Depth from grade where sch 80 PVC must begin

[cwatrous76]

Hello all. Does anyone know the depth below grade where PVC conduit that will be exposed to physical damage above grade must transition to PVC Schedule 80? The application is for a solar farm 5MW that is fenced in in its entirety. The conduit(s) in question will contain 1500VDC or 600VAC conductors and leave underground in grass that must be mowed/weedwhacked. Any code references would be appreciated. I was told at one time that sch 80, if necessary, must start 18" below grade but 300.5 doesnt seem to support this. Any other references?

 

[infinity]

IMO if it will be subject to damage on the portion that is above grade then that's where the sch80 must start.