We recently purchased a new home that has a 16 x 30 steel building in the back yard approximately 100ft from the main panel. The building has power run to it off a 60 amp breaker in the main panel. The current sub panel is an old fuse box that desperately needs an upgrade. It appears that at some point a previous owner started to upgrade the electrical but never finished. There are three #2 aluminum feeders 2 hot and one neutral but no ground. I am going to replace the sub panel with a new GE panel and want to know the proper way to add a ground. Can I just run #4 wire from the panel to two grounding rods outside of the building or do I need to run a new wire back to the main?
The big reason I'm asking is that if I am going to pull a new wire I'm going to end up redoing the whole feed with a larger wire so that I can run a 125 amp sub panel. The building will eventually become my new shop with power hungry equipment. If I can safely get it up and running now for a minimum of expense that would be great otherwise I will have to plan for the larger project. Thanks for the advice.