How to use it
- Choose Supply: single-phase or three-phase, and type the Supply voltage, 230 V or 400 V.
- Type the Current the cable carries and its One-way length, from the supply to the load.
- Choose the Conductor material, then type the Cross-section in mm² or the AWG number, not both.
- Read Voltage drop as a percentage of the supply and the volts lost. Enter the Conductor temperature to see the resistance of a cable under load.
The formulas
A cable has a resistance, and the current through it drops a voltage along its length. The resistance of a conductor is R = ρL ÷ A, where ρ is the resistivity of the metal and A its cross-section. Heat raises the resistance of copper and aluminium by about 0.39 % for every kelvin above 20 °C.
R per metre = ρ ÷ A × (1 + α × (T − 20))
Single-phase: ΔV = 2 × I × R
Three-phase: ΔV = √3 × I × R
R resistance of one conductor over the length (R = R per metre × L)
ρ at 20 °C: copper 0.0172, aluminium 0.0282 (Ω·mm²/m)
α at 20 °C: copper 0.00393, aluminium 0.0039 (per kelvin)
Percentage = ΔV ÷ supply voltage × 100- Single-phase: the supply voltage is between live and neutral. The current goes out on one conductor and comes back on the other, so the length counts twice.
- Three-phase with a balanced load: the drop between two lines is √3 times the drop of one conductor.
What the calculator does not cover
- The reactance of the cable and the power factor of the load. The result is for a resistive load (cos φ = 1); for motors and long runs the reactance adds to the drop.
- The legal limit. Wiring rules set the largest drop allowed for a circuit, and they differ by country and by use. This tool gives the drop; check the rule that applies to your installation before you choose a size.
- Cable current ratings. The current a cable may carry depends on its insulation, how it is installed and how many cables are grouped, and is not part of this calculation.
Worked example: 4 mm² copper, 30 m, 20 A at 230 V
At 70 °C the resistance of one metre is 0.0172 ÷ 4 × (1 + 0.00393 × (70 − 20)) = 0.005145 Ω/m, so one conductor over 30 m is 154.3 mΩ.
For single-phase the drop is 2 × 20 A × 0.1543 Ω = 6.174 V, which is 6.174 ÷ 230 × 100 = 2.68 % of the supply.
The voltage at the load is then 230 − 6.174 = 223.8 V.
Frequently asked questions
Why does the voltage drop grow with temperature?
The resistance of copper and aluminium rises as they get warmer, by about 0.39 % per kelvin near 20 °C. A cable under load runs warmer than its reference temperature, so its drop is larger than the figure at 20 °C.
Why does the single-phase drop count the length twice?
The current goes out on one conductor and returns on the other. Both carry the full current, so the drop is 2 × I × R, where R is the resistance of one conductor over the length.
How do I use an AWG size instead of mm²?
Type the AWG number in the AWG box and leave the mm² box empty. For example, 12 is about 3.31 mm² and 1/0 is about 53.5 mm². The calculator converts the gauge with the standard AWG formula for solid round wire.
Is copper or aluminium better for a cable?
Copper has the lower resistivity, so for the same cross-section it drops less voltage. Aluminium is lighter and cheaper, but it needs a larger cross-section to give the same resistance.
What is the largest voltage drop allowed?
That depends on the wiring rules that apply to your installation and on the use of the circuit, so this calculator does not state a limit. Find the rule for your installation and compare the percentage with it.
Sources
- Wikipedia: Electrical resistance and conductance (R = ρL/A)
- Wikipedia: Electrical resistivity and conductivity (resistivity and temperature coefficient at 20 °C)
- Wikipedia: American wire gauge (diameter formula and areas)
Figures and text last checked on Tuesday, November 10, 2026.