuit that measures perfectly at the panel can still leave a motor or appliance underpowered at the far end of a long run. This calculator applies the standard voltage-drop formula to a chosen wire gauge, length, and current so the loss can be checked before installation rather than discovered afterward.
The Formula
Percent Drop = (Vdrop / Source Voltage) × 100
The multiplier accounts for the return path: single-phase circuits use 2 (current travels out and back through the conductor), while three-phase circuits use √3 ≈ 1.732. Conductor resistance per foot is drawn from standard copper wire tables.
Copper Conductor Resistance by Gauge
| Wire Gauge (AWG) | Ohms per 1000 ft |
|---|---|
| 14 | 2.525 |
| 12 | 1.588 |
| 10 | 0.999 |
| 8 | 0.628 |
| 6 | 0.395 |
| 4 | 0.249 |
| 2 | 0.1563 |
| 1/0 | 0.0983 |
| 2/0 | 0.0779 |
| 4/0 | 0.049 |
A Worked Example
A 100-foot single-phase run of 12 AWG copper carrying 20 A from a 120 V source:
| Configuration | Voltage Drop | Percent Drop |
|---|---|---|
| Single-phase (multiplier 2) | 6.352 V | 5.29% |
| Three-phase (multiplier √3) | 5.501 V | 4.58% |
Both exceed the commonly recommended 3% maximum — this run would call for a heavier gauge or a shorter path.
Where This Calculation Matters
- Long outbuilding or subpanel runs — feeding a detached garage or barn often means hundreds of feet of conductor, where drop adds up fast.
- Motor performance — motors are sensitive to undervoltage, which can cause overheating and reduced torque even when the load isn't excessive.
- Code compliance — while not always a hard code requirement, a 3% branch-circuit / 5% combined limit is a widely followed design guideline.
- Solar and battery wiring — low-voltage DC runs (12V or 24V systems) are especially sensitive to voltage drop percentage-wise.
How to Use This Calculator
- Select the wire gauge (AWG) from the dropdown.
- Enter the one-way run length in feet.
- Enter the current the circuit will carry, in amps.
- Enter the source voltage.
- Select Single-Phase or Three-Phase, then select Calculate.
Related Calculations
Not sure the gauge is adequate to begin with? Start with the Wire Size Calculator, or check total circuit demand with the Electrical Load Calculator.