Is NM Cable Acceptable in a Wet Location?

For example, has a limited amount of NM cable been installed in outside locations, such as inside a Type 3R enclosure or inside electrical metallic tubing (EMT) where the EMT has been installed exposed on an outside wall, and has there been an issue with this type of installation? What would be an acceptable, inexpensive substitute for NM cable that would solve the National Electrical Code prohibition of NM cable in a wet location? Is NM cable installed in EMT located outside considered to be a wet location, even inside the raceway?

These and many other questions need to be considered and answered. Code-Making Panels should be considering these issues for the next Code cycle relating to the installation of NM cables, since many parts of the country are not complying with the requirements in Article 334 of the NEC.

I will start out by citing the installation issues with Article 334 and any prohibitions located within the article. For example, 334.10 covers “Uses Permitted” for NM and nonmetallic-sheathed cable (NMC.) Section 334.2 defines these two cable assemblies. Type NM cable is defined as insulated conductors enclosed within an overall nonmetallic jacket, while Type NMC is insulated conductors enclosed within an overall corrosion-resistant nonmetallic jacket. The only difference between the two definitions is that NMC is corrosion-resistant and NM cable is not. NM cable can only be installed in dry locations, while NMC can be installed in dry, moist, damp and corrosive locations. In addition, 334.12(B)(4) limits NM cable from being installed in wet or damp locations.

The answer to most of the questions posed in the first paragraph would be easily resolved by installing NMC. NMC is being manufactured and listed by some manufacturers as a dual-rated Type NMC plus underground feeder cable (Type UF-B), as covered in Article 340. The problem from an installer’s point of view is that UF-B cable is very difficult and time consuming to strip off the outside jacket compared to stripping NM cable. The Type UF-B cable, based on 340.112 and used as a substitute for Type NM cable, must have individual conductor insulation that is rated at 90°C (194°F), which is the same as required in 334.112 for NM cable. While both are rated at 90°C, the ampacity for each is still based on the 60°C column of Table 310.16 of the 2020 NEC.

Now that we understand the requirements in Articles 334 and 349, we should consider the location of the installation. In the Southwestern and Southern part of the country, dwelling unit services are often located on the outside of the house. A common installation procedure in these areas is to place the NM cables into a bundle, using a chase nipple in the back of the Type 3R panelboard, bring the NM bundle into the panelboard, at which time outside jacket would be stripped off and the conductors terminated in the panelboard.

A Type 3R panelboard is tested and listed for installation in rain, snow and sleet in accordance with UL 50 and Table 110.28. There are two main Code issues with bringing a bundle of NM cable into the chase nipple and into the panelboard. The first one is in 312.5(C) that states where a cable is installed in a cabinet or cutout box, each cable shall be secured to the cabinet or cutout box. Using a chase nipple is not a method of securing the NM cable to the enclosure. The second issue is the use of NM cable in a damp or wet location, as previously discussed. While this type of installation is very common in areas of the South and Southwest that do not get snow, these installations would technically be a violation of the NEC .

Another common application is to drill a hole in the freeze boards located outside between the trusses or drill a hole in the fascia boards and stub NM cable into a connector in the back of a weatherproof box for outside lights or receptacles. The NM cable would be in a damp or wet location and a violation of the NEC. There are installations where the NM cable is installed into the weatherproof box or an LB condulet and, using EMT, extended along the eaves or fascia. Based on 300.9, the interior of the raceway is a wet location.

The question of whether nonmetallic cable can be installed in wet locations is a little more complicated than yes or no. You’ll need to consider location and a host of other issues before installation to ensure you do not violate the NEC requirements while still ensuring that the installation is safe.

About the Author

Mark C. Ode

Fire/Life Safety, Residential and Code Contributor

Mark C. Ode is a lead engineering associate for Energy & Power Technologies at Underwriters Laboratories Inc. and can be reached at 919.949.2576 and Mark.C.Ode@ul.com.

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