I guess I'm thinking of:
110.12 (A) Unused Openings
Unused openings, other than those intended for the operation of equipment, those intended for mounting purposes, or those permitted as part of the design for listed equipment, shall be closed to afford protection substantially equivalent to the wall of the equipment. Where metallic plugs or plates are used with nonmetallic enclosures, they shall be recessed at least 6 mm (1/4 in.) from the outer surface of the enclosure.
230.62 Service Equipment — Enclosed or Guarded
Energized parts of service equipment shall be enclosed as specified in 230.62(A) or guarded as specified in 230.62(B).
230.62(A) Enclosed
Energized parts shall be enclosed so that they will not be exposed to accidental contact or shall be guarded as in 230.62(B).
Or:
110.3 Examination, Identification, Installation, Use, and Listing (Product Certification) of Equipment
110.3(A)(2) Mechanical strength and durability, including, for parts designed to enclose and protect other equipment, the adequacy of the protection thus provided
110.3(B) Installation and Use
Equipment that is listed, labeled, or both shall be installed and used in accordance with any instructions included in the listing or labeling.