The Carlon conduit was designed for UV exposure and burial (though a coat of latex paint enhances the former).
3/4" PEX would be better. Around these parts, 24" would be in frost zone. Shouldn't be too hard to pull through in case of replacement. I'm not familiar enough with PEX to know if you could use pulling lube.
You'd be better with a POU heater in the barn. 1&1/4" is too small to provide sufficient annular insulation.

There is no need to run the PVC.

PEX tubing, Polyethylene roll pipe and PVC are all three approved for direct burial water supply lines.

Plastic pipes of all varieties are immune to acid soil conditions and in all probability they will outlast the structure your connecting to. By example, on our family farm we ran 1" poly to the barn in 1957 and it has been conveying about 2000gal a day. After 52 yrs of continuous service that line is still just as good as the day we laid the pipe.

Code minimum for a water supply line to a structure is 3/4".

"There is no need to run the PVC. "

True enough, but conduit allows possibilities, with some forward thinking advising larger diameter.
Cheaper to put conduit and seperate waterline in the ditch than hiring the backhoe again.

Thanks for all your advice,

I was favoring protecting the PEX in conduit because the PEX manufacturer pamphlet advises against direct burial in clay soil or soil with a lot of large rocks. I have clay soil. Now, my understanding is that most soil contains some clay so what exactly constitutes "clay soil" according to the manufacturer does not seem that clear to me. Running the 1 ¼ conduit for 50ft indeed costs an additional $30 - seemed an acceptable precaution.

The barn is very small (250sqf) and I use it mostly as a garage/shop. I started out this project digging a ditch to take electricity to the barn. Then, I figured that, since I went through the manual effort (yes indeed as lazypup points out it took me 2 days to dig the trench by hand, including tunelling under 5ft of concrete) of opening the trench, it would be nice to also lay a water line and put a sink and/or a hose bib at the barn so that I can wash my hands etc. I chose PEX vs the simple poly pipe because, my understanding is that, PEX would also keep the water potable. Im obviously not a plumber, but from what I read, I suspected that a 1/2 " line was barely enough for a sink and hose bib. ¾" would be better, but Im not sure if I will be able to feed a ¾ PEX line through a 1 ¼" conduit. ½ seems to be going in fine but ¾ may have too much friction. Could upgrade to 1 ½" conduit for an additional $10, but where do the upgrades stop?

Well, you can't beat the poly pipe like Lazypup says. 100psi coil will tolerate small rocks ( though better to bed in sand or fines) won't have but two joints and is pretty cheap.
If you take a ball of your soil and squeeze the moisture out, let it dry a couple days then put in a jar of water for a few days and it pulls out still hard as rock, you have clay. Otherwise, it's a mix of soil triangle components.
I'm not a plumber by profession but do own a hoe. If an hour of $60 machine does the job, then day labor @$10x2 better be done in three hours. No, I don't know anyone around here that works for a sawbuck.

I'd run the water in poly, if it makes you feel better, upgrade to 160. If you have the PVC, use it for conduit. The poly can be used for conduit too. That's a trick used to upgrade a two pipe jet to a submersible in a well with minimal shovel time. I'm assuming you already have UF or USE cable in the ditch, a size bigger than you think you need unless you have a short pencil.

Having a conduit gives you an option. You might not know the future use but I guarantee it will cost more then and your back won't be so amenable.

I have to agree with the pup and alphonse

Run it in poly and use 1" so that you are oversized a little. It's fast, cheap and you shouldn't have to worry about upgrades later. Bed it in sand if you are worried about the soil.

For hot water, use a point of use water heater. A small electric tankless should do what you need if you have the capacity on the circuit you are running out there. If not, a small electric tank would work also.

Question: Are burrowing critters known to chew through PEX or poly? If so, it seems to me that running it in conduit would be cheap insurance, providing an extra layer of protection.

They certainly could, although I don't know if there'd be much incentive for them to do so.

I agree, the conduit is cheap insurance. PEX is also still relatively new, we've seen lots of plastic pipe systems come and go.

I'd go one more and run a conduit for data/telephone, with room and a pull string for extra for later.

"Question: Are burrowing critters known to chew through PEX or poly?"

Here's a SWAG, not to be construed with fact: Squirrels are real PVC connoisseurs, with mice being only incidental consumers, meaning, if they're building a nest and there's wire, ducting or other material in the way, it will be chewed up.
If you bury below frost line it eliminates much of our furry friend's interest. In the case of transmission lines (think pipelines...gas & product movers that generate heat from friction) creatures will make burrows adjacent/ on top of the line. This becomes rarer when buried below the level of organic activity in the soil.

The AHJ is not going to be very happy with putting a water line in a conduit with electrical lines.

Buried electrical conduit is already a wet location, but you are going to have issues with things like conduit fill factors, and just the listing of the electrical conduit.
It is not listed for plumbing use.

brickeyee, I don't think anyone was suggesting that the water line be run in the same conduit as electrical. :-)

>you are going to have issues with ... the listing of the electrical conduit. It is not listed for plumbing use.

Well, it wouldn't be for plumbing per se -- just as a protective outer shield for it. But I tend to agree that regular white PVC should probably be used anyway to avoid any possible confusion as to the conduit's designated purpose.

"Well, it wouldn't be for plumbing per se -- just as a protective outer shield for it."

That would be a plumbing use.
Just us some PVC water pipe if you are that worried, and then pull the PEX through that.

How it all turned out, in case someone else wants to do the same project:

This project started by planning to take electricity (a 20amp circuit) from my house to my barn. Then I said: "Since Im going through the trouble of digging the trench, why not also run a phone/network cable then why not water for a sink someday then why not warm water for the future and so.

I ended up running four 1 1/4" conduits to my barn. One for electric, one for phone/cable/network and two for water (cold/hot)

I ran two 3/4" PEX water lines inside two of the conduits. The ½ PEX line would have gone in easily but the ¾" was a different story. I had to both push on one end and pull from the other. I suspect that the only reason I was able to finally do it is because I only used 45degree elbows (5 elbows on each conduit run). Had I used 90 degree elbows, ½" PEX may have been the only realistic option.

I did use electric PVC because of the gentler elbows. Regular white PVC elbows are too sharp to pull PEX through. Could have used ABS, I suppose, but decided on electric PVC. The run is short (50ft) so the use is obvious looking at the two ends where the electric cables and water lines come out. In retrospect, I should have marked (written on) the entire conduit length to indicate use, since how the 4 pipes are routed may not be obvious to someone digging just a hole. But in any case, the water lines are at the bottom 24" under, the electric line is just above at 21" and the data/network lines are at about 14" (I separated data from electric lines to prevent possible interference). No frost issues even at 14" here in central California (we have many problems but frost is not one of them).

I also had to construct an insert to put in front of the PEX in order to push/pull it, to prevent the PEX edges from catching at the PVC conduit joints. It was a rather fallic looking insert made from a wooden dowel, shaped on a grinding wheel and a metal edge to reduce friction. I do not know if they make something commercially for this purpose. I immagine its a common problem.
Also, I dont know if there is something to lubricate the PEX to help out, but I did not use anything and certainly stayed away from oils as they do degrade most plastics (including PEX?).

If I had to do it again, I would have taken lazypups advice, skipped the conduit and burried the PEX water lines directly, in spite of the PEX manufacturers advice to avoid burying the PEX in clay soils (my type of soil). I had to tunell under concrete for 3 ft so opening a small hole for the ¾ PEX would have been much easier than opening a 1 ½" hole in the dirt for the 1 ¼" conduit. The cost of the conduit is not much (around $30 for 50ft run) but it does take considerable amount of time to assemble it correctly in the trench and tunel it under the concrete. I would have still had to deal with the two conduits for electric and data though. If the PEX water line ever goes bad, I may be glad I did it, especially if Im 70 by then, Ill appreciate not having to redig the trench.

Actually, after reading the comments, I would have run the PEX in 1 or 1¼" poly, but I could not find any at the usual stores (HD/Lowes).

I ran PEX because I wanted to keep the water potable. I call it barn but use it mostly as a shop. No animals, it is only 250 sqf. PEX has been used for over 20 years in quality an regulation obsessed places like Germany. Id say it is a safe and reliable option.

I have anecdotally heard of mice chewing through exposed poly in rural settings, especially if they can hear the water running in it. Perhaps theyve figured it out in dry climates.

I have put a poly string in all the currently unused conduits.

Took me 2 days to dig the 50ft, 24" ditch alone by hand, including tunneling under concrete for 3ft, and one day to buy the materials and assemble the PVC in the trench. Then a couple of hours to fill back the trench.

I am not sure how I will handle the hot water if I ever hook up this water supply. I may try out to see how hot remains the water once it reaches the barn and if the results are not satisfactory I may install a small electric water heater now that I have also run conduit for electric. I guess, if I end up not using the second PEX line for hot water, I could then turn it into a second cold water line to increase my max water flow.

Running the actual electrical cables to the barn is still pending, subject to resolution of some grounding issues (I have an active post in the electrical forum).

Thanks for the advice.

The next question is how are you going to create a drain for a sink?

Most residential codes prohibit connecting two buildings on one septic tank or municipal sewer line and the codes absolutely forbid installing a fixture without a suitable drain.

You may install a hose bib but not a fixture unless you have an approved sanitary drain.