As most know I will do what I can to keep a debate alive looking for that 1000 post thread.
This thread along with the debate of SE-R installed underground would be a good thread to make the 1000 post.
To stay in line with this thread I have a couple of sections of the NEC that might have some bearing on this.
Some have made the comment that 338 and 340 are for direct burial conductors although 310.7 addresses direct burial conductors.
338.2 defines a USE conductor as an underground conductor and nowhere in 338 does it refer to the USE conductor as a conductor for direct burial.
340.10(1) states
For use underground, including direct burial in the earth. For underground requirements, see 300.5.
Notice it is not referring us to table 300.5 but to the section which would include (B) listed for use in wet locations. This section also mentions both "Direct Burial" and "Underground" as being two separate places.
310.9 Corrosive Conditions
Conductors exposed to oils, greases, vapors, gases, fumes, liquids, or other substances having a deleterious effect on the conductor or insulation shall be of a type suitable for the application.
Who can say just what the PH level of the soil is 6 to 24 inches under the earth.
This is why I have posted Table 310.13 and the Application Provisions listed in the table.
Then we have the UL?s letter designations assigned to wire. If it is listed for direct burial it will be marked ?FOR DIRECT BURIAL,? ?DIRECT BURIAL,? ?DIR BUR? or ?DIR BURIAL? and for underground use it will be marked with a ?U?
These things cannot be squelched by 300.5(B) as this section does nothing more than say that a conductor installed underground in a raceway or enclosure must be listed for a wet location.
If the only requirement was for the conductors to be listed for a wet location then I would install THW conductors to a gas pump and 501.20 could be deleted form the code book.
It will take something more that 300.5(B) to convince me that the other sections of the code and UL?s listing does not come into play in some way with underground conductors.