I am helping my son in California install a new 200 amp panel. He originally was going to come in overhead w/ power so did not run conduit in the slab. Now we have dug a trench & are bringing up 3/0 wire into the main panel attached to the sheer. How do we water-proof the entrance areas from the panel? We've got 2 ea. 2-1/2" knock-outs in the back of our Square D panel. Our local Home Depot does not have 2-1/2" romex clamps, so we are thinking of using a male fitting (grey PVC) but don't want rainwater entering house. Any suggestions?

Not sure about anyone else, But I am not sure where you need waterproofing. The knockout out the back going into the home? What type of wiring method or raceway are you using?