I am reading that wire inside Schedule 40 PVC conduit running underground must be THWN, however it is proving very hard to find at local big box stores.
Something like this https://www.homedepot.com/p/Southwire-100-ft-10-Black-Stranded-CU-THHN-Wire-22973284/204812489 does not mention THWN, but in the Specs you see that it is rated for wet locations. Is this what I need? Why can't they just add THWN on the label? What am I missing?
Thanks in advance!
The wire needs to be rated as and stamped THWN. You need that "W" in there, that designates the wire is suitable for wet locations. As you know, the inside of a buried conduit must be treated as a wet location.
However it's not a problem here, in fact I don't know if there's any THHN on the market now that's not also rated THWN.
That wire is actually dual rated as THHN and THWN, and the jacket will be stamped with both. They just have THHN on the label and product description. You can verify this in the Product Brochure PDF linked on the homedepot page in your post.
The wire must be marked "THWN" or "THWN-2" along its length to be used outdoors. No markings, no use. (I suppose XHHW would also be OK, but I don't think it's chemically possible to be THHN-XHHW).
It is possible to make wire dual-rated THHN/THWN. THHN is allowed 90C operating temperature and THWN is not. But obviously any dual-rated wire must be good for 90C! Therefore, a new wire type was declared, "THWN-2" which is THWN with a 90C rating. (you don't get to use the 90C rating in actual practice, but it's the basis of derates such as 310.15(B)(3)(a) which affect how many circuits can share a conduit.)
Now, for cable manufacturers, the cost difference to make a wire THHN-rated, versus dual-rated THHN/THWN-2, is rather small. So naturally, most of them make their THHN dual-rated. However, as much as you may love big-box stores, they squeeze their vendors very, very hard on price. The large volume makes it worth it for vendors to make "cheapie" price-shaved versions of their products for big-box. So it's quite possible the dual-rating was shaved off.
Why can't they just add THWN on the label? What am I missing?
Dual rated cable is commonly marketed as THHN or THHN/THWN depending on manufacturer, but the part number, if it contains a rating, usually only contains THHN. Generally the PVC insulation is what gives it its wet rating. Marketing is inconsistent as it is not UL mandated. The marking on the wire however is, it WILL have a wet temp rating or a "boat cable" BC code. If it is marked as outdoor rated, wet is implied.
That wire is Southwire SIMpull THHN, which is PVC insulated with a low friction Nylon outer jacket.
It is rated THWN for gauges 14 through 10, and THWN-2 for 8 and larger.
If you are looking for a single common rating that will meet or exceed all specs, look for XHHW, though it is thicker which gives it better durability. But you will probably not find that so easily at a local store.
