I am adding a sub panel to a new garage addition. New 125 amp panel with a double 50 amp breaker in the existing panel. The new sub panel will be 75 feet from the existing main panel. My concern is the correct wire gauge because of the length of the run. The cable will be inside underfloor then in the wall of existing garage just above the block foundation then into the new addition.
I also need something cleared up about the earth ground. Should I install a new copper rod for the new panel or should I extend the existing ground from the old existing panel.
I read something on another board about a parallel ground condition and they didn't explain why it was a bad thing. What is correct and why?
Thank you for your time.
Sincerely,
Scott Sliwinski

I would use #2 aluminum 4 wire SER cable.

I get that the garage addition will be attached to the main house, if so no ground rod is needed. If the garage is a separate building, then a ground rod is needed, and will connect to only the equipment ground bar in the new panel.

In both cases, the neutral is to be insulated and isolated from the equipment ground.