Swapping out a 225a 3-phase panel. The old panel is completely locked up but is connected via 2.5" conduit. The new panel doesn't have anything larger than a 2" knockout. Can I just drill a 2.5" hole?
If you have a 2.5” metal cutting hole saw you can.
If not you may have to ether saw and grind the hole.
There are also punch and die sets you can buy where you first drill a smaller hole and then there is a bolt that goes thru the hole and connects the punch and die. You tighten the bolt and it cuts the hole.
Another option would be a hand nibbling tool. They cost around $16. I don't know if one could be rented?
Then I started thinking how I might do it with tools I already have. Mark the circle, drill a hole in the center and make radial cuts with a metal blade in a jig saw. Clean up the best I can with the jig saw then use a round file to finish.
I have the hole saw, it was more a question of whether there is some proscription against doing so (though I have never seen a 2.5 inch knockout, so probably not).