Voltage Drop Calculator

Voltage Drop

Results for guidance only. Verify against current standards and manufacturer data.
Not a substitute for engineering judgement or a licensed professional.

Methodology

Voltage drop is calculated from conductor resistance, which depends on material resistivity, one-way cable length and cross-sectional area. Three-phase circuits use $V_d = \sqrt{3}\,I\,(R\,\mathrm{PF} + X\sin\phi)$ (line-to-line). Single-phase and DC circuits use Vd = 2 × I × (R·PF + X·sinφ), accounting for both the active and return conductor. DC circuits use $V_d = 2\,I\,L\,R$ instead: a DC circuit has no power factor and no reactance, so both are hidden in DC mode and take no part in the calculation. Reactance (X) is optional and defaults to zero — entering a value from a cable datasheet includes the reactive component of voltage drop. Resistivity values used are approximate operating-temperature figures (75°C) for copper (0.0211 Ω·mm²/m) and aluminium (0.0345 Ω·mm²/m) conductors — for compliance-critical work, use tabulated AS/NZS 3008.1 resistance values which account for specific cable constructions.

Worked Example

A 3-phase 415V circuit draws 40A through a 50m run of 6mm² copper cable at PF 0.85. Resistance $R = \frac{0.0211 \times 50}{6} = 0.176\ \Omega$. $V_d = \sqrt{3}\times40\times0.176\times0.85 = 10.4\ \mathrm{V}$, which is 2.5% of 415V — within the 5% guidance limit.

Assumptions & Limitations

• Cable length entered is one-way (route) length, not the conductor loop length — the calculation accounts for the return path internally
• Uses simplified resistivity figures — for AS/NZS 3000/3008 compliance certification, verify against full tabulated cable data
• Reactance (X) is optional and defaults to zero if not entered — for larger conductors and longer runs at low power factor, entering a reactance figure from a cable datasheet or AS/NZS 3008 tables gives a more accurate result
• Assumes single circuit, not grouped with derating applied