Understanding Electrical Wires
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 (as in the case of a bare copper grounding wire) or it may be insulated with a continuous layer of nonconductive material, usually made of a special vinyl or nylon plastic. In most cases, it is the type of insulation used around the metal wire 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 (2017 edition). The individual conductors are generally bundled together in a sheathed cable (such as the familiar NM cable) or a plastic or metal conduit (such as EMT). But it is the insulation used on the individual wire conductors inside the cable or conduit that defines if it is suitable for use in wet or damp locations.
Wire Coding
Wire insulation—the plastic jacketing around individual wire conductors—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 insulationX: thermoset; another type of plastic used for insulationH: heat-resistant; two Hs indicates higher heat resistanceN: nylon-jacketed; insulation has a durable coating of nylon for added protectionW: 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 it 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.
THHWTHWTHW-2THWNTHWN-2XHHWXHHW-2UFUSEUSE-2
NM Cable in Wet or Damp Locations?
Even professional electricians express some disagreement over whether or not standard NM (non-metallic) cable (usually designated as NM-B) can be used in damp locations. The answer is no—standard NM-B cable is designated for use only in dry locations. Neither the sheathing itself nor the vinyl-plastic insulation on individual conductors in NM cable is a moisture-proof material. For wet or damp locations, UF-B (underground feeder) cable is designated as acceptable for damp locations, such as when burying it directly in the earth. UF cable sheaths the individual conductors in solid, water-proof plastic vinyl, making it fully moisture-proof.
