I want to add a 100 amp sub-panel from a 200 amp main in an attached garage. It will be about 100 feet from sub to main. What gauge wire should I use? The sub panel is to hook up a 7.5 horse air compressor and a welder.
The NEC provides wire sizing based on the type to be used and the application.
But here is a useful question that's already got all the answers I believe you'll need: