The formulas
Single-phase: Vd = 2 × I × R
Three-phase: Vd = √3 × I × R
17.2 Ω·mm²/km is copper's resistivity at 20°C per IEC 60228. A is the conductor's cross-sectional area in mm², L is one-way run length in metres, and I is load current in amps. Single-phase uses a factor of 2 because current travels out and back through two conductors; three-phase uses √3 to account for the phase relationship between conductors.
Why 5%, and why temperature matters
Most electrical codes recommend keeping total voltage drop under 5% from source to load, split between distribution and final circuits. This calculator uses resistance at 20°C — a cable running hot under sustained load has measurably higher resistance than at 20°C, so a tight final design should apply your code's temperature-corrected resistance value rather than this baseline figure.
Frequently asked questions
What's an acceptable voltage drop percentage?
Most codes recommend total drop under 5% of nominal voltage from the origin to the point of use, often split as roughly 3% for distribution circuits and 2% for final circuits — check your local code for the exact split.
Why does three-phase use √3 instead of 2?
In a balanced three-phase system, the voltage drop calculation accounts for the 120° phase relationship between conductors, which works out mathematically to a √3 (≈1.732) multiplier instead of the factor of 2 used for a single-phase out-and-back circuit.
Does cable temperature really change the result much?
Yes, meaningfully on a tight design — copper's resistance rises roughly 0.4% per °C above 20°C, so a cable operating at 70-90°C under load can have 20-30% higher resistance than this calculator's 20°C baseline. For a margin-critical design, apply your code's temperature-corrected resistance value.