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Voltage drop calculator

Single-phase and three-phase voltage drop for copper and aluminium cable, in AWG or mm², with the smallest standard size that stays within the drop you choose. The formula is shown, nothing leaves this page.

FreeNo signupAWG · mm²

Supply

Conductor material

Size system

Conductor size

Cable length, one way

Length unit

Load current, amps

Supply voltage, volts (line-to-line for three-phase)

Conductor temperature, °C (20 = no correction)

Allowed drop, % (for the size finder)

Nothing leaves this page. Length is the one-way distance from the supply to the load.

All standard sizes for these inputs

Size	Area, mm²	Resistance of one conductor, Ω	Drop, V	Drop, %	Within limit?

The formulas

Single-phase / DC:  V_drop = 2 · ρ · L · I / A
Three-phase:        V_drop = √3 · ρ · L · I / A
% drop = V_drop / V_supply × 100

ρ  resistivity, Ω·mm²/m   (copper 0.0172, aluminium 0.0282, both at 20 °C)
L  one-way length, m      I  current, A      A  conductor area, mm²
Temperature:  ρ(T) = ρ20 · (1 + α · (T − 20)),  α = 0.00393 (Cu), 0.00403 (Al) per °C

This is the resistive, steady-state approximation: it ignores cable reactance, power factor, skin effect, bundling and start-up surges, which matter on large conductors, long runs and motor loads. AWG areas are the standard values from the AWG diameter formula; metric sizes are the preferred cross-sections of IEC 60228.

This page does not give ampacity (current-carrying capacity) or permitted drop limits. Both depend on the installation method, insulation, ambient temperature, grouping and your local electrical code. The 3% default is only a starting value you can change. Check the local code and the cable maker's data for both.

Planning aid; verify against local code and the manufacturer's data; the qualified person on site is responsible.