Cable Size Calculator
How to calculate the cable cross-section
- Choose copper or aluminium and the type of circuit.
- Enter the voltage, the current and the one-way length of the cable.
- Set the voltage drop you allow; the minimum cross-section appears with the next standard size in mm² and AWG.
Frequently asked questions
What is the formula?
Cross-section (mm²) = k × length × current × resistivity ÷ allowed voltage drop, with k = 2 for DC and single-phase (out and back) and √3 for three-phase. The resistivity is 0.0172 Ω·mm²/m for copper and 0.0282 for aluminium at 20 °C.
Is voltage drop the only thing to check?
No. A cable must also carry the current without overheating, which depends on its insulation, how it is installed and how many cables run together. Check the result against the current ratings in your local wiring rules (such as DIN VDE 0298-4, BS 7671 or NEC 310.16) and use whichever size is larger.
How much voltage drop is acceptable?
Common limits are 3% for lighting and 5% for other circuits, measured from the supply to the load. Check your local wiring rules.
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