Wire Size Calculator

Selecting the Correct Wire Size Using AWG Standards

Wire sizing determines how much current a conductor can safely carry without overheating and how much voltage drops over a given distance. The American Wire Gauge (AWG) system assigns smaller numbers to larger wires: 14 AWG is standard for 15-amp residential circuits, 12 AWG for 20-amp circuits, 10 AWG for 30-amp circuits, and 6 AWG for 50-amp circuits like ranges and ovens. This calculator determines the appropriate wire gauge based on your circuit amperage, run length, voltage, and acceptable voltage drop percentage.

Voltage drop is the loss of electrical pressure as current travels through a conductor, and the National Electrical Code (NEC) recommends keeping it under 3% for branch circuits and 5% for the total system (feeder plus branch). The formula is: voltage drop = (2 x length x current x resistance per foot) / 1000. Longer wire runs and higher currents require larger (lower AWG number) wire to keep voltage drop acceptable. For example, a 20-amp circuit running 100 feet at 120 volts needs at least 10 AWG copper wire to stay under 3% voltage drop, even though 12 AWG would be adequate for the ampacity alone.

The NEC ampacity tables (Table 310.16) specify the maximum current each wire size can carry based on insulation type and installation conditions. Copper wire carries more current than aluminum of the same gauge, but aluminum is lighter and cheaper, making it common for large feeder and service entrance cables. When running wire through conduit with multiple conductors, ampacity must be derated -- three or more current-carrying conductors in a conduit require reducing the rated ampacity by 20% or more. For outdoor, underground, or high-temperature installations, always verify the appropriate ampacity adjustment factors in the NEC before selecting wire size.