THHN and XHHW-2 are types of insulation used for electrical conductors in various applications. They differ in their construction materials, temperature ratings, and some specific use cases. Here is a brief comparison:
- THHN (Thermoplastic High Heat-resistant Nylon-coated): THHN insulation is made from a thermoplastic material called polyvinyl chloride (PVC). Over the PVC insulation, there's a layer of nylon jacket/covering, which provides additional protection to the insulation and allows for easier pulling through raceways by lowering the coefficient of friction when used with pulling lubricant compounds.
- XHHW-2 (Cross-linked High Heat-resistant Water-resistant): XHHW (XHHW-2) insulation is made from cross-linked polyethylene (XLPE) and is also referred to as Thermoset insulation. Cross-linking involves chemically bonding the polyethylene molecules, resulting in a more robust and heat-resistant material compared to non-cross-linked polyethylene.
- THHN: Typically, THHN wire has a temperature rating of 90°C (194°F) in dry locations and 75°C (167°F) in wet locations. Traditionally most THHN is also THWN-2 as well so it then gets a rating of 90°C (194°F) in both dry and wet locations.
- XHHW: XHHW wire also has a temperature rating of 90°C (194°F) in both dry and 75° C (167°F) in wet locations. However, most manufacturers produce their XHHW as XHHW-2 which gives it a rating of 90°C (194°F) in both dry and wet locations. Cross-linked polyethylene insulation offers better thermal stability compared to THHN and THWN-2 insulation.
- THHN: While THHN insulation can resist moisture to some extent, it is generally recommended for use in dry locations or indoor applications.
Interesting to note that THHN is also rated for dry and damp locations per 310.10(B) but when dual rated as THWN-2 it picks up the wet location rating for use in wet locations, such as, but not limited to, raceways outside and raceways underground.
- XHHW: XHHW (XHHW-2) insulation offers excellent moisture and water resistance, making it suitable for both indoor and outdoor applications, as well as in wet locations.
- THHN: The nylon jacket on THHN insulation provides some degree of chemical resistance, but it may not be suitable for environments with constant exposure to corrosive substances.
- XHHW: The XLPE insulation in XHHW offers excellent resistance to chemicals, oils, and various environmental factors, making it suitable for industrial applications and harsh environments.
- THHN: THHN wire is generally more flexible and easier to install compared to XHHW wire due to its thermoplastic insulation and nylon jacket.
- XHHW: Though not as flexible as THHN wire, XHHW wire still offers good flexibility for installation, especially considering its cross-linked insulation's robust properties.
In summary, while THHN, or THHN/THWN-2 when dual rated, and XHHW (XHHW-2) wires are both suitable for a variety of electrical applications, XHHW (XHHW-2) wire has better moisture, heat, and chemical resistance, making it more appropriate for outdoor and harsh environments. THHN wire is more flexible and easier to install but is best used in dry or indoor applications, and is considered the workhorse of the insulated conductors.
