How many Romex cables can go through one hole?

The NEC does not set a numeric limit. NEC 300.4(A) governs hole location and protection, and NEC 334.80 requires ampacity adjustment where more than two NM cables pass through the same opening that is sealed with thermal insulation, caulk, or foam.

How far from the stud edge must a bored hole be?

NEC 300.4(A)(1) requires the edge of the hole to be at least 32 mm (1 1/4 in.) from the nearest edge of the wood member, or the cable must be protected by a steel plate at least 1/16 in. thick.

Does passing through a single stud trigger derating?

No. NEC 310.15(C)(1) applies where conductors are bundled without maintaining spacing for more than 750 mm (30 in.), which the thickness of a stud does not approach.

Can two Romex cables go through the same bored hole?

Yes. Two runs of 10-2 NM-B through a single one inch hole in a wood stud is a compliant installation. There is no code rule limiting the number of cables per bored hole. The rules that do apply are about where the hole is, whether the cable needs a nail plate, and whether the bundle is long enough to trigger ampacity adjustment.

Where the hole can be

Section 300.4(A)(1) of the 2023 NEC covers cables and raceways through wood members. Holes bored in studs, joists, rafters, or similar members must be located 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 that distance cannot be maintained, the cable or raceway must be protected from screws and nails by a steel plate or bushing at least 1.6 mm (1/16 in.) thick and of appropriate length and width installed to cover the area of the wiring.

In a nominal 2x4 stud the face is 3 1/2 in. deep, so a one inch hole drilled dead center leaves about 1 1/4 in. on each side and just barely clears. A one inch hole in a 2x4 that is drifted off center will not clear, and it needs a nail plate. In a 2x6 you have room to work with. Section 300.4(A)(2) covers notches in wood members and requires the same 1/16 in. steel plate protection.

When derating kicks in

Section 310.15(C)(1) requires ampacity adjustment where the number of current carrying conductors in a raceway or cable exceeds three, and it applies the same factors where conductors are installed in a cable or raceway longer than 750 mm (30 in.) without maintaining spacing. Table 310.15(C)(1) gives the factors, starting at 80 percent for four to six current carrying conductors.

Two 10-2 cables passing through one hole is four current carrying conductors, but only for the thickness of the stud. That is nowhere near 30 in. of bundled length, so no adjustment applies. The situation that actually triggers derating is a stack of cables running together through a series of holes, or a bundle strapped tightly along a joist for several feet, or the classic case in Section 334.80 where NM cables are installed in contact with thermal insulation in a wall.

The 334.80 trap

Section 334.80 states that where more than two NM cables containing two or more current carrying conductors are installed without maintaining spacing between cables, through the same opening in wood framing that is to be sealed with thermal insulation, caulk, or sealing foam, the ampacity of each conductor shall be adjusted in accordance with Table 310.15(C)(1). Note the trigger is more than two cables. Two cables through a fire caulked opening is fine. Three cables through the same fire stopped or foamed hole is not, and they get the 70 percent factor for seven to nine current carrying conductors if all three are two conductor cables.

Section 334.80 also fixes the ampacity of NM-B at the 60 degree Celsius column of Table 310.16 for the final answer, even though the conductors are 90 degree rated and the 90 degree column may be used as the starting point for adjustment.

Field takeaway

Two cables in one hole is fine. Keep the hole edge 1 1/4 in. off the stud face or add a nail plate, and count your cables before you foam the penetration, because 334.80 starts adjusting at three.