we do 24" cover with sch 40 here, 18" radius 90s, usually 2" conduit is standard for us. 30" deep trench, 3" sand on the bottom, sand on top to about a foot from the top, then underground utility warning tape, then backfill the rest of the way with native soil, till the last 4" would be left open for AC patchpave. Some of this may be above and beyond NEC, just what we are doing on our job.
