THHN vs THWN Cable: Dual-Rated Wire Explained

Decoding the Letter Codes

UL wire type designations are constructed from letter codes defined in UL 83 (Thermoplastic-Insulated Wires and Cables) and referenced by NEC Article 310. Each letter or pair of letters indicates a specific property:

Applying this to the full designations:

• THHN — Thermoplastic + High-Heat-resistant + Nylon jacket. Originally rated 90°C dry/damp locations only.
• THWN — Thermoplastic + Heat-resistant + Wet-rated + Nylon jacket. 75°C wet locations.
• THWN-2 — Updated THWN specification: 90°C in wet locations. The "-2" suffix is the significant upgrade.

Why Modern Wire Carries Both Marks

In the 1970s and 1980s, THHN and THWN were separate wire types with different insulation thicknesses and ratings. As UL updated its standards, the specifications converged. Manufacturers found they could build a single construction that satisfied both UL 83's THHN requirements (90°C dry, nylon jacket) and THWN-2 requirements (90°C wet, nylon jacket, appropriate PVC wall thickness).

The result is that essentially all standard building wire manufactured since the mid-1990s carries both marks printed on the jacket. The dual mark "THHN/THWN-2" is not a marketing claim — it means the wire has been UL Listed to both standards simultaneously. You are buying one product that is rated for both applications.

THHN vs THWN vs THWN-2: Quick Comparison

The 75°C vs 90°C Ampacity Column: The Critical NEC Rule

This is where most installation confusion occurs. NEC Table 310.16 lists ampacities at 60°C, 75°C, and 90°C. The rule from NEC 310.15(B)(7) (310.15(B)(3) in the 2023 NEC) is:

The ampacity of a conductor must not exceed the lowest-rated temperature limit of any connected termination, conductor, or device.

In practice this means:

• If all terminations (breaker lugs, panel lugs, load terminals) are rated 75°C — use the 75°C column. This is the most common case.
• If all terminations are rated 90°C — you may use the 90°C column. This is rare; most standard panelboard lugs are 75°C.
• The 90°C wire rating is primarily used as a starting point for derating calculations under NEC 310.15(B)(3) (conduit fill) and ambient temperature correction. After applying derating factors to the 90°C ampacity, the result must not exceed the 75°C column value for standard 75°C-termination equipment.

Example: 2 AWG THHN/THWN-2 copper has a 90°C ampacity of 130A and a 75°C ampacity of 115A. If installed in a conduit with five current-carrying conductors in an ambient of 40°C, you apply derating factors to the 130A value — but the final derated result cannot exceed 115A (the 75°C column value) if terminations are 75°C-rated.

XHHW-2: When to Choose It Over THHN/THWN-2

XHHW-2 uses cross-linked polyethylene (XLPE) insulation instead of PVC. Both types carry the same 90°C wet/dry rating and are used in conduit installations. XHHW-2 advantages:

• Lower friction coefficient — easier conduit pulls, especially for long runs or many bends
• Better moisture barrier — XLPE is less permeable than PVC, beneficial for conduit systems prone to water infiltration
• Better chemical resistance — oils and many solvents that attack PVC have less effect on XLPE
• Better low-temperature flexibility — important in cold climates where wire must be installed at or below freezing

THHN/THWN-2 advantages over XHHW-2:

• Lower cost — PVC insulation is less expensive than XLPE at equivalent gauge
• Wider color availability — standard NEC color code sets are easier to source in THHN
• Better flame resistance — PVC formulations in THHN tend to have slightly better flame spread ratings

Common Installation Notes

• THHN/THWN-2 is approved for installation in conduit (EMT, RMC, IMC, PVC), cable trays, and as single conductors in raceways. It is not approved for direct burial without conduit.
• The nylon jacket on THHN/THWN-2 provides pull lubrication but is not a separate insulation layer — it is not counted toward the insulation wall thickness for voltage rating purposes.
• THHN/THWN-2 is available in copper and aluminum conductors. Aluminum conductors require appropriate anti-oxidant compound at terminations and must use Al-rated connectors and lugs.