I and many other building inspectors have always considered cables in bored holes to be bundling. Article 310.15(B)(2)(a) covers bundled cables. Derating is required for more than 3 current carrying conductors. But, if we do the math, and take into account the "small conductor" rules, we find that 4 cables are allowed to be installed in every bored hole. This works for #14, #12 and #10 NM and AC cables, 2 or 3 conductor.
What is maybe more important, is the size and placement of bored holes.
Building code requires that bored holes be a minimum of 2 inches back from the edge of joists and rafters, and 5/8 inch away from the edge of studs. NEC requires nailer plates if the hole is closer than 1-1/4 inch.
Building code also limits the size of the holes in rafters and joists to 1/3 the actual measured size of the joist or rafter. For a 2 x 8 that is a 2-1/2 inch hole, for a 2 x 10 that is a three inch hole. Again the holes have to be kept at least 2 inches away from the edges of the joist or rafter. Also, never bore a hole in a truss or engineered lumber.
Earl
