Wire Size Calculator

How wire sizing works

Choosing the right wire gauge involves two constraints: the wire must have enough ampacity to safely carry the load current, and the voltage drop over the circuit length must stay within NEC recommendations (3% for branch circuits).

Ampacity vs. voltage drop

Ampacity is the maximum current a conductor can carry continuously without exceeding its temperature rating. NEC Table 310.16 provides ampacity values at 75°C. However, a wire that meets ampacity requirements may still cause excessive voltage drop on long runs — so both constraints must be checked.

Copper vs. aluminum

Copper has lower resistance and higher ampacity per gauge than aluminum, but costs more. Aluminum is commonly used for larger feeders (e.g., service entrances, sub-panel feeds) where the cost savings outweigh the larger conduit needed for the bigger wire.