Cable voltage drop calculator
On a long line, some voltage is lost to the conductor's resistance — the load ends up with less than the source. The calculator estimates the loss from material, cross-section, length, and current.
Input data
Result
Fill in the fields — the verdict will appear here
The limit for power and lighting circuits is usually no more than 5%. Reactive resistance of the conductor is ignored (acceptable for cross-sections up to roughly 50–95 mm²).
How it's calculated
- Conductor resistance is calculated from its length and cross-section: the longer the line and the thinner the conductor, the bigger the loss.
- The multiplier depends on the network: in single-phase, current travels there and back, so the length doubles; in three-phase, √3 replaces the 2.
- The loss is compared to the supply voltage and converted to a percentage — the limit for power and lighting circuits is usually no more than 5%.
Example: Copper, three-phase, 50 m, 20 A current, 4 mm² cross-section at 380 V → a drop of 7.58 V, which is 1.99% — comfortably within limits.
ΔU = k·ρ·L·I / S, where k = 2 (single-phase) or √3 (three-phase); ρ — resistivity (copper 0.0175; aluminum 0.0294 Ω·mm²/m).
This is an estimate
Verify final values against current standards, the project design, and equipment nameplates.