I just bought a lower-mid priced 120V/20A welder (Forney Easy Weld 140MP), and the manual details that the welder be plugged into a 20A outlet (with a 30A circuit breaker), or you can use a provided adapter to plug into a 15A outlet (with a 20A circuit breaker) for welds requiring lower power draw.
I’ve researched a bunch on breaker/receptacle amperage and building code, and thus am now aware that multiple 15A receptacles can be on a 20A circuit.
That aside, is the suggestion from the manual common practice? I would assume that a 20A outlet be on a 20A breaker, not a 30A breaker. I would assume that a 15A outlet would be on a 15A breaker (unless I was specifically informed otherwise).
I’d figured that a 20A welder on a 20A circuit would be a-ok. I definitely don’t have 30A circuits to plug into in my humble garage.