How Wire Gauge Is Determined
This calculator uses the standard voltage drop formula based on wire resistance, current, and one way distance, then checks each standard AWG wire size until it finds the smallest gauge that keeps voltage drop under your selected limit. Smaller AWG numbers mean thicker wire and less resistance, so as your amperage or distance increases, a heavier gauge is needed to keep the voltage drop acceptable.
This tool estimates voltage drop only. It does not replace ampacity tables for wire insulation temperature ratings, conduit fill limits, or local electrical code requirements. Always verify final wire selection against your local code and manufacturer ampacity charts before installation.
Why Voltage Drop Matters
As current travels through a wire, resistance causes some voltage to be lost as heat before it reaches the load. Too much voltage drop means motors run hotter and less efficiently, lights dim, and electronics can behave erratically. The National Electrical Code recommends keeping voltage drop under 3 percent for branch circuits and under 5 percent total from service to the farthest outlet.
Copper vs Aluminum Wire
- Copper: Lower resistance, smaller gauge needed for the same load and distance, more expensive per foot
- Aluminum: Higher resistance, requires a heavier gauge for the same performance, lower material cost, common in long outdoor or service entrance runs
Common Situations Where This Matters
Long extension cord runs to a shop or barn, sizing wire for a sub panel, running power to a well pump, wiring a garage or workshop from the main panel, and any DC system like solar or automotive wiring where voltage is already low and drop percentage matters more. In all these cases, distance is often the factor people underestimate when picking wire size.