Wire / Cable Size Calculator
Undersized wire runs hot and starves your load. Enter the current, run length, and voltage, and get the minimum cable cross-section in mm² — with the nearest AWG — that keeps voltage drop inside your limit.
Minimum wire size
0 mm²
≈ AWG —
Estimates for planning only. This sizes for voltage drop, not ampacity — a wire can pass the drop check yet be illegal or unsafe for the current. Always verify against your local electrical code and consult a licensed electrician.
How this wire size calculator works
It solves the voltage-drop equation for cross-section: area (mm²) = k × ρ × length × amps ÷ allowed volts dropped, where k is 2 for DC/single-phase (out and back) or √3 for three-phase, and ρ is the conductor's resistivity (copper 0.0175, aluminum 0.0282 Ω·mm²/m). The result is rounded up to the next standard metric size, with the nearest AWG equivalent shown, and the "actual drop" chip recalculates the drop at that chosen size.
Remember this covers one of the two sizing requirements. The other — ampacity, the current a wire can safely carry without overheating — comes from code tables that depend on insulation, bundling, and ambient temperature. Meet both, and always let the larger wire win.
Wire size FAQ
How do I calculate what wire size I need?
Two checks: ampacity (per code tables) and voltage drop (≤ ~3%). This tool sizes for drop: area = 2 × resistivity × length × amps ÷ allowed volts, rounded up to a standard size. Confirm ampacity against your local code.
Why does wire length change the size I need?
Resistance grows with length and current flows out and back — doubling the run doubles the loss. Low-voltage (12/24 V) systems are hit hardest because each volt lost is a big share of the total.
What voltage drop is acceptable?
3% is the common branch-circuit target (5% total with feeders). Sensitive electronics and long 12 V solar runs often aim for 2%. Excess drop wastes energy and makes motors underperform.
Is aluminum wire smaller or larger than copper for the same load?
Larger — about 60% higher resistivity means roughly 1.6× the cross-section (about two AWG sizes up). It's lighter and cheaper per amp, which is why utilities use it for feeders.