Most 120-volt MIG and flux-core machines top out around 125 to 140 amps, which by Miller's 1 amp per 0.001 inch rule covers steel up to about 1/8 inch in a single pass. 240-volt machines reach 200 amps and more, with longer duty cycles at those outputs. Dual-voltage machines run on either, with lower output on 120 volts; their makers state both ratings.
Why input voltage limits output
A welder turns input power into a low-voltage, high-current arc. The power it can draw from the wall caps the arc it can make. A household 120-volt circuit is usually protected at 15 or 20 amps; a dedicated 240-volt welder circuit is often 30 to 50 amps. That's why 120-volt machines cluster around 125 to 140 amps of output and 240-volt machines reach 200 amps or more.
Miller's rule of thumb, 1 amp of output for each 0.001 inch of steel, turns that into thickness: about 125 to 140 amps welds up to roughly 1/8 to 9/64 inch in one pass, while 200 amps reaches 3/16 inch and 250 amps reaches 1/4 inch.
What each can weld
| Input | Typical top output | Single-pass steel at that output |
|---|---|---|
| 120 V household outlet | about 125 to 140 A | up to about 1/8 to 9/64 in |
| 240 V dedicated circuit | about 180 to 250 A or more | about 3/16 to 1/4 in |
Makers' thickness claims often go further, especially with flux-core wire or multiple passes; our product pages quote each claim with its wording. The duty cycle at the output you'll use matters as much as the peak: a 120-volt machine is often rated 20% or so near its maximum.
Dual-voltage machines
Many inverter welders accept 120 or 240 volts, usually through an adapter plug or a cord swap. On 120 volts they deliver a lower maximum output; on 240 volts the full rating. Makers usually state duty cycle and output for each input separately, and the 240-volt figures are what a box headline uses.
Machine pages show the maker's input voltage and output for each.
Before you plug in
- Check the circuit and plug the maker specifies; many 120-volt welders want a 20-amp circuit, not the 15-amp outlet in most garages.
- Avoid long, thin extension cords, which drop voltage and reduce output. The maker's manual states the minimum cord gauge for a given length.
- For 240 volts, match the receptacle type (often NEMA 6-50 for welders) and breaker size to the maker's requirements.
- Have a licensed electrician install any new 240-volt circuit.
Which to choose
Choose 120 volts if you weld sheet metal, exhaust, light brackets and repairs up to about 1/8 inch, need portability, or have no 240-volt supply. Choose 240 volts, or a dual-voltage machine, if you'll weld 3/16- to 1/4-inch steel regularly, build trailers or farm equipment, or want long beads near full output.
Our amperage selector can filter to machines that run on 120 volts, and shows the amps your thickness needs.
A worked example
You want to weld 1/4-inch brackets onto a truck bumper. That's about 250 amps by the rule for a single pass. A 140-amp 120-volt machine would need bevelled joints and several passes, and would spend much of the job waiting on its duty cycle. A dual-voltage machine on a 240-volt circuit at 200 amps or more is the better tool.