The protection plate is required to be a minimum of 1/16". The exception allows for the plate to be less than 1/16" if it is listed and marked.
300.4(A) Cables and Raceways Through Wood Members.
(1) Bored Holes. In both exposed and concealed locations,
where a cable- or raceway-type wiring method is installed
through bored holes in joists, rafters, or wood members,
holes shall be bored so that the edge of the hole is not less
than 32 mm (1 1 ⁄ 4 in.) from the nearest edge of the wood
member. Where this distance cannot be maintained, the
cable or raceway shall be protected from penetration by
screws or nails by a
steel plate(s) or bushing(s), at least
1.6 mm ( 1 ⁄ 16 in.) thick, and of appropriate length and width
installed to cover the area of the wiring.
Exception No. 1: Steel plates shall not be required to
protect rigid metal conduit, intermediate metal conduit,
rigid nonmetallic conduit, or electrical metallic tubing.
Exception No. 2: A listed and marked steel plate
less than
1.6 mm ( 1 ⁄ 16 in.) thick that provides equal or better protec-
tion against nail or screw penetration shall be permitted.