Voltage Drop Calculator
Estimate the voltage lost along a wire run from length, current and conductor size.
Results update as you type.
About this calculator
Voltage drop is the voltage lost to the resistance of a conductor over its length. For a single-phase run the drop is Vdrop = 2 × ρ × L × I / A, and for three-phase it is √3 × ρ × L × I / A, where ρ is resistivity (copper ≈ 0.0175, aluminium ≈ 0.0282 Ω·mm²/m), L is the one-way length, I the current and A the conductor cross-section.
Frequently asked questions
How is voltage drop calculated?
For single-phase, Vdrop = 2 × ρ × L × I / A. The factor 2 accounts for the current travelling out and back; three-phase uses √3 instead of 2.
What voltage drop is acceptable?
A common rule of thumb is to keep total drop under 3% for branch circuits and 5% overall. Larger conductors (bigger area) reduce the drop.
Copper or aluminium — does it matter?
Yes. Aluminium has about 60% higher resistivity than copper, so an aluminium run of the same size drops proportionally more voltage.
Results are estimates for general guidance only, not financial, medical or tax advice.