According to NEC, (National Electrical Code), #4 THHN is suitable for 95 amps, #4 THWN is suitable for 85 amps and #4 THW is suitable for 70 amps. THHN insulation is 90 degrees C, THWN is 75 degrees C and THW is 60 degrees C. So insulation type is important but 99.99 percent of the wire theses days for these applications is THHN. I don't think your going to see a significant voltage drop in just 50 feet either, if any, even if under a 60 percent load. Happy Camping in the barn though, that's where mine is right now.