Reno, I've seen the aftermath of a gross difference in distance of parallel feeders (about 5 feet on 4/0) one raceway got so hot the paint all burned off of the wall behind it. Wish I'd had a camera then, melted globs of insulation running out of the end of the conduit. One of the guys I was working with at the time said that since ti was AC power the Eddy currents contribute to the heat as well.
