I am in the process of building a house at a rural site which already has 200A service to a new outdoor feed-thru panel. Currently there are only two 20A circuits powering two GFCI outlets for construction use. I plan on installing a 200A panel as a subpanel in the house (as the existing outdoor panel is the main panel). Several people have suggested I use a direct bury 4/0-4/0-4/0-2/0 aluminum bundle, but after doing lots of online research it seems like I should do it the right way and run THWN or similar in conduit. Especially since there are trees nearby. It's easy enough to find wire gauge requirements and voltage drop tables, but I can't seem to find a definitive word on what size ground wire would be needed. The house will already have a "ufer" ground from the footings brought up via 4awg copper, but since the house panel will be a subpanel, I know it has to be bonded to the existing main panel. So my question is, what size ground do I need between the existing main and the house's subpanel?