Im moving my service entrance from my house to a shop 75' away (in Tucson AZ, with TEP). The new service will be 400A (well 320/400) with the shop eating 200A and a backfeed to the house underground to its original panel (but moving the feed from overhead wire to underground). This panel will have the ground bus and neutrals separated with 4 wires feeding the unit: 350 KCMIL AL for the HOTs, a 4/0 AL for the Neutral (using the 70% rule) and a #6 cu ground wire. The wires are suitable for direct burial, but i'm encasing them in conduit anyways and burying at 36" depth.
So the question is: the 6AWG Cu looks tiny compared to the massive current carrying conductors. Did I interpret the NEC code incorrectly?