If you have THHN underground, you're already in trouble. Underground conduit is by definition a wet area and requires wet-rated conductors no matter what the conduit is.

There's no requirement in the NEC for any such separation. Neither is there any provision for such in the International Fuel Gas Code (essentially, the NEC equivalent for gas piping).

If you're talking about outside supply lines to a house or neighborhood, the utilities themselves will often have standards about collocation

Got me. I was typing too fast. It is a THWN XLPE USE-2 rated 2awg cable that will be direct buried (pulled in to a 200' directional bore). I know we can pull a 1" water line with it but unsure about also pulling a gas line.

This is to feed an outbuilding.

Thanks,

Eh? You can't direct bury individual conductors. You either need to enclose them in conduit or pull an appropriate cable. I'm hoping you mean you're pulling the conduit and intend to use USE-2 inside it.

Make sure that you pull four conductors (two hots, neutral, and ground) to the outbuilding. Your outbuilding is going to need its own grounding system as well.

What? The code no longer allows direct burial of USE2? When did that change? Last I heard it was still approved for that use at up to 75 deg C.

If he's using USE-2 multiconductor cable he can do it. If he's using single conductors USE-2 then he cant. The issue isn't that you can't bury USE-2 but rather that you can't use individual conductors that aren't either in a conduit or part of a cable. Since he only indicated a single gauge I made the (possibly incorrect) assumption that he had single conductors.

It is 3 #2 conductors twisted together. Two neighbors have had the same stuff installed for supplying power to their docks.

OK, I was initially wrong about the conductors. As long as they are direct buried close together, that is acceptable.

You're now going to have to tell me what's on the far end of the conductors? I'm still assuming you're putting a panel in some structure? It needs to have a ground wire run with the other feeder conductors.

Note that you also need to be down 24" and use an approved conduit or other means to bring the conductors up to and above the grade (and into the structure).

Electric is a feed from a panel in our garage to a post mounted outlet at the end of our driveway. Main power feed is from a pole very near our house. The bore will be about 7' deep to avoid frost. The water and gas lines will both be supply lines from the street to our house.

The pole is still a structure (I'm guessing something like an RV outlet here?). Requries four wires and a grounding system under the later NEC versions.

You need to protect the wires with something like sched 80 PVC from 18" below grade up to the panel.