Size a conductor for both limits that matter — ampacity so it won't overheat, and voltage drop so the load actually gets its volts. Scans every standard gauge and marks the smallest that passes, with copper-versus-aluminum and run-length effects laid bare.
Tip: the FLA from the motor bench drops straight in here.
Resistance rises ~0.4%/°C above 20°C. 75°C is the usual table basis at full load.
NEC continuous loads: conductor ampacity ≥ 125% of load.
Every standard size, smallest first. Green row is the smallest that clears both ampacity and your voltage-drop target. Grey rows fail one or both — the reason is in the status column.
Two independent limits. A conductor must satisfy both: ampacity (it carries the current without overheating — a fire-safety floor) and voltage drop (enough volts reach the load — a performance floor). On short runs ampacity governs; on long runs voltage drop governs and forces a much bigger wire than safety alone would.
Voltage drop. Three-phase Vd = √3·I·L·R/1000; single-phase and DC Vd = 2·I·L·R/1000 (the 2 is the round trip down and back). R is conductor resistance in Ω/km (or Ω/1000 ft), L the one-way length. Equivalent K-form: Vd = 2·K·I·L/CM with K = 12.9 (Cu) or 21.2 (Al).
Temperature. Copper resistance rises about 0.393%/°C, aluminum 0.403%/°C, above 20°C. At 75°C operating temperature that's roughly 22% above the cold value — this tool corrects the table resistance to your stated temperature.
Aluminum vs copper. Aluminum's resistance is about 1.6× copper for the same size, so an aluminum run needs roughly two AWG sizes larger for equal drop — but it's cheaper and lighter, which wins on long feeders. The table lets you flip between them and compare.
Targets. The 3% branch / 5% total figures are NEC informational-note recommendations (and similar IEC/BS limits), not hard code — but motors, electronics, and lighting all suffer below them, so they're treated as design rules. Ampacity and final protection must follow the actual code and any derating for conduit fill, grouping, and ambient temperature, which this tool does not apply.