Voltage Drop Calculator
Percentage voltage drop with a 5% compliance check.
Inputs
Result
Formula
| Three-phase current | P ÷ (√3 × U × cosφ) |
| Resistance | ρ × L ÷ S |
| Three-phase drop | √3 × I × R × cosφ |
| Single-phase drop | 2 × I × R × cosφ |
| Percentage | ΔU ÷ U × 100 |
| Max length | L × 5 ÷ percentage |
5 kW at 380 V on 50 m of 4 mm² copper at 0.85 power factor: 8.94 A, 0.2188 Ω, a 2.88 V drop and 0.76% — a pass. The same cable at 30 kW drops 4.55%, close to the limit.
FAQ
Why 2 for single phase and √3 for three?
Because a single-phase circuit has an out and a return leg, so there are two lengths of resistance and the drop doubles. In three phase the currents are 120° apart and vector summation gives √3 ≈ 1.732, not 2 or 3. Swapping these factors is the commonest calculation error and shifts the answer by over 15%.
Why not just use a bigger cable on long runs?
Because cost turns sharply once the size grows: capital rises linearly with area while losses fall with the square of the inverse, so there is an economic size. Beyond it the saved energy no longer pays for the copper. The usual answer for loads over 200 m is a local distribution board, or distributing at higher voltage and transforming on site.
What does low voltage actually do?
Motors suffer most: a 10% drop cuts torque by about 19% since torque goes with the square of voltage, while starting current rises, starts take longer, windings run hotter and life halves. Lighting shows it as dimmed incandescent lamps, reluctant fluorescents and unstable LED drivers. The code limit protects equipment, not just brightness.
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Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.