Hi,
To do it proffecionally you must know the starting current,the length of wires,ambient temp. ...etc just to clarify for examble under ground cable has less current density over cables on a tray you will never be able to do it witout the book I suggest fo you NEC code (National electrical code) you will find all the necessary information to choose the type and size of your cables for your specific application. If you don like to go through that book as a minimum I suggest you a booklet made by Clockner Moller (german company producing contactors ,switch gears....etc) made for their customers in a simple way. But sure I preferr the NEC book but you will pay effort, if you will do it once or twice you can send me your COMPLETE data and I can calculate the size of ewires for you.
Regards