My experience has included a number of times that moisture was coming into the panel's cabinet by way of the service entry cable or conduit. By running the service entry conductors into the bottom of the cabinet I limit the damage that such water will do to rust in the bottom of the cabinet rather than corrosion of the panel parts.
Locally most of the panels are installed in basements so using the bottom of the cabinet for service entry conductors leaves the top of the cabinet and the wiring troughs free for branch circuit and feeder conductors.
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Tom H
