Here is a reply from Mark Ode from UL
A weep hole or drain hole, usually located in each corner of the
enclosure must be left open and not used for a No. 4 grounding electrode
conductor, even though it fits extremely well, since the purpose of the
holes are to ensure that any water that may enter the enclosure for any
reason has the ability to drain out of the enclosure. By inserting
something into these holes and encumbering drainage, you may adversely
affect the operation of the equipment.
There are some panels that have the corner drain holes with an additional
small knockout inside of a larger knockout where either the small knockout
can be removed and a No. 4 inserted into the hole or the larger knockout
can be removed with a connector installed and then the No. 4 inserted into
the connector and tightened down. These knockouts were installed so that
the user would not hopefully feel the need to use the weep holes for this
purpose.
