The section on NM (sheathed electrical cable) cables in attics refers to this section.
"320.23 In Accessible Attics
Type AC cables in accessible attics or roof spaces shall be installed as specified in
320.23(A) and (B).
(A) Where Run Across the Top of Floor Joists Where run across the top of floor joists,
or within 2.1 m (7 ft) of floor or floor joists across the face of rafters or studding, in attics
and roof spaces that are accessible, the cable shall be protected by substantial guard strips
that are at least as high as the cable. Where this space is not accessible by permanent
stairs or ladders, protection shall only be required within 1.8 m (6 ft) of the nearest edge
of the scuttle hole or attic entrance.
In accessible attics, Type AC cable installed across the top of floor joists or within 7 ft
of the floor or floor joists across the face of rafters or studs must be protected by guard
strips. Where the attic is not accessible by a permanent ladder or stairs, guard strips are
required only within 6 ft of the scuttle hole or opening.
(B) Cable Installed Parallel to Framing Members Where the cable is installed parallel
to the sides of rafters, studs, or floor joists, neither guard strips nor running boards shall
be required, and the installation shall also comply with 300.4(D)."
(D) Cables and Raceways Parallel to Framing Members and Furring Strips In both
exposed and concealed locations, where a cable- or raceway-type wiring method is
installed parallel to framing members, such as joists, rafters, or studs, or is installed
parallel to furring strips, the cable or raceway shall be installed and supported so that the
nearest outside surface of the cable or raceway is not less than 32 mm (1 1 / 4 in.) from the
nearest edge of the framing member or furring strips where nails or screws are likely to
penetrate. Where this distance cannot be maintained, the cable or raceway shall be
protected from penetration by nails or screws by a steel plate, sleeve, or equivalent at
least 1.6 mm ( 1 / 16 in.) thick.