I made a comment while teaching a class that nonmetallic (NM) cable was not permitted in an outdoor, wet location. An attendee took exception to that statement and asked me to provide National Electrical Code (NEC) justification for the assertion. Initially, the answer seemed very easy to justify, but there were extenuating circumstances that were more difficult to address. So, I decided to use this opportunity to research the pros and cons of these installations and applications.
The simple answer to the question of whether NM cable can be used in outdoor, wet locations would be better dealt with as a question of whether NM cable can be used in a wet location. The quick answer to that question—no—is found in 334.12(B)(4), which states that NM and NMS cables are not permitted in wet or damp locations. NM cable is defined as insulated conductors enclosed within an overall nonmetallic jacket, and NMS cable is defined as insulated power or control conductors with signaling, data and communications conductors within an overall nonmetallic jacket. NMC cable is permitted by 334.10(B)(1) to be installed in exposed and concealed work in dry, moist, damp or corrosive locations, but it is not manufactured at this time. However, Type UF-B (underground feeder) cable, covered by Article 340, can be used as a substitute for NMC. Type UF-B cable may be used underground, including direct burial for outside lights and other loads or utilization equipment in wet and damp locations. Multiple conductor UF-B cable may be used for interior branch-circuit wiring in residential buildings at conductor temperatures not to exceed 90°C with an ampacity limited to that for 60°C conductors as specified by 340.10(4) in the NEC. The voltage rating for UF-B cable is 600 volts.
The next question involves NM cable used in a dwelling unit and connected to a panelboard or all-in-one panel located on an exterior wall outside of the dwelling in a damp or wet location. Is the panelboard enclosure considered to be a wet or damp location, and is the inside of the enclosure a wet or damp location? Section 312.2 states that, “in damp or wet locations, surface-type enclosures within the scope of this article shall be placed or equipped so as to prevent moisture or water from entering and accumulating within the cabinet or cutout box, and shall be mounted so there is at least 6 mm (¼ in.) airspace between the enclosure and the wall or other supporting surface. Enclosures installed in wet locations shall be weatherproof.”
Article 100 defines “weatherproof” as constructed or protected so that exposure to the weather will not interfere with successful operation. However, “watertight” is defined as constructed so that moisture will not enter the enclosure under specified test conditions. UL 50E provides the environmental test conditions for enclosures with a rain test for Type 3R enclosures, stating a Type 3R enclosure shall be considered to have met the requirements if, at the conclusion of the test, there is no accumulation of water within the enclosure and no water has entered the enclosure at a level higher than the lowest live part. Water may, however, enter above live parts in an enclosure if (1) the Type 3R enclosure that allows water to enter is marked or provided with instructions indicating the areas in which live parts will or will not be located; (2) the equipment is so constructed that no water is visible on the live parts, insulating material or mechanism parts; and (3) no water has entered any space within the enclosure in which wiring may be present under any proper installation conditions. Based on these criteria, the NM cable conductors should not be exposed to water within the enclosure.
In many dwelling units, NM cable is installed in walls and attics and stubbed out through fascia (freeze) boards or soffits to terminate in weatherproof boxes to supply power to floodlights. Where these boxes are not placed in appropriate locations to illuminate the area, NM cables are often installed in EMT, as well as other raceways, to extend the circuit conductors out to the necessary location on the fascia board. Section 300.9 states that, where any raceways are installed in an above-grade wet location, the interior of the raceway shall be considered to be a wet location. Insulated conductors and cables installed in wet locations above-grade shall comply with 310.10(C), providing specific insulated conductors listed for use in a wet location or those that have a “W” in the insulation type. If in a wet location, use UF-B which is listed for use in a wet location, rather than take the chance of a violation notice from the inspector.