The National Electrical Code (NEC) is a nationally recognized document that outlines recommendations and specifications for electrical installations, including when you can use wiring for damp or wet locations. It is revised every three years, and at the time of this writing, it is the 2017 NEC that is current. While most city and state electrical codes follow the NEC, only the local electrical codes (enforced by the authority having jurisdiction, or AJH) are law in a given jurisdiction. Therefore, the local AJH is the best place to learn about specific requirements for damp or wet applications.
An electrical wire consists of a solid or stranded metal core and is also known as a conductor. A wire may be a bare conductor or it may be insulated with a continuous layer of nonconductive material, usually made of a special plastic. In most cases, it is the type of insulation used that makes a conductor suitable for dry, damp, or wet applications. While any wire that is approved for damp or wet locations can also be used in dry locations, there are some dry-location wires that do not have the appropriate insulation to be used in damp or wet locations. Appropriate uses for various wire types are given in Article 310.10 of the NEC.
Wire insulation is identified by a code that includes letters and sometimes numbers. The simplest way to determine whether a wire is suitable for damp or wet locations is to look for a "W" in the coding on the wire's insulation. This is easy to remember because the W essentially stands for "wet." However, there are some wires that are suitable for wet locations that don't carry a W in their coding. Some of the most common code letters found in wiring for residential applications include:
- T — thermoplastic; referring to the type of plastic insulation
- X — thermoset; another type of plastic used for insulation
- H — heat-resistant; two Hs indicates higher heat resistance
- N — nylon-jacketed; insulation has a durable coating of nylon for added protection
- W — wet locations
Decoding the Code
As an example, two of the most common types of wire used in residential projects are THHN and THWN. THHN is thermoplastic, highly heat-resistant (up to 90 degrees C), and nylon-jacketed. It is not suitable for wet locations. THWN is similar to THHN but is rated only for a maximum of 75 degrees C and is suitable for wet locations. It's important to note that the application can affect the temperature rating of a wire. For example, THHW wire is rated for 90 degrees C in dry locations but is rated only for 75 degrees C when used in wet locations.
Short List of Wet Wires
The following are some of the most commonly used types of wires suitable for wet locations. Again, note that some do not have a W in their coding. Also, a number following the letter designation usually indicates a higher value for one or more of the ratings; for example, THWN is rated for 75 degrees C in both wet and dry locations, while THWN-2 is rated for 90 degrees C for both wet and dry locations.