Originally Posted by mopowers
Well, I found the owner's manual and it says it recommends a 40 amp breaker and draws about 20 amps.
What happens if I run it on a 30 amp breaker?
That's fine. The 40 amp breaker is simply the maximum breaker allowed. There is no code requirement that says you can't go to a lower amperage breaker. The main thing is that the breaker holds when you strike the arc and load the welder.
The branch circuit conductor ampacity must not be less than nameplate I-eff (effective current) or the rated primary current (I-Max) times the duty cycle factor of table 630.11(A). This gives you minimum ampacity. You then check table 310.16 for a wire in conduit or a cable assembly that is not less than that ampacity. You then take the table value of the conductors ampacity and multiply it by 2 to reach the maximum circuit breaker rating for the branch circuit conductors. If this value is not more than 200% (2x) of the I-max or rated primary current shown on the welder nameplate then the welder does not require integral (separate) ocpd.
40 amps max breaker for the branch circuit conductors suggests 20 amp I-eff or 20 amps for supply conductors after adjustments for duty cycle using primary rated current from the nameplate. 14 awg copper would be allowed to be used for this arc welder on a maximum 40 amp breaker unless specified by the manufacturer to be different.
In general for DIY residential use ... use the I-max (rated primary current) from the nameplate without adjustments to size your branch circuit conductors then stick the appropriate breaker in the panel. 50 amp I-eff or 50 amp rated primary current (I-max) = #8 awg copper in conduit or #6 nm-b cable with 50 or 60 amp breaker. This is just a rule of thumb used by many to avoid a mistake trying to follow art. 630.