Wireing help PLEASE!!!

I had my square d ps set up so the power coming in is a switched power source and the out goes to my relay, so today i pulled out of my driveway and the switched wire overheated and melted off the plastic and set my carpet on fire. Luckily it wasnt my truck but what i need to know is a better way of wireing it. Im sure i should have used bigger wire and a smaller fuse but this time i think im gonna hardwire it with a switch. Can someone help me please.