When a homeowner decided to finish the basement in his modest-sized home, he enlisted the help of trained professionals, hiring a general contractor to finish the downstairs into a bedroom and family room. Unfortunately, the project did not proceed without complications.

The remodeling project included installation of a new electrical panel and rerouting of several circuits in addition to the wiring for lights and receptacles in the newly finished space. The GC subcontracted with an electrical contracting firm to perform modifications to, and installation of, the basement electrical system.

The structure of the basement consisted of a slab-on-grade floor, and a structural steel beam ran overhead for the length of the basement to support the upper levels. Joists for the floor above ran perpendicular to the beam, with a 2×6 nailer board on the top of the beam providing a spacer and method of attachment for the wood joists (see Fig. 1). The drywall enclosure, forming the soffit around the beam and duct work, was secured to the joists using a 90° sheet metal (approximately 25 mil, or 0.025 inches) bracket with the horizontal portion connected to the joists and the drywall screwed to the vertical portion, immediately adjacent to the steel beam (Fig. 2). Numerous 1¼-inch drywall screws were used to secure the drywall to the bracket. Generally, the wiring within the basement was installed by the electrical contractor between joist spaces and through penetrations drilled through the joists. However, one run of 14/3 non-metallic sheathed (NM) cable was installed and secured using two staples to the 2×6 plate that ran along the top of the steel beam (Photo 1).

The accident

As the basement was nearing completion — after the drywall was installed, textured, and painted — the electrical contractor energized the newly installed electrical panel. A few minutes later, one of the workers noticed the smell of smoke. While the source of the smoke was not immediately visible, the electrician noticed heat coming from a particular area of the ceiling and discovered a fire behind the drywall. They immediately called the fire department, and due to a quick response, the fire was extinguished while still contained to the basement.

The investigation

Following the fire, I was hired by the general contractor to determine the cause of the accident. After a thorough forensic investigation and analysis of the evidence, I was able to draw certain conclusions. The fire spread was limited to an area between the joists above the soffit that was built and covered with drywall by the general contractor to enclose ductwork and the structural steel beam that extended across the family room area. Specifically, active combustion was limited to the two joist bays nearest the east wall of the family room exclusive of the joist bay directly above the wall. For the purposes of this article, the joist bay directly above the wall will be designated as joist space 1, the adjacent joist space as 2, and the next space adjacent to joist space 2 as joist space 3, as shown in Fig. 1. The deepest char was found in the joist space 2. Analysis of the burn patterns, and particularly a comparison of the depth of char in joist space 3 against the depth of char in joist space 2, as well as the relative damage within the joist spaces, led me to conclude that the origin of the fire was in the joist space 2 in the vicinity of the nailer board on top of the beam (Photo 2).

Careful examination of the debris removed from the area of the fire revealed a screw that had been subjected to intense heat (Photo 3). Examination of the unburned portions of the soffit structure provided a final clue to the incident (Photo 4). Based on location of the fire origin and the remaining evidence that was collected, it became clear that the cause of the fire was a drywall screw that penetrated the 14/3 NM cable that had been mounted to the 2×6 nailer on top of the steel beam. The screw formed an unintentional electrical path from the energized conductor to the neutral or ground, although the exact path the current traveled could not be identified because the pieces were dismantled during the fire suppression effort. The path had sufficient electrical resistance to prevent the circuit breaker from tripping, but was able to carry sufficient current to generate extreme heat, resulting in ignition of the surrounding combustible materials.

The lesson

The possibility of a drywall screw penetrating the non-metallic sheath of residential wiring is anticipated by the National Electrical Code (NEC). Because of the risks associated with damage to NM cables, the NEC requires that NM cable be protected against damage. Specifically, the Code requires:

“300.4 (D) Cables and Raceways Parallel to Framing Members. In both exposed and concealed locations, where a cable- or raceway-type wiring method is installed parallel to framing members, such as joists, rafters, or studs, the cable or raceway shall be installed and supported so that the nearest outside surface of the cable or raceway is not less than 32 mm (1¼ in.) from the nearest edge of the framing member where nails or screws are likely to penetrate. Where this distance cannot be maintained, the cable or raceway shall be protected from penetration by nails or screws by a steel plate, sleeve, or equivalent at least 1.6 mm (1/16 in.) thick.”

This requirement was violated by the electrical subcontractor in this case, in that the cable was mounted on the surface of the framing member without any protection.

Several options were available to the electrical contractor in this case during installation of the wiring, which would have eliminated the dangerous condition that led to the fire. To prevent this accident, the cable should have been installed in compliance with the Code by one of the following approaches: passing the cable through holes drilled in the joists a minimum of 1¼ inches from the bottom edge, enclosing the cable in rigid metal conduit, or undertaking another appropriate protective measure such as mounting a metal plate (minimum 1/16 inches thick) or wood framing member at least 1¼ inches wide between the cable position and the drywall. The failure of the electrical contractor to comply with the NEC requirements caused this fire.

Palmer, Ph.D., P.E., C.F.E.I., is manager of electrical engineering and fire investigations at Knott Laboratory, LLC, Centennial, Colo.