Wire Insulation Types Guide: THHN, XHHW, NM-B & More

Wire insulation is far more than just a protective coating, it determines where wire can be installed, its temperature rating, and its ampacity. Understanding wire insulation types is essential for selecting the right wire for each application and ensuring safe, code-compliant electrical installations. The designation printed on wire insulation provides all the information needed to determine its proper use.

Decoding Wire Insulation Designations

Wire insulation types are identified by letter codes that indicate their properties. Understanding these letters helps you select appropriate wire for any application.

Common Building Wire Types

THHN (Thermoplastic High Heat-resistant Nylon-coated)

THHN is the most common wire type for conduit installations in dry locations. It features thermoplastic (PVC) insulation with a nylon jacket for added durability and ease of pulling. THHN is rated for 90 degrees C in dry locations but only 75 degrees C in wet locations unless also rated THWN.

• Temperature rating: 90 degrees C dry
• Locations: Dry and damp only
• Applications: Conduit runs, commercial wiring, industrial applications
• Notable features: Thin insulation allows more conductors in conduit

THWN and THWN-2 (Thermoplastic Heat and Water-resistant Nylon-coated)

THWN adds wet location capability to THHN. THWN-2 maintains the 90 degrees C rating in wet locations, making it the most versatile choice for general purpose wiring. Most wire sold today is dual-rated THHN/THWN-2, providing maximum flexibility for various installations.

• THWN temperature rating: 75 degrees C wet and dry
• THWN-2 temperature rating: 90 degrees C wet and dry
• Locations: Wet, damp, and dry
• Applications: Indoor and outdoor conduit, wet locations

XHHW and XHHW-2 (Cross-linked High Heat-resistant Water-resistant)

XHHW uses cross-linked polyethylene (XLPE) insulation, which provides superior heat resistance, chemical resistance, and moisture resistance compared to thermoplastic insulation. XHHW-2 maintains its 90 degrees C rating in wet locations.

• XHHW temperature rating: 90 degrees C dry, 75 degrees C wet
• XHHW-2 temperature rating: 90 degrees C wet and dry
• Locations: Wet, damp, and dry; some types direct burial rated
• Applications: Industrial, direct burial, high-temperature environments

USE and USE-2 (Underground Service Entrance)

USE wire is specifically designed for direct burial applications, typically for service entrance conductors between the utility transformer and the meter. USE-2 is rated for 90 degrees C in wet locations and can also be used for above-ground service entrance conductors when marked for such use.

Cable Assemblies

NM-B (Non-Metallic Sheathed Cable or Romex)

NM-B, commonly known by the brand name Romex, is the standard cable for residential branch circuit wiring. It contains two or three THHN-rated conductors plus a bare ground wire, all enclosed in a PVC jacket. NM-B is rated for 90 degrees C but is typically limited to 60 degrees C ampacity due to the outer jacket.

• Temperature rating: 90 degrees C (conductors), 60 degrees C (ampacity due to jacket)
• Locations: Dry locations only, inside or along building structures
• Restrictions: Not permitted in conduit, not for wet or damp locations
• Applications: Residential and light commercial branch circuits

UF-B (Underground Feeder Cable)

UF-B cable is designed for direct burial without additional protection. The insulation is molded directly around the conductors rather than wrapped, providing excellent moisture resistance. It can be used for outdoor circuits and as a substitute for NM-B in wet or damp locations.

• Temperature rating: 90 degrees C (conductors), 60 degrees C (ampacity)
• Locations: Direct burial, wet, damp, and dry locations
• Burial depth: 24 inches without protection, 12 inches under concrete
• Applications: Outdoor circuits, underground wiring, wet location branch circuits

MC Cable (Metal-Clad Cable)

MC cable contains insulated conductors within a spiral metal armor, providing both physical protection and a grounding path. It is widely used in commercial construction where the metal armor provides protection without installing separate conduit.

• Temperature rating: Based on conductor insulation (typically 90 degrees C)
• Locations: Dry locations, some types rated for wet locations
• Advantages: Fast installation, integrated ground path, physical protection
• Applications: Commercial buildings, industrial installations

AC Cable (Armored Cable or BX)

AC cable, often called BX, is similar to MC but uses its metal armor as the only grounding path. A thin bonding strip inside the armor helps ensure good electrical connection. AC cable is being replaced by MC in many applications because MC includes a dedicated ground wire.

Specialty Wire Types

TFFN and TFN (Fixture Wire)

Fixture wire is used for connecting lighting fixtures and similar equipment. It has thinner insulation and is rated for lower ampacity than building wire. TFFN has a nylon jacket while TFN does not. Fixture wire is limited to 18 AWG and 16 AWG sizes.

MTW (Machine Tool Wire)

MTW is designed for wiring machine tools and other industrial equipment. It can withstand oil, coolant, and other industrial fluids. MTW is often dual-rated with THHN/THWN for maximum versatility in industrial applications.

Welding Cable

Welding cable is extremely flexible multi-strand cable designed for high-current, low-voltage applications. It features extra-fine stranding (Class K or higher) and rubber insulation for maximum flexibility. It is used for welding equipment leads and other high-flex applications.

Temperature Ratings and Ampacity

Wire temperature ratings directly affect ampacity, the maximum current the wire can safely carry. Higher temperature ratings allow more current because the wire can safely operate at higher temperatures without insulation damage.

Choosing the Right Insulation Type

Selecting wire insulation depends on the installation environment, temperature requirements, and code requirements for the specific application.

For Residential Branch Circuits

NM-B (Romex) is standard for most residential applications due to its low cost and ease of installation. Use UF-B for outdoor circuits and any wiring in wet or damp locations. Switch to individual THHN/THWN-2 conductors in conduit for exposed locations or where physical protection is required.

For Commercial Installations

THHN/THWN-2 in conduit or MC cable are the standard choices for commercial buildings. The choice between them depends on labor costs, local practices, and specific installation requirements. XHHW-2 may be preferred in high-temperature locations or where chemical exposure is possible.

For Industrial Applications

Industrial environments often require wire with enhanced properties, such as oil resistance, chemical resistance, high temperature capability, or extreme flexibility. Consult wire manufacturers for specialized types such as MTW, welding cable, or specialty high-temperature wires for these applications.

Installation Considerations

• Always verify wire ratings match the installation environment
• Use wire rated for the highest temperature that may be encountered
• Account for derating when running multiple conductors together
• Never exceed the temperature rating of the lowest-rated component
• Follow NEC requirements for wire type based on location and installation method
• Consider future expansion when selecting conduit and wire types

Understanding wire insulation types ensures you select the right wire for safe, efficient, and code-compliant installations. When in doubt, choose wire with higher ratings than minimum requirements and the modest additional cost provides insurance against future problems and makes the installation suitable for a wider range of conditions.