I do not advise that any attempts be made to 'seal' the conduit. I do like the idea of a junction box NEXT to the sump, though.
Indeed, I favor a grossly oversized conduit, and might even like it if there were separate conduits for the pump and float cables.
Let's look at this a bit closer. First, what is a 'junction box?? I most certainly do NOT mean any sort of metal box with a gasket. Rather, I am referring to the fiberglass or concrete 'handholes' that have open bottoms. These are set atop a layer of gravel, and (ideally) have the conduits use a sweep to enter them vertically, from the bottom.
Will an overfilled sump also fill the box? Yes. Rain and ground water will also enter. The open bottom means that they will also drain.
The reason for large conduit is that, over time, the cable insulation swells considerably. This can make it impossible to replace the entire length of cable.
Splices in the box are made using some sort of waterproof method; I prefer simple wire nuts, dipped in Scotch-Kote, and left pointing 'up.'
If you have to splice in the pit, do it leaving enough cable that the next time you can do it from ABOVE the pit. I like the Ideal UF splice kit for power leads; it has a 4-port set screw arrangement that allows for ready replacement the next time, or for easy rotation reversal of three phase pumps.
If you have to replace an existing junction box in a pit, use a PVC one. The 'home centers' carry very affordable ones by Carlon; I think a 12x12x4 cost me about $25. These are nice NEMA-4 boxes with proper gaskets. I use Scotch-kote to secure the gasket to the cover. Where cables enter, I use the fittings that have rubber grommets in them.
