The last step of the building electrical distribution – the branch circuit – is commonly run inside walls through bored holes in framing members. National Electrical Code (NEC) Article 300.4 provides specific guidance for the installation of wiring in walls where they are “subject to physical damage.” Since the wiring system is typically covered with sheetrock or other covering, the wires cannot be directly damaged unless the wall is penetrated. A screw or nail used for attaching objects to the wall, if long enough, can damage the wire. There are three potential problems associated with penetrating a wire:
- Full short circuit that would be cleared by the circuit breaker.
- An intermittent short circuit causes an arcing fault. This may not manifest immediately and become a problem later.
- Energized screw (or nail), which is a shock hazard.
Type NM cable is the most common method of wiring residential (and some commercial and institutional) buildings. The Code provides two very important safeguards. The wires are required to run in bored holes in framing members, providing a distance of 1.25″ to the edge of the framing member. And where run parallel to the framing members, wires are required to be attached to the framing member at a distance not less than 1.25″ to the edge.
Allowing 0.5″ for the sheetrock, if the screw or nail penetrates the wall less than 1.75″, we have an installation with low probability of problems, assuming that the wiring was installed according to NEC article 300.4. But compliance with Article 300.4 is difficult to accomplish in the field (more on this later). Building codes safely limit the size of drywall screws in new installations to 1.25″. This is good for new construction, but after the building is delivered to an occupier, the control is lost. The following hidden dangers exist:
- Installing a bracket on the wall may add 1/8” of depth. A screw can reach the NM wire if it penetrates 1.875″ with a 0.5″ drywall if the wiring is installed per the Code. The typical installer is not aware of this limit.
- It is common to drill a pilot hole prior to installing anchors on walls. The drill bit may penetrate the wire.
- Starting with the 1971 version, the NEC required the bored hole for installation of wiring to be drilled at the center of the stud, but there were no requirements for wires that were installed parallel to framing members. The 1.25” spacing for wires running parallel to framing members was added in the 1990 NEC. Clearly, older houses pose a risk as the depth of the wire attachment is not known.
- Wires that are fished in the walls can be at any location between studs. These wires are particularly susceptible to nail guns. Such nails are fired at high speed and can penetrate the wire, while a screw may just push the fished wire back.
- A common reassurance for installers is a device called a stud finder, which is used to identify the location of the framing members. These devices are typically equipped with a “wire detection” feature. A test was performed utilizing some commercially available stud finders. They did not consistently identify wires in walls. Adding thickness, by placing a plate of wood or other material on the wall, made the stud finder even more ineffective in locating energized wires.
- Professional wire and breaker detector tools are available in the $1,000 to $3,000 price range (these are outside the realm of a do-it-yourselfer and probably even most electricians). The device applies a high-frequency signal to the wire (from a receptacle, for example), which is then traced using a detector unit. The device can effectively trace a branch circuit at the vicinity of the point where the signal is applied. But other branch circuits in the area could be missed in the process.
Probably the most useful tool for identifying wire locations is an endoscopic inspection camera. The lighted camera is inserted into the wall cavity through a small hole. The area between the wall studs can then be inspected for wires.
And with some planning, the building owners can facilitate wire location identification:
- During the construction and before sheetrock is installed, pictures taken from the electrical rough-in can be invaluable. These pictures cost very little to acquire and show the exact location of wiring. They must be transferred to new owners during the home sale process.
- Circuit diagrams show the sequence in which various devices are installed in a branch circuit and are useful for identifying wire locations. Unfortunately, they are typically not available for residential buildings.
One of the dangers associated with accidental penetration of wires is the arcing fault. The National Electrical Code has evolved to protect buildings from this hazard. This is accomplished through the application of arc fault circuit interrupters (AFCI). AFCI requirements were placed into the Code in 2002 for the dwelling unit bedrooms. Later versions of NEC have expanded the AFCI requirements. For the older homes, it would be a good judgment on the part of the owners and electricians to upgrade the circuit breakers.
However, the other hazard associated with accidentally penetrating a wire (electric shock) is not relieved with AFCI breakers. Educating the public – homeowners, cabinet installers, drapery installers, etc. – is the only mitigating solution.
The 1.25″ boring distance requirement in Article 300.4 is difficult to accomplish in the field because it requires the electrician to hold the drill at 90 degrees to the stud during the boring operation. Placing the drill bit at the exact center of the stud, the 1.25″requirement is violated on the other side of the stud if the drill is held at an angle less than 90 degrees. With the large number of holes being drilled, it is difficult to detect this violation, and the required steel protective plate has not been installed. Therefore, this requirement is not commonly enforced. However, it is incumbent upon the authorities having jurisdiction to aggressively enforce Article 300.4 as it is the only way that the home (facility) owners can have some assurance about the maximum size of screws and nails that can be driven into walls.