Wire size calculator — by voltage drop

The calculator works the allowed volts (V × drop% ÷ 100) backwards through resistance per kilometre to the smallest conductor area that meets them: A = ρ × 1e9 ÷ R_req, with copper at about 0.0172 Ω·mm²/m and aluminium at 0.0282. It then rounds up to the nearest standard metric size and its AWG equivalent.

NEC 210.19 recommends keeping branch-circuit drop to 3% and total drop (feeders plus branch) to 5%, which is the default 3% used here. On 12 V and 24 V runs the limit bites fast — 3% of 24 V is only about 0.7 V of slack — so voltage drop usually drives the conductor size, not ampacity.

The required cross-section rarely matches a stock conductor, so the tool rounds up to the smallest standard metric size (0.5, 0.75, 1, 1.5, 2.5, 4, 6 mm² and up) that meets it, and picks the smallest nominal AWG whose real area still covers it. Rounding up never undersizes the conductor.