Google pulled up this page using the search string "voltage drop gauge":
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at the bottom is a calculator to figure voltage drop.
For 80 feet of 10 gauge @ 240V w/ 20A load, the drop is almost 4V or 1.6% to 236V.
For 8 gauge wire the drop is only 2.4V or 1%.
For a transformer welder that maxes out at 20A, I would think that neither drop would be a problem.
If you surf over to Miller:
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The manual for a MM175 has the follwing guidlines: Input V: 230V Input current: 20A Max recommended fuse rating: 30A Min input conductor size: 14 gauge Max rec. input cond. length: 67' Min grnd cond. size: 14 gauge
I would say if you go longer than 80' or you anticipate a load closer to 30A, go up to 8 gauge.
Hope this helps.
Jeff Dantzler