Voltage Drop in Industrial Cables: Formula, Limits & Worked Example
Every metre of cable has resistance, and that resistance steals voltage. On long runs the loss can starve motors, overheat windings, and make contactors chatter.
Here's how to compute voltage drop, the limits codes impose, and how to fix an over-budget run.
The formula
Vd = (2 × L × I × ρ) ÷ A for single-phase copper (ρ ≈ 0.0175 Ω·mm²/m). Use √3 instead of 2 for three-phase.
Worked example
A 50 m run carrying 30 A through 6 mm² copper drops (2 × 50 × 30 × 0.0175)/6 = 8.75 V — about 3.6% on 240 V, near the limit.
Code limits
IEC 60364 suggests ≤3% for lighting and ≤5% for power feeders. Exceed it and the fix is a larger conductor or a shorter route.
Related calculators
Frequently asked questions
What's an acceptable voltage drop?
Typically ≤3% for lighting circuits and ≤5% for power feeders.
How do I reduce voltage drop?
Increase conductor cross-section or shorten the cable run.
This guide is for educational purposes. Always verify against the relevant standard before final design.