I am glad I found this question, because this almost my exact setup to my detached garage. My setup( from previous owner) is such: 60 amp 2 pole in main panel 3 wire 4 awg out to garage ( in plastic conduit) service panel in garage is a 6 slot square D with NO main breaker, and one neutral/ground bar. 6awg copper from the single bar to an 8 ft copper ground rod. no ground wire carried from the house, just 2 hots and a neutral , and the neutral is not bare (i.e. identical to the 2 hots, except yellow stripe on insulation.) There is nothing metal between the 2 structures, and they are 30 ft apart.