This cable size calculator finds the conductor cross-section needed to keep the voltage drop within a limit, from the current, cable run, and allowable drop. Enter the values above.
Formula (DC / single-phase, copper)
A = (2 × ρ × L × I) ÷ Vdrop (ρCu ≈ 1.68×10⁻⁸ Ω·m)
Example
20 m run, 15 A, 0.5 V drop: A = (2 × 1.68×10⁻⁸ × 20 × 15) ÷ 0.5 = 20.2 mm².
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Frequently Asked Questions
How do I calculate cable size?
Divide twice the resistivity times length times current by the allowable voltage drop to get the cross-section area.
Why multiply by two?
The current travels out and back, so the total conductor length is twice the cable run.
Does cable size depend on voltage drop?
Yes. A tighter voltage-drop limit or longer run requires a larger cross-section.