What is different about THHN vs THWN? I cant believe that the insulation in THHN isnt waterproof. Does moisture cause some sort of degradation over time? Or does it mess with the dielectric properties? Does it only become an issue at higher temperatures (the -2 suffix seems to imply that)? Spent a while googling and couldnt find anything. Just ignore that THHN might not even be available anymore.
The NEC itself gives you information about insulation characteristics, so that's a good starting point. Keep in mind that insulation, like electrical equipment and even the code, has evolved over the years; we didn't just start in the past 10 years. Nobody would want a high-leg delta service today, but it was part of the past because it was a simple way to add a small 3ph capacity to an existing 1ph service, which is actually where it came from.
Most modern wire is dual-rated as THHN/THWN or THWN-2 these days. THHN has two H's for greater heat levels, but no W, so it's not
rated for wet use, but might survive it. THWN has the W for wet use, but only one H. A -2 suffix means the wet temperature rating (usually lower) is equal to the dry rating. Check out Art. 310 and websites like Southwire's for a lot of info.
