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It is possible to make a super-long extension cord. You MUST give due regard to durability, ampacity, and voltage drop. Still, such a thing can come in quite handy for situations like this. A few years ago I bought a spool of 250 feet of 10/3 SOOW hard service cord at a "going out of business sale" of a failed big box retailer. It cost me $100. It has paid for itself several times over. The resistance of 10AWG wire is .00118 ohms per foot, making the two-way resistance of my "extension cord" about 0.56 ohms. At 10 amps (typical of heavy duty 120 volt power tools), the voltage drop is 5.6 volts or about 5%. This is a little higher than the 2 to 5 % goal of the NEC, but provides perfectly acceptable results for use in a pinch. At 240 volts, you can go up to 20 amps with this same 5% drop. At 30 amps, it would be 7.5 percent -- or a drop from 240 volts to 228 volts. Still very much within the operating range of most 240 volt equipment. I've used this to run a 3HP well pump (through a VFD -- long story). Let us consider your welder. You probably won't be using the limits of its amperage (nor its maximum duty cycle) to weld a fence. Let's say you set the welder dials to 100 amps. At the welder's typical output of 30 volts, and allowing for some inefficiency, this would translate to about 20 amps from a (voltage sagged) 220 volt supply. It will draw higher current as you strike the arc, but not for very long. A 10AWG long extension cord should operate the welder fine FOR THIS APPLICATION. The other posters are correct in saying that gas torches, or generators, or motor operated welders, would be better and would be the "right" way to do it. But if I had the cord, I would give it a try before paying $$$ at the rental store. For anyone reading this, please don't use the above discourse to justify doing stupid things like trying to weld up 1/2 inch plate for hours on end, or connecting 5 horsepower motors to long cords (remember the 5 to 6x motor start current), or hooking up your whole shop to an extension cord, or going over the ampacity of the cord, or using NM wire for portable applications. If you burn your house or shop down misusing electricity, it's your own fault.
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