Originally Posted by 737Pilot
The garage is detached. I do have a grounding rod at the subpanel. Right now there is also a 100 breaker in the subpanel that the URD wires attach to. I am going to be running A/C and 220 compressor/welder in the garage. I originally told the electrician that I wasn't gonna have A/C in the garage...I have since changed my mind. That's why I want to know the correct wire size for 100A. Thanks for all the help.
since you have a 125 amp panel, you can feed it with the proper wire to up the main breaker (that is what the 90 amp breaker is being used for) all the way to a 125 if you want. (and available). Make sure the breaker in the panel in the house, or whatever has the same sized breaker as well as you will not have any more power available than the smaller of the 2 breakers.
as to the 3 wire or 4 wire; there is more than it was legal pre-2008 code. Even then there were situations where you had to use 4 wire even then.
The most notable is; are there any other metallic paths between the garage and the house (or wherever the main panel is)? If so, you must use 4 wire system.
I would suggest running 4 wire simply because it removes a possible problem in the future, even if there isn't one now.