Im trying to add a 600 watt max active subwoofer in my truck.
what gauge power wire should i use?

 

Wire for the sub, right? I would use 14 gauge wire. 16 gauge might work well, but if you are pushing that much of a load RMS, you may want the larger gauge. Don't cheap out by using lamp cord or something else generic. Even though it's a sub, lampcord or other generic wire doesn't usually distinguish polarity. You could reverse the phase if you don't mark it before installing it. While it's not a problem, it does create a chore if you don't have a multimeter to ring out the lines if you forget to mark it. The larger the wire, the less resistance it will have. The lower the resistance, the less heat it will carry which will reduce the risk for burning it. I have always used shielded signal wire for my audio.