I was a guest during a National Electrical Code Panel 3 discussion for the 2023 NEC process and one of the public inputs and comments under discussion was about the potential damage to wiring methods used in dwelling units.
The person who submitted the public input and comment was a manufacturer of a bushing designed to be inserted into bored holes within the wood structure of walls and ceilings. He was concerned about lag bolts, screws and other hardware for cabinets in kitchens and bathrooms; shelving brackets for storage in pantries; and TV mounting brackets installed after the drywall is put in.
He explained that the requirement in 300.4(A)(1) and (2) for bored holes in wood joists, rafters and walls, where the edge of the bored hole must be no less than 1¼ inches from the nearest edge of both sides of the wood member, was not far enough for the normal mounting of equipment and cabinets on finished walls. He stated that long lag bolts were being used that would penetrate through the ½-inch drywall and well into the 2-by-4-inch wood structure of walls and ceilings, penetrating into the bored holes and the NM cables located in the wood structure and potentially causing fires.
He used NFPA and State of Minnesota dwelling unit fire statistics to indicate fires that occurred over a period of years could have been caused by screw and lag bolt penetration into existing NM cables in walls and ceilings. None of the fire investigations indicated that the fires were actually caused by long lag bolts and screws, but the fires could have been caused in that manner. The manufacturer was attempting to get text inserted into the NEC requiring either nail plates or bushings installed in every bored hole and at all notches in wood.
Let’s look at the requirements in Section 300.4 that have been in the NEC for many years and then at actual installation practices to determine if there is a safety issue that would warrant such measures.
Section 300.4(A)(1) states that “in both exposed and concealed locations, where the 1- and ¼-inch distance from the edge of the bored hole to both edges of the 2-by-4-inch stud cannot be maintained, the cable or raceway must be protected from penetration from screws or nails by a steel plate, or by a bushing, that is at least 1/16-inch thick and of an appropriate length and width installed to cover the area of the wiring.”
In 300.4(A)(2), if there is no weakening of the building structure, cables or raceways are permitted to be laid in notches in wood studs, joists, rafters or other wood members where the cables or raceways are protected against nails or screws by a steel plate that is at least 1/16-inch thick and of appropriate length and width, installed to cover the area of the wiring. The steel plate must be installed before building finish is installed.
There are two exceptions to the requirements in 300.4(A)(1) and (A)(2). The first exception states: “Steel plates shall not be required to protect rigid metal conduit (RMC), intermediate metal conduit (IMC), rigid nonmetallic conduit (PVC), or electrical metallic tubing (EMT).”
The idea behind this exception is that unless a nail or screw is installed directly into the wood, penetration into RMC and IMC will not happen, and with PVC or EMT, penetration will not normally cause problems for the conductors installed within these two wiring methods.
The second exception was inserted into the NEC several Code cycles ago. It permits a thinner steel plate than the normal 1/16-inch-thick plate where the listed and marked steel plate provides equal or better protection against nail or screw penetration. These plates are normally case-hardened and will not permit penetration, even with self-tapping screws. Any bushing that is installed in a bored hole must also be thick enough and substantial enough to withstand any screw or nail to prevent penetration of the cable or raceway.
Two additional protection methods are also incorporated into dwelling units. The first method of protection is the branch-circuit protective device that will provide ground-fault and short-circuit protection at the service or panelboard. The second method is arc-fault circuit interrupters that are installed on all new homes, which can be added to older homes as needed. Most bored and notched holes in wood framing members are not at the height in a wall where TVs and cabinets are going to be mounted, and the screws are not normally long enough to penetrate into raceways and cables.
Until substantial data can be compiled, requiring sleeves, plates or bushings in every bored hole should not become a requirement.