Cable size calculator
Size a copper or aluminium conductor by voltage drop – with a recommended standard cross-section.
Mode
Conductor material
Recommended cross-section: 2.5 mm²
Minimum cross-section: 1.66 mm²
Actual drop: 1.99 %
The calculation covers voltage drop only. The actual cross-section must also satisfy current-carrying capacity for the installation method, protective devices and applicable standards (e.g. BS 7671) – final sizing belongs to a qualified electrician.
How the calculation works
The cross-section is calculated from the allowed voltage drop: A = 2 × L × I × cos φ / (γ × ΔU) for single-phase and DC circuits; for three-phase, √3 replaces the 2. L is the one-way cable run, γ the material conductivity (copper 56, aluminium 35 m/(Ω·mm²)) and ΔU the allowed drop in volts. The result is rounded up to the nearest standard size and the actual drop is calculated.
The recommended drop is up to 3% for lighting circuits and up to 5% for other loads. Important: this calculation covers voltage drop only – the final cross-section must also satisfy current-carrying capacity for the installation method, protective devices and standards (BS 7671 in the UK). Electrical installation design belongs to a qualified electrician.
Worked examples
| Input | Result |
|---|---|
| Cu, 1-phase, 230 V, 16 A, 20 m, 3% | min. 1.66 mm² → 2.5 mm² (drop 2%) |
| Cu, 1-phase, 230 V, 10 A, 50 m, 3% | min. 2.59 mm² → 4 mm² |
| Al, 1-phase, 230 V, 16 A, 20 m, 3% | min. 2.65 mm² → 4 mm² |
| Cu, 3-phase, 400 V, 32 A, 40 m, 3%, cos φ = 0.8 | min. 2.64 mm² → 4 mm² |
Frequently asked questions
What voltage drop is acceptable?
The common recommendation is up to 3% for lighting and up to 5% for other loads (measured from the distribution board). The longer the run and the higher the current, the larger the cross-section needed.
What is the difference between copper and aluminium conductors?
Aluminium has lower conductivity (35 versus 56 m/(Ω·mm²)), so it needs roughly 1.6× the cross-section for the same drop. It is used mainly for larger sizes and service connections.
Does the calculator account for current-carrying capacity?
No – it only calculates voltage drop. The conductor must also carry the current safely for its installation method (ampacity tables) and match the protective device. Always consult an electrician for installation design.
Why is the length multiplied by two?
Current flows to the load and back, so in a single-phase circuit the resistance of both conductors applies – the route length doubles. For a balanced three-phase load, √3 is used instead of 2.
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Last updated: 16 July 2026