Voltage Drop Calculator
Select the wire size and material, then enter the one-way length, load current and supply voltage. The result shows the voltage lost in the wire and the voltage reaching the load.
Result
- Voltage at the load
- 114.21 V
- Wire resistance
- 1.9315 Ω per 1,000 ft
- Power lost in the wire
- 86.9 W
The drop is above the 3 percent commonly recommended for branch circuits. Resistance is calculated for a solid conductor from its cross-section and resistivity. AC reactance is not included, which matters for large conductors and long runs.
Formula
- Single-phase or DC: V_drop = 2 × L × I × R
- Three-phase: V_drop = √3 × L × I × R
- R = ρ × (1 + α × (T − 20)) ÷ A, per unit length
- AWG diameter (in) = 0.005 × 92^((36 − gauge) ÷ 39)
How the Voltage Drop Calculator works
Every wire has resistance, so some voltage is lost along its length. The loss equals current times the resistance of the complete path. In a two-wire circuit the current travels out and back, which is why the one-way length is doubled.
Resistance rises with temperature. Copper at 75 °C has about 22 percent more resistance than at 20 °C. The US National Electrical Code suggests keeping the drop to 3 percent on a branch circuit and 5 percent for feeder and branch combined.
Worked example
A 15 A load 100 ft from the panel on 12 AWG copper at 75 °C: the wire is 1.93 Ω per 1,000 ft, so the drop is 2 × 100 × 15 × 0.00193 = 5.79 V. On a 120 V circuit that is 4.83 percent, leaving 114.2 V at the load.
Frequently asked questions
What is an acceptable voltage drop?
The National Electrical Code recommends no more than 3 percent on a branch circuit and 5 percent total for feeder plus branch. These are recommendations for efficiency, not mandatory limits in most cases.
How do I reduce voltage drop?
Use a larger wire, shorten the run, raise the supply voltage or reduce the load current. Each step up of three AWG sizes halves the resistance.
Why is the length doubled for single-phase?
Current flows through the hot conductor to the load and returns through the neutral, so it passes through twice the one-way length of wire.
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