Wire Size & Voltage Drop Calculator

Wire gauge is selected based on the maximum amperage it must carry (from NEC Table 310.15(B)(16)) and the voltage drop over the circuit length. The NEC limits continuous loads to 80% of breaker rating. This calculator shows the minimum NEC gauge for your amperage and flags when voltage drop exceeds the recommended 3% for branch circuits.

Voltage drop is the reduction in voltage caused by the resistance of the wire carrying current. As current flows through a wire, some energy is lost as heat, reducing the voltage at the load end. NEC recommends keeping voltage drop under 3% for branch circuits and 5% total for feeder plus branch.

AWG (American Wire Gauge) is the US standard for wire diameter. Counterintuitively, a lower AWG number means a larger, heavier wire with more current-carrying capacity. AWG 14 is common for 15A household circuits, AWG 12 for 20A, AWG 10 for 30A, AWG 8 for 40-50A, and AWG 6 for 50-60A.

Copper is the standard for most wiring because of its low resistivity, durability, and reliability of connections. Aluminum is used for large-gauge feeders and service entrance cables because it is lighter and cheaper at large gauges. Aluminum requires special handling: anti-oxidant compound, aluminum-rated connectors, and periodic tightening of connections.

Solid wire has one conductor and is stiffer and better for permanent wiring in walls and panels. Stranded wire has multiple smaller conductors twisted together, making it more flexible and better for appliance cords, motor connections, and any application with movement. Stranded wire can also be easier to route through conduit.