You may want to go to a larger gauge than what is required for the current because 50 feet is a pretty long run. The larger the conductor, the less resistance to current flow per foot. If you think about it the wire is just like a resistor with a specific resistance per foot. Take for example 10 gauge wire. It has a resistance of .0018 ohms per foot. Over 50 feet it has a resistance of .059 ohms. If you are drawing 30 amps and calculate the power dissapated by the cable using P=Current squared x Resistance, you are dissapating 53.1 watts of power in heat or about 1 watt per foot. Thats a little high. If you go to 8 gauge wire you will dissipate only 33 watts.