I classify mig welders by the amperage they generate at a given duty cycle
-- the number of minutes in every ten the mig welder can operate. The higher the
amps, usually the shorter the duty cycle.
Thickness of metals being welded
Thin metals require less amperage; thicker metals require more amperage. For
example, to MIG weld 18 gauge steel in a single pass takes 70 amps, where
welding quarter-inch steel in a single pass requires about 180 amps. Of course,
multiple passes are made to weld thicker metals. However, this takes time, so
you have to watch your mig welder's machine's duty cycle and spend time
waiting that you could be spending mig welding.
How much amperage does your mig welder need?
For light repair work on steel, stainless steel and aluminum – from sheet
metal to material 3/16 in. thick – a 130 to 150 amp MIG welder with a 30 percent
duty cycle can perform many welding jobs. For heavier repair or fabrication
jobs, consider 200 to 250 amp MIG welders with a 40% to 60% duty cycle.
You must have enough amperage to get proper penetration on the first pass. You
can't make up for a poor first pass with more passes.
Actual amperage used depends on the type of weld, welding position, electrode
diameter, type of shielding gas, and other factors. Remember, thicker material
can be welded by making multiple passes.