Using THHN for this application would be a violation since THHN is not rated for wet locations. I would have to admit that all of the THHN I have seen is rated as THHN/THWN but make sure you are using THWN. I pretty much gave up using CU THHN/THWN for any thing #2 and larger quite a few years ago and switched to XHHW. The insulation on XHHW does not start to fall apart within a couple of years after being installed when exposed to sunlight and it also holds up much better in underground application. Many of the high tech company's and schools in my area will not permit any THWN to be used for underground applications.
E-Scott - Why did MA decide not to adopt 310.8(D)?
Curt
