My understanding is all buried wire in conduit must be THWN-2.
Per Southwire's spec sheet here the end of the first paragraph reads:
All Sizes Rated Both THHN and either THWN (sizes 14, 12, and 10 AWG) or THWN-2 (sizes 8 AWG and larger).
It is baffling me that there doesn't seem to be THWN-2 in 12 AWG.
What do folks do for 20A circuits buried in conduit?
Background, I'm doing all this in conjunction with running 4-8's for a separate 35A 240V circuit to a mini-split. And figured I should fix the previous homeowners error of running 12/2 romex in that buried 3/4" PVC conduit.
Upsizing a 20A circuit to 8 THHN just to get a THWN-2 rating doesn't make sense to me, and I'd run into fill issues. I would then need to upsize the conduit (not out of the question, but seems an extreme path to take).
Running 12 AWG UF-B in the conduit seems to be the other code compliant. While permitted that seems extreme as well and I'm not sure how to calculate fill using 12 UF-B + 4 8 THHN. Again, likely having to upsize the conduit.
I assumed I could just run 2-12 THHN/THWN (20A circuit A) and add the 4-8 THHN/THWN-2 (35A circuit B), and using one of the 8s as a common ground on both circuits. 6 wires total, fill would be 33%.
I'm wondering if there is some footnote in the NEC that says wire under 8 AWG actually only need to be rated THWN in buried conduits. Or if someone actually makes 12 AWG THWN-2, just not SouthWire?