Hi according to the ugly book I just bought page 117 a separate building/structure supplied by feeder circuit shall have equipment grounding conductor sized iaw table 250.122 run with the supply conductors and connected to the building disconnecting means and grounding electrode. So what I gather from this now you have to run 4 wires in a trench. The ground and neutral is tied/bonded together in the panel boxes at both ends, along with the ground rods. Am I close? So if I plan to run a 100 amp feeder circuit, a 2-2-2-4 aluminum cable would be required? Thanks for the help. Dave

You will need a 4 wire cable, as you already know. The ground and neutral must be separated at the sub panel and a ground rod needs to be used as well. The ground and neutral are only tied together at the main panel, not at any of the sub panels. The size of the cable may need to be larger depending on the length of the run. The Ugly book has that info as well.