Several problems arise when installing an outlet box on the exterior of a building. If the box is flush, then it is entirely within the building, is not in a wet location and simply requires a weatherproof cover for a 15 or 20A, 125 or 250V receptacle. In a damp location that is protected from the weather, the cover must be weatherproof when the receptacle is not in use. In a wet location, the cover must be weatherproof whether the receptacle is in use or not. Weatherproof covers are available for toggle switches.
The exterior wall of the building is the demarcation point between a wet (exterior) or damp (protected by an overhang) and a dry (interior) location. Where the box is on the surface, an FS or FD box is generally used, as these boxes have no openings other than for raceway or cable entries. If the wiring method is raceway, the threaded hub in the rear of an FDA box is used, and the conductors must be suitable for a wet location. One can check suitability by looking for a W in the insulation identification. The raceway must be sealed against the movement of warm moisture-laden air to the cooler air in the box on the exterior.
A matter of opinion
When using cable wiring, there are other issues to take into consideration. If the cable connector is threaded into the box hub, it is in a dry location. For example, according to some, if the conductors within the cable are marked THWN, then the cable can enter the wet location within the box. Others disagree on the basis that Type NM cable is permitted for dry locations only [334.10(A)(1)], without regard for the type of wire in the assembly. Still others feel that the interior of a properly covered and sealed box exposed to the weather is a dry location. This key question is whether the interior of a properly sealed and covered enclosure that is exposed to the weather is a dry location.
If the entry to a cast box is not through a hub, but through a tapped hole in the back of the box, then the NM cable connector is in a wet location, and these connectors are suitable for dry locations only unless indicated otherwise on the carton by a Underwriters Laboratories (UL) mark.
Exterior lighting included
Similar problems accompany lighting fixtures on the exterior wall. The issue of covering luminaires (fixtures) in specific locations appears in 410.4 (A): “Luminaires (fixtures) installed in wet or damp locations shall be installed so that water cannot enter or accumulate in wiring compartments, lampholders, or other electrical parts.” This will generally require applying a bead of sealing material where the rim of the fixture canopy meets the wall surface to prevent rainwater from getting behind the fixture and entering the building wall. The luminaire manufacturer’s installation instructions will generally include instructions for applying this weather protection.
A luminaire mounted on an exterior wall that is exposed to the weather is required to be marked “Suitable for Wet Locations.” When wired with Type NM Cable, the same question arises: Can the wiring method that is permitted only for dry locations be used on a wet-location luminaire? In residential construction, it is almost universally done this way.
Unlike a box supplying a switch or receptacle, the box for a luminaire is not covered or sealed against the entrance of moisture except for the bead of sealing material installed where the luminaire canopy meets the wall. How effective is this, and how long will it last?
An even worse and too-common practice is to install a pancake (½ in. deep) box on the surface of the wall. This is a box suitable only for interior use in a dry location, but it is often installed in a wet location.
These may seem to be minor matters. Nit-picking, as it were, but when the electrical contractor is of one mind and the electrical inspector is of a different mind, then these matters are not trivial at all.
The main idea is to keep rainwater out of the building. Water leaking into the interior of wood frame walls may take some time to become evident, but by then it may have developed into a major structural problem.
SCHWAN is an electrical Code consultant in Hayward, Calif. He can be reached at firstname.lastname@example.org