Calculating alternator requirements

There are reference charts, materials, etc., that have amp draws for various electrical loads such as head lamps, tail lights, blower motors, and other stock equipment. You can also reference loads for cooling fans, sound systems, power seats, a/c clutches and on. Tally up the loads on one side of some paper. Then look up wire gauges and what current they can handle and see what you have in the vehicle. It comes down to protection and distribution. Think of it as sizing water pipe for a home or sprinkler system; current becomes gpm. The more water you need, the bigger the pipe to get it there. However unlike pipes which if too small you run out of water; in conductor layout, too small means hot terminals and leads to melted wiring; especially if the alternator far exceeds the system's capacity.