These days the caution is that some THHN is 'only' THWN, not THWN-2. So the conductors are only rated for 70C in wet conditions (eg. conduit out of doors).
A caution: Several retailers list Southwire's regular THHN product as THWN-2 (Home Depot, Lowe's, Amazon). However, in common sizes like 10-14 AWG, it's *not* THWN-2. Southwire's web page about their THHN says it's just THWN for 10-14 AWG, and THWN-2 only if: (1) 8 AWG or larger, or...
