Metal inert gas (MIG) welding uses an electrode wire to strike an electrical arc with the metals being welded which melts and fuses the metals together. This MIG wire also melts in the welding process and serves as a filler material, and, in the case of flux core MIG wire, creates a shielding gas to protect the weld bead from contaminants. Choosing the correct size MIG wire for your project is based on the type and thickness of metals being welded.
Determine the wire type needed for the metals that you are welding. For most projects the wire you choose will be made from the same metal that you are welding. For welding carbon steel, you can opt for a flux core MIG wire that produces its own shielding gas during the arc welding process. Flux core wires are thicker in diameter than solid core wire.
Calculate the amperage required for the weld based on the thickness of the metals being joined. For example, for 1/16th inch metal sheeting, convert the fraction (1/16) to its decimal equivalent .062. As a rule of thumb, you will need one ampere for each one-hundredth of an inch of metal. Therefore, for 1/16th inch metal, you will need 62 amps of arc welding power.
Select a MIG wire diameter based on the amperage range rated for the wire. This information should be listed on the MIG wire packaging. Generally, .023 inch MIG wire can be used between 30 and 90 amps, .030 inch MIG wire should be used from 40 amps up to 145 amps, .035 inch MIG wire is used between 50 and 180 amps, and .045 inch MIG wire is used from 75 amps up to 250 amps.
Refer to a manufacturer's MIG wire diagram chart for exact wire diameter specifications. Many MIG wire manufacturers publish charts identifying the specific MIG wire diameter required based on whether the wire is flux or solid core and the metal type.