Distribution d'énergie

Cable Voltage Drop Calculator (Cu & Al)

Estimate conductor voltage drop and percentage loss for DC, single-phase, and three-phase circuits from the current, run length, cross-section, and material, and compare it with the usual 3 % / 5 % limits. A resistance-based estimate — for long or heavily loaded cables, add the reactance term from the cable data.

System

Courant de charge (A)

Voltage (V)

One-way length (m)

Cross-section (mm²)

Conductor

Allowed drop

Voltage drop

— V

Percentage drop

—

Voltage at load

—

Against limit

—

How it is calculated

The conductor resistance for a one-way length is R = ρ × L / S, and the number of loaded conductors sets the multiplier:

Three-phase: ΔV = √3 × I × ρ × L / S Single-phase/DC: ΔV = 2 × I × ρ × L / S Percentage: ΔV% = ΔV / V × 100

where I is current (A), L the one-way run (m), S the cross-section (mm²), and ρ the resistivity at operating temperature — about 0.0225 Ω·mm²/m for copper and 0.036 for aluminium (roughly 70 °C). Single-phase and DC use a factor of 2 for the go-and-return conductors; three-phase uses √3 on the line-to-line voltage.

ⓘThis estimate uses resistance only. For large cross-sections and long runs the inductive reactance adds to the drop — include the cable’s X and the load power factor (ΔV = I(R cos φ + X sin φ)) for a rigorous figure.

Typical limits

Common design limits are about 3 % for lighting and 5 % for other uses, measured from the origin of the installation to the load. Excessive drop dims lighting, weakens motor torque on start, and wastes energy as heat. If the drop is too high, increase the cross-section, shorten the run, or raise the distribution voltage — do not simply accept it.

Questions fréquentes

Do I use one-way or round-trip length?

Enter the one-way route length. The formula already applies the factor of 2 for single-phase and DC, and √3 for three-phase, so the return path is accounted for.

Why does aluminium drop more than copper?

Aluminium has about 60 % higher resistivity, so for the same size and current it drops more. It is often sized one or two steps larger than copper to reach the same performance.

Is the drop worse on motor starting?

Yes. Starting current is several times the running current, so the momentary drop is much larger and can prevent the motor developing enough torque — check the starting condition separately.

Protect and switch the circuit

With the conductor sized for voltage drop, confirm the breaker and its coordination for the same cable.

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Normes référencéesIEC 60364-5-52 (wiring systems, voltage drop guidance) · conductor resistivity at operating temperature.

Resistance-based estimate for guidance only. Final conductor sizing must account for reactance, power factor, temperature, installation method, and the applicable wiring rules, verified by a qualified person.