Foreword: I've been using UF wire for about 10 years now, and I've really grown to hate it. It's hard to work with, hard to strip, and hard to pull through conduit. I prefer all my underground wire to go through conduit, because WHEN something happens (and it will ... I've had lightning fry underground wires thrice in 10 years), it's much easier to replace when you don't have to dig it up again.
With that said, I'm starting a new project of lining my driveway with lamp posts. Due to voltage drop (600+ feet), I need 6 AWG wire. 6 AWG UF is HUGE and would require ENORMOUS conduit. I've never used THWN before, but after much research, it looks far easier to work with that UF. I've decide to go with THWN, instead.
The wire I'm getting is rated THHN/THWN-2/MTW/AWN, so it's suitable for dry AND wet locations to 90ºC. I know, obviously, that it isn't suitable for direct burial and MUST be in conduit from the breaker box to the lamp post. However, this is where things become less clear. I've looked through the Code thoroughly, and I haven't found any guidance that clearly applies to what happens once the wire enters the lamp post. I'll run conduit through the ground and up INTO the lamp post. This gets the THWN wire above grade. However, once I get into the lamp post, what then? Can I simply run the THWN up through the lamp post and join it to the conductors for the light fixture within the lamp post body? Must the light fixture wires be joined to the THWN in a junction box? Must I run UF wire through the lamp post from a junction with the THWN to a junction with the light fixture wires?
Any guidance on this would be appreciated. I'd hate to find out after pouring the concrete and mounting the lamp post that it doesn't pass the inspection.