Where Run Across the Top of Floor Joists, or Within $\mathrm{ft}$

Where run across the top of floor joists, or within _________ $\mathrm{ft}$ of the floor or floor joists across the face of rafters or studding, in an attic that has permanent stairs, nonmetallic-sheathed cable shall be protected by substantial guard strips that are at least as high as the cable.