The wiring for any device must accommodate its current requirements. Welders draw large amounts of current and require substantial wiring. Table 310-16 of the National Electric Code specifies the wire sizes used for specific current draws.
Different conductor sizes with insulation support different amounts of current. For example, 8-gauge wire will handle up to 40 amps of current, while 50 amps will require 6-gauge wire.
Circuit Breaker Sizes
Circuit breaker sizes are determined by the amount of current the devices they service require. For example, a device that requires 27 amps would use a 30-amp circuit breaker. The wire gauge used to wire a circuit must also accommodate the current allowed by a circuit breaker. A circuit with a 20-amp circuit breaker needs 12-gauge wire at a minimum.
It's always best to size welder circuits and wiring based on the input current requirements of the welder. For example, 240-volt, 40- to 50-input amp welders will require a 50-amp circuit breaker and 6-gauge wiring. Welders operating at 30 to 40 input amps require a 40-amp breaker and 8-gauge wire. Small welders operating below 30 input amps can use 10-gauge wire with a 30-amp circuit breaker.