Wire Size Calculator
Enter the load current, one-way length and allowed voltage drop to estimate the minimum copper cross-section by BS7671 voltage-drop method, for home and light-commercial wiring selection.
Input Data
Results
At a glance:Select by voltage drop: allowed drop ΔV = V × pct/100; required copper cross-section A = 2·ρ·L·I / ΔV (ρ copper 0.0172 Ω·mm²/m, 2 is the outgoing+return path). Round up to a standard gauge (1, 1.5, 2.5, 4, 6, 10, 16…). This calculator only sizes by voltage drop and does NOT include current-carrying capacity (overload/heat) or protection coordination; in practice verify current rating per BS7671 as well.
Formula
ΔV = V × drop%
Cross-section = 2 × ρ × L × I / ΔV
$$\Delta V = V \cdot p$$$$A = \frac{2 \rho L I}{\Delta V}$$How to Use
- Enter the load current (A).
- Enter the one-way length (m).
- Enter the system voltage and allowed drop%.
- The tool outputs the minimum cross-section and suggested gauge.
Case Studies
Living-room socket circuit
20 A, 15 m, 230 V, 3%.
Min cross-section about 2.2 mm²; choose 2.5 mm².
Remote water pump
32 A, 40 m, 230 V, 5%.
Min cross-section about 4.8 mm²; choose 6 mm².
FAQ
Why is voltage drop alone not enough?
Real wire selection must meet voltage drop, current-carrying capacity (no overheat) and MCB coordination at once — take the strictest of the three.
What resistivity for copper?
Room-temperature copper is about 0.0172 Ω·mm²/m; at high temperature or continuous load you may increase it slightly for safety margin.
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References
Content review: Calculatorism Editorial Team. Results are for reference only; please refer to the relevant authorities for the official figures.