Sealing conduit ends against flood water - duct seal, foam, or just let it drain?

I ran a run out to my garden shed about eight years ago and went thru basially the same debate. Started with duct seal because it was cheap and easy. It held okay for maybe two summers, then I opened the box one spring and found it had hardened and pulled away from the pipe like old chewing gum. Water had been getting in for who knows how long.

What acutally worked for me was a combination of fixing the water problem and letting gravity do the rest. I dug a bit of gravel around the box and graded the dirt so surface water ran past it instead of pooling there, then I drilled a small weep hole in the lowest point of the box bottom. The conduit stays dry most of the time now, and the couple times water does sneak in, it drains right back out instead of sitting there.

I also swapped the open box for a proper weatherproof one with gasketed fittings where the conduit enters, which made a bigger difference than anything I stuffed into the pipe ends. The fittings are rated to seal around the conduit, and they have held up fine without ever cracking or drying out. Skip the foam. A friend of mine filled his conduit with the stuff and when he needed to pull a new circuit to his garage door opener a few years later, he ended up digging the whole run up. Not worth it.

I'm with you on the drain idea, and honestly I think it's the only approach that holds up long term. I've got a run out to my barn that crosses a spot that stays soggy most of the winter, and I spent two years fighting it with sealant before I gave up and accepted that the pipe is going to get wet no matter what I do. Even a perfectly sealed run breathes with temperature swings, so you get condensation on the inside that has nowhere to go. My fix was dumb simple: I made sure the low point of the run was where I wanted it, drilled a couple of small weep holes there so anything that collects can drip out, and mounted the box up on a couple of stacked patio blocks so the ends sit well above grade. Then I spent an afternoon with a shovel and some leftover gravel from the driveway project, pulling the grade away from the box so the swamp forms ten feet off instead of right at it. Five years now, opened the box last fall to add a circuit and the conductors looked clean, no corrosion on the lugs, no standing water. The pipe staying dry is a nice-to-have, but keeping the water moving and keeping the box itself high and dry is what actually matters.

One thing I'd add is to think about the slope of the run itself. If your trench bottom undulates, every dip in the pipe becomes a permanent little water trap regardless of what you do at the box, becuase water finds the low point wherever it physically is, not where you want it to be. When I pulled my run I laid the conduit on a bed of sand and kept a consistent fall the whole way, and it made a noticable difference in how much standing water I see when I pop a cap off either end. Also, have you checked whether water is actually wicking in through the trench backfill around the box rather than through the openings? Loose soil acts like a sponge and can hold water against that plastic long after the surface dries out.

One thing to check is the other end of the run. Water that gets in your low spot will keep traveling toward whichever opening is lower, and on my 55 foot run to the shop I found standing water had made it all the way back to the panel side. I pulled the conductors out, taped a strong pull string to them before I did anything else, then poured a half cup of water down the high end just to watch where it came out. That told me exactly where my true low point was. Turned out my box wasn't the lowest spot, a dip about ten feet from the shop was. Worth tracing before you commit to fixing only one end.

Not sold on the gasketed fittings being the big fix, honestly. I sealed my box up tight like that years ago and all it did was turn the inside of the box into a condensation trap. Pipe breathes with the temperture swings like the barn guy said, and if the box is sealed good, that moisture has nowhere to go except dripping onto your connections. I drilled my own weep hole after that and the box has stayed dry since. A box that can drain is better than a box that can't breathe, in my experiance.