A underground conduit is ALWAYS a wet location no matter what guarantees you can make.

I've never seen buried cable eaten. I've had squirrels eat the hell out of some of my aerial stuff.

For a simple light, I'd use UF.

UF is quicker to run; the conduit gives opportunity to pull more conductors later, if sized correctly for future expansion. With a run of 350', you may need to account for voltage drop.

Unless you're going to install some kind of subduct into the conduit with your THWN, I think you'll find it is near impossible to pull more conductors through conduit that's already got wires in it. We always pull all the expected (current and future) stuff together or pull the current and subduct together.

At 350' you do start to have to worry about voltage drop, but if we're talking about a light, it's probably not drawing more than a couple of amps anyhow, so it's not going amount to much. At that distance with #12, you can draw 5A before you'll drop it 3% in voltage (and all it means would be the light would be a little dimmer in most cases).

One other thought. Will you always be able to remember where this cable is buried and know that you will never dig in any of those locations. At my place we are frequently adding to the landscaping and digging deep holes for trees and shrubs. All of the underground wiring I've installed has gone in conduit for protection.

Thats why the code requires direct bury to be 24" in such situations. If you're going to likely run into some burried utility. Some of the stuff (cable TV, phone) may not even be burried 24" That's why almost every jurisdiction has a one call utility locating service available (DIGSAFE, Miss Utility, etc...).

24" burial depth is not required for 1 or 2 family dwelling units. It can be as little as 12" if certain conditions are met.

Thanks Ron et al! Load on this line will be very low, at most a couple 100 w light bulbs worth, more likely LED lamps at a few watts plus a few strings of LED Christmas lights a few weeks each year. I think I will go with the UF and bury it at 24". It will be pretty obvious that there's a buried electric line because there will be s couple of intermediate light posts all in a row.

Rigid and intermediate metal conduit only require 6 inches of cover.

This may be a stupid question... but why does rigid or intermediate metal conduit not rust?

@ brickeyee

I did not reference that because the OP asked about UF and PVC, not RMC or IMC. I've never run threaded raceway at a single family dwelling. Just curious- has anyone used RMC or IMC for a home?

It's galvanized, but it will rust over time and exposed to the elements and corrosive conditions. In some locations, it's necessary to use PVC coated rigid to preserve the raceway.

It is much faster to dig that shallow trench out to a front light post and then just put the plugs of sod back in.

Use a garden spade, AKA 'drain spade.'

The blade is longer and narrower.

That's a lot of digging. I need to run an irrigation line to the new orchard at the same time. Digging 500 ft of trench by hand was the sort of task my father liked to give me when I was much younger. I think I'll stick with the machine.

No sod here, we're going through the woods.

With roots and possible rocks sandy soil and other things there is absolutely no way I would direct bury it unless it never gets below freezing there.