I realize this is sort of late in the thread, but here's what the wire designation letters mean.
T=Thermoplastic insulation, 60C
H=15C higher. HH=30C higher.
W=Wet locations. Ok to submerge in water. The inside of a conduit installed in a wet location is considered a wet location.
N=Nylon outer covering.
THHN=90C insulation, dry locations only.
THWN=75C insulation, OK for wet locations.
Be careful with the 90C temperature rating though, the vast majority of terminations are rated for a maximum of 75C, and this is the ampacity column you must use in table 310.16 The 90C rating is useful when de-rating due to multiple conductors in the same conduit.