What size wire for 30 amp at 125 ft?

Over the short distance of 25 feet no voltage drop calculations are needed to be made. A #10 copper conductor with an insulation rating of 60, 75 or 90 degrees C is rated at 30 amps respectively. 30 x 80% = 24 amps. If the load is under 24 amps then this size is the one to use. If the load current is greater than 24 amps then use a conductor one size larger. That would be a #8 which is rated at 45 amps x 80% for maximum wire loading = 36 amps.