Voltage Drop Calculator: Copper and Aluminium Wire
Estimate the voltage drop along a copper or aluminium cable from its size in mm² or AWG, the one-way length, the current and the supply voltage, for DC, single-phase and three-phase runs.
Enter the wire size, length, current and supply voltage to estimate the drop.
Estimate only, not an electrical-code check.
Copper resistivity at 20 °C: 1.724 × 10⁻⁸ Ω·m.
Reactance, power factor and conductor heating are ignored.
A hot conductor drops more: copper at 75 °C has about 20 % more resistance than at 20 °C.
About the Voltage Drop Calculator
The Voltage Drop Calculator estimates how many volts a cable loses between the supply and the load. Choose copper or aluminium, give the wire size in mm² or AWG, type the one-way length of the run in metres or feet, the current and the supply voltage, and pick DC or single-phase, or three-phase. The calculator shows the drop in volts, the drop as a percentage of the supply and the voltage left at the load.
The result is an estimate only, not an electrical-code check. It uses the DC resistance of the conductor at 20 °C and ignores reactance, power factor and the heating of the cable. A conductor that runs hot has a higher resistance and drops more than the estimate. The page never shows a pass or a fail: your local electrical code, and the electrician who signs off the installation, decide what is acceptable.
How to use it
Wire and run
- Choose the conductor material and the system.
- Pick mm² and type the cross-section, or pick AWG and choose a gauge from 4/0 to 30.
- Type the one-way length: the distance from the supply to the load, not the total length of both wires. The formula already counts the return path.
Load
Type the current the load draws and the supply voltage. For three-phase, type the line-to-line voltage, such as 400 V or 480 V.
The formulas
With L the one-way length, I the current, ρ the resistivity and A the cross-section:
The resistivity at 20 °C is 1.724 × 10⁻⁸ Ω·m for copper (100 % IACS) and 2.82 × 10⁻⁸ Ω·m for aluminium. An AWG size n has a diameter of d = 0.127 mm × 92^((36 − n)/39), so 12 AWG is 2.0525 mm across and 3.3088 mm² in area.
Worked examples
- Copper, 2.5 mm², 20 m, 16 A, 230 V single-phase. The drop is 2 × 20 × 16 × 1.724 × 10⁻⁸ ÷ 2.5 × 10⁻⁶ = 4.41 V, which is 1.92 % of the supply, leaving 225.59 V at the load.
- Copper, 12 AWG, 100 ft, 20 A, 120 V. 100 ft is 30.48 m, and the drop is 6.35 V or 5.29 %, leaving 113.65 V.
- Aluminium, 16 mm², 50 m, 40 A, 400 V three-phase. The drop is 6.11 V or 1.53 %.
- Copper, 1.5 mm², 10 m, 5 A, 12 V DC. The drop is 1.15 V, or 9.58 % of a 12 V supply: low-voltage runs need thick wire.
How much drop is acceptable?
Many wiring guides aim for a drop of 3 % to 5 % between the supply and the furthest load. When an estimate is above 5 %, the calculator shows a note. To cut the drop, use a thicker conductor, shorten the run or raise the supply voltage.
Privacy
Nothing is uploaded; every calculation runs in your browser and never leaves your device.
Related tools
Work out the current a load draws with the Ohm's Law Calculator, convert run lengths with the Length Converter, or convert cross-sections with the Area Converter.
Frequently asked questions
How is voltage drop calculated?
For DC and single-phase circuits the drop is twice the one-way length times the current times the resistivity, divided by the cross-section. Three-phase runs use the square root of three instead of two.
Should I enter the one-way or the round-trip length?
Enter the one-way length from the supply to the load. The factor of two in the single-phase formula already accounts for the current flowing out and back along two wires.
Why does aluminium drop more than copper?
Aluminium has a resistivity of about 2.82 × 10⁻⁸ Ω·m against 1.724 × 10⁻⁸ for copper, so the same size of aluminium wire has roughly 64 percent more resistance and drops more voltage.
Does this calculator check my installation against the code?
No. It is an estimate only, not an electrical-code check. It ignores reactance, power factor, ampacity and temperature, and it never reports a pass or a fail.
What is a good voltage drop percentage?
Many guides aim for 3 to 5 percent between the supply and the furthest load. The calculator shows a note above 5 percent, but your local code sets the actual limit.
Why is the estimate low for a hot cable?
Resistance rises with temperature. Copper at 75 °C has about 20 percent more resistance than at 20 °C, so a fully loaded cable in a warm space drops noticeably more than the estimate.