I?m having a little debate with a local electrician over TTHN vs. THWN. He is saying that TTHN is rated for use in conduit in wet locations like under slabs or used in PVC schedule 40, he is saying that is basically waterproof. I?m saying if it doesn?t have a W or a U in the labeling it isn?t rated for that used and I need some code references to help prove my point.