How do you seal the pipe where it goes outside? You are talking about sealing the inside of the pipe, right? I have a 1" pipe leaving my basement underground (about 12" below grade) to my lamppost. I did not seal the inside...caulk? foam?
Condensation could form inside the pipe. Ground water can also come thru if it wasn't glued well. You don't want that water coming into your basement so sealing the pipe is a good idea.
You can use duct seal for this purpose, or just fill it up with goobs of silicone. Both of those are removable in case you ever have to repair a broken pipe or pull another set of wires into the pipe.
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