Voltage Drop Calculator
Enter the wire size, the length of the run, the current and the supply voltage. You get the voltage drop, the percentage, the voltage at the load and the power lost in the wire.
Your numbers
How we calculate it
A wire has resistance R = ρ × L ÷ A, where ρ is the resistivity of the metal, L the length and A the cross-section. The voltage drop is V = I × R. In a DC or single-phase circuit the current goes out and comes back, so the resistance is for twice the one-way length. In a three-phase circuit the drop between the lines is √3 × I × R for one conductor.
The AWG gauge sets the diameter: d = 0.127 mm × 92^((36 − n) ÷ 39), so 12 AWG is 2.053 mm. This calculator uses 1.7241 × 10⁻⁸ Ω·m for copper (100% IACS) and 61% of that conductivity for aluminum, at 20 °C. For 15 A over 50 feet of 12 AWG copper at 120 V, the round trip resistance is 0.159 Ω, the drop is 2.38 V (1.99%) and 35.7 W is lost in the wire.
Assumptions and limits
- A common recommendation is to keep the drop to 3% on a branch circuit and 5% on the feeder and branch circuit together. These are guidelines, not rules for your wiring. Follow your local electrical code and have an electrician check the design.
- Resistance rises with temperature, so a hot wire drops more than shown. Long runs, motor starting current and AC effects in large conductors are not included.
- The current is taken as steady. The drop is shown against the supply voltage you enter, so enter the line voltage for three-phase.
Frequently asked questions
What is an acceptable voltage drop?
A common guideline is a total drop of 5% or less, with no more than 3% on any one circuit. Sensitive and motor loads can need less.
How do I reduce voltage drop?
Use a thicker wire (a lower AWG number or a bigger mm² size), shorten the run, lower the current or raise the voltage. Copper drops less than aluminum of the same size.
Why is the length doubled for single-phase?
The current flows through the wire to the load and back through the return wire, so it passes through twice the one-way length. Enter the one-way length and the calculator doubles it.
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Worked example (a fixed example using the starting numbers, it does not change as you type)
Inputs: Circuit = DC or single-phase (out and back); Conductor = Copper; Wire size = 12 AWG; Current (amps) = 15; One-way length of the run = 50; Length unit = Feet; Supply voltage = 120.
- Voltage drop 2.38 V
- Voltage drop percent 1.99 %
- Voltage at the load 117.62 V
- Wire resistance 0.1588 Ω
- Power lost in the wires 35.7 W
- Check Within the commonly recommended 3% for a branch circuit
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