How it works
Arcuracy mathAmps = thickness in inches × 1,000 (Miller's rule of thumb: 1 amp of output for each 0.001 inch of material, so 1/8 in needs about 125 A). MIG wire size comes from Miller's amperage table; wire speed = amps × Miller's multiplier for that wire size. The lens shade is OSHA's minimum for the process and current.
LimitsThe 1 amp per 0.001 inch rule is a starting point for single-pass welds on mild steel. Joint type, position, and thicker sections that pull heat away all change the setting, and above about 1/4 in many machines weld in multiple passes. The machine's own setting chart and the wire or electrode maker's range come first.
Lens shadeOSHA 29 CFR 1910.133 sets minimum shades: for example shade 10 for MIG and flux-core from 60 to 500 A, shade 8 for stick from 60 to 160 A, and shade 8 for plasma cutting under 300 A. OSHA's rule of thumb is to start too dark to see the weld zone and go lighter only until you can see it, never below the minimum.
MachinesMachines appear when the top of their maker-stated output reaches the amperage, closest first, with the stated duty cycle point nearest above it. Plasma cutters are matched on the maker's cut capacity instead.
Questions welders ask
How many amps do I need to weld 1/4 inch steel?
About 250 A by Miller's 1 amp per 0.001 inch rule for a single pass. Many 140 to 200 A machines weld 1/4 in in multiple passes; their makers state how.
What size MIG wire should I use?
Miller's table: .023 in for 30 to 130 A, .030 in for 40 to 145 A, .035 in for 50 to 180 A, .045 in for 75 to 250 A.
What shade should my welding helmet be?
At least OSHA's minimum for your process and current, for example shade 10 for MIG from 60 to 500 A. Start darker and go lighter until you can see the weld zone.
Can a 120 V welder handle 1/4 inch steel?
Usually not in one pass. Most 120 V MIG machines top out around 140 A; check the maker's stated thickness and duty cycle.