Cable Voltage Drop Calculator

Formulas

R = ρ(T) × L / A · ρ(T) = ρ₂₀ × (1 + α × (T − 20))
3-phase: ΔV = √3 × I × R · 1-phase: ΔV = 2 × I × R

Values used: copper ρ₂₀ = 0.01724 Ω·mm²/m with α = 0.00393 per °C, aluminium ρ₂₀ = 0.0282 Ω·mm²/m with α = 0.00403 per °C. For three-phase, enter the line-to-line voltage; for single-phase, the phase-to-neutral or two-wire supply voltage.

Typical limits

• Many Indian and international practices work to a total drop of about 3% for lighting and 5% overall from the supply point to the farthest load. Your own specification may differ.
• For a motor, also check the drop while it starts. The starting current is several times the running current, and a deep dip can stop it from starting.

What this estimate leaves out

• It counts conductor resistance only. For large cables, above about 50 mm², reactance adds to the drop, especially at low power factor, so the real figure is higher.
• It does not check the cable's current rating, grouping or derating, or short-circuit withstand. Use it alongside a proper cable sizing table.

To measure the actual drop, read the voltage at the source and at the load under load with a digital multimeter. The Ohm's law tool helps with the maths.