A welder nameplate read two ways — duty cycle sizes the wire, 200% sizes the breaker
Duty cycle derates the wire (Table 630.11(A)); a separate 200% rule caps the breaker (630.12(A)). That gap is why a 50 A breaker on 10 AWG wire is legal here — but only on a dedicated welder circuit.
What this diagram shows
A welder nameplate showing rated primary current of 40 amps and a 50 percent duty cycle, with two chains fanning out from it. The blue chain multiplies 40 amps by 0.71, the Table 630.11(A) duty-cycle multiplier for 50 percent duty, giving 28.4 amps, which sets the minimum conductor at 10 AWG. The amber chain multiplies the same 40 amps by 2, the 200 percent maximum overcurrent protection rule in 630.12(A), giving an 80 amp ceiling. A green banner concludes that a 50 amp breaker with a NEMA 6-50 receptacle on 10 AWG wire is legal, because 50 is under the 80 amp ceiling and 10 AWG clears the 28.4 amp floor — the breaker is allowed to exceed the wire ampacity only on a dedicated welder circuit.
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Welder Circuit Calculator
What breaker and wire for your welder? Free NEC Article 630 calculator from the nameplate primary amps and duty cycle — the rule most pages skip.
