Since most extension cords are fairly small wire, there's a large voltage drop for long lengths. Running a separate permanent line to the location would be better, because you can use much larger conductor, and set up a dedicated circuit breaker on that line for safety. Large voltage drops cause the motor on your AC compressor to draw much higher amperage. That makes them run hotter, and can force thermal shutdown. Repeatedly getting into that range will certainly shorten the life of that compressor motor. A large enough conductor, probably much like the wire in your 50 amp cable, or larger, helps reduce that voltage drop.
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