THHN cannot be buried directly in the ground, so you’ll have to upgrade on a single conductor to USE-2 wire. It’s used mostly on solar installations and has rubber insulation.
If your installation is directly in the earth instead of in conduit, then you’ll have to upgrade to a UFB cable which is an underground feeder cable. It’s color-coded and flat instead of round with a bare copper ground wire. The insulation covers each conductor individually to keep it water and moisture-tight directly underground.
Tray cable is also an underground cable for larger sizes since UFB cable only goes as large as 6 AWG 3 conductors. Tray cable goes as big as 500 mcm 4 conductors with ground. It has all-black numbered THHN conductors and an overall PVC jacket. We sell it per foot on our website so that you can get the exact length you need and avoid any waste.