I have an outdoor run that will use THWN wires in gray PVC electrical conduit. I plan to seal around and inside the conduit at the (above-ground) building penetration. This is for energy efficiency and to keep the critters out. But along the rest of the outdoor run, is there any code requirement to seal together the conduit pieces? I’m thinking in particular the bits that will be buried. I find in practice that these pieces assemble quite tightly together. By the time I have attached the far end to a junction box mounted on a wall and put in any supports along the conduit it’s near impossible to take the pieces apart. So is there any point in gluing it all together?
Yes, the NEC instructs to follow the product instructions:
110.3(B) Installation and Use. Listed or labeled equipment shall be installed and used in accordance with any instructions included in the listing or labeling.
Sometimes finding the instructions requires a bit of diligence, but they are out there.
And then there's this:
352.48 Joints. All joints between lengths of conduit, and between conduit and couplings, fittings, and boxes, shall be made by an approved method.
And Article 100 defines approved:
Approved. Acceptable to the authority having jurisdiction.
So even if you can't find where instructions say you must, you will still have your Inspector to deal with.
It's not glued, it's solvent-welded. What you call "glue" melts the PVC, causing the joined pieces to become the same piece of plastic. Then the solvent evaporates, leaving continuous plastic.
But along the rest of the outdoor run, is there any code requirement to seal together the conduit pieces? I’m thinking in particular the bits that will be buried.
Oh my gosh, are you kidding me? That's what needs gluing the most!
You must assemble the conduit route complete before pulling any wire into it. Some people think you assemble the conduit over the cable stick by stick, but this is bad for a variety of reasons. You get the wire in there using a technique called "pulling", which also requires the conduit route to be constructed to be "able to be pulled". That is a Code requirement in any case. We regularly have questions on here where the fix would be easy if the person could pull in replacement or additional wire - but often they cannot because of prior idiocy.
Anyway, the reason PVC needs gluing is that the pulling forces can be considerable. If the joints were not glued, then pulling forces could pull the conduit apart underground (and you'd never know) - and then you have a big gap and the wires are being pinched.