Most welders come with a fairly short cord, mine is six feet long. Sometimes I need to move the welder to the far side of the garage to make a weld. The first step in making an extension cord, is identifying the type of receptacle in your shop. My garage has a NEMA 6-50 that is on a 30 amp circuit breaker.
I purchased a plug and receptacle to match the receptacle on my wall and the plug on my welder.
I wanted a 25 foot extension cord. Using a wire size calculator, I determined that a 240 volt, 30 amp circuit, 25 feet in length, would require 10 gauge copper wire. There are smart phone wire size calculator apps. Additionally, most wire and cable manufacturers have online wire size calculators. A 240 volt welder requires a three conductor circuit. Two of the conductors are hot and the third is a ground wire. A ground wire is used to clear a ground fault. If a hot wire were to short to a metal part of the welder, a large current will flow through the ground wire and trip the circuit breaker (clear the fault). This is to help prevent a shock to the operator. I chose 10-3 SOW cord. The SOW cord is flexible, rated for 600 volts, and is oil and weather resistant.
I used a utility knife to strip the outer insulation from the SOW cord. It is important not to cut into the insulation of the inner conductors. You can get an idea of the depth of cut by bending the wire. I cut most of the way through the insulation and then tear the remainder by hand.
After stripping the outer insulation, I cut off the inner filler material with diagonal cutters.
Sometimes an electrical component has a strip gauge that shows how much insulation to strip from the conductor.
If there is no strip gauge, strip enough insulation so that the conductor can make full contact with the terminal.
The connection points on plugs and receptacles are often color coded. These blades for my plug are a perfect example. The left is brass colored for a hot wire. The center is silver for a neutral conductor. A 240 volt welder circuit does not use a neutral conductor, so a second hot conductor will be attached to this blade. The right blade is colored green. The ground wire will be connected to this blade.
This picture shows the wired plug. Line 1, a hot wire, is the black wire on the left. The ground wire is the green wire connected to the center prong. Line 2, a hot wire, is the white wire on the right. Red is usually used to indicate line 2, but 3 conductor SOW cord does not come with a red wire. This plug is molded and has pegs so that the blades can only be inserted into the correct slots. The strain relief portion of the plug clamps down on the outer insulation as seen in the lower left corner.
This picture shows the completed receptacle portion of the extension cord.
This is the completed 25 foot welder extension cords. If you decide to build one for yourself and have any doubts about the construction, get the assistance of an electrician.