THHN and XHHW are the two most common building wire types used in commercial and residential electrical installations. Both are rated for 600V and 90°C in dry locations, but they use fundamentally different insulation systems — PVC with a nylon jacket (THHN) versus cross-linked polyethylene (XHHW) — which gives each distinct advantages depending on the environment.
Full name: Thermoplastic High Heat-resistant Nylon-coated
Insulation: PVC (thermoplastic) with nylon jacket
Temp rating: 90°C dry, 75°C wet (THWN-2: 90°C wet)
Voltage: 600V
Full name: Cross-linked polyethylene High Heat-resistant Water-resistant
Insulation: XLPE (thermoset / cross-linked polyethylene)
Temp rating: 90°C dry, 75°C wet (XHHW-2: 90°C wet and dry)
Voltage: 600V (some rated 1000V)
| Property | THHN | XHHW |
|---|---|---|
| Full designation | THHN / THWN-2 | XHHW / XHHW-2 |
| Insulation material | PVC + Nylon jacket | XLPE (cross-linked polyethylene) |
| Insulation type | Thermoplastic | Thermoset |
| Temperature rating — dry | 90°C | 90°C |
| Temperature rating — wet | 75°C (THWN-2: 90°C) | 75°C (XHHW-2: 90°C) |
| Voltage rating | 600V | 600V (some 1000V) |
| NEC Table 310.4(1) listing | Yes | Yes |
| Wet location use | THWN-2 variant only | Yes (rated water-resistant) |
| Direct sunlight (UV) | Limited (requires conduit) | Better UV resistance |
| Conduit fill advantage | Slightly thinner profile | Thicker insulation |
| Typical sizes available | 14 AWG – 1000 kcmil | 14 AWG – 1000 kcmil |
| Common applications | Commercial/residential conduit runs | Industrial, wet locations, healthcare |
| Relative cost | Lower | Higher |
Both THHN and XHHW share the 90°C column but are typically derated to 75°C ampacity values per NEC 110.14(C), based on the temperature rating of terminations (lugs, breakers, outlets). The 90°C rating is still useful for ampacity correction and adjustment factor calculations.
NEC Requirements: Both THHN and XHHW are listed in NEC Table 310.4(1). For ampacity purposes, NEC 110.14(C) typically limits both to 75°C column values unless all terminations are rated for 90°C. The 90°C rating is useful for ampacity correction and adjustment factor calculations per NEC 310.15(B). The THWN-2 and XHHW-2 variants are both rated 90°C in wet locations.
THHN alone is rated for dry locations only at 90°C. However, most modern THHN is dual-rated as THHN/THWN-2, which is rated for wet locations at 90°C. Check the wire markings — if it says THWN-2, it can be used in wet and outdoor locations in conduit.
XHHW has better moisture resistance and more durable insulation since XLPE won't melt like PVC can. For wet locations or demanding environments, XHHW is the better choice. For standard dry conduit runs, THHN works well and costs less.
The -2 suffix means the wire maintains its full 90°C temperature rating in wet locations. Without the -2, both THHN and XHHW are derated to 75°C in wet conditions.
Yes. Both wire types can be installed in the same raceway. Use the ampacity column from NEC Table 310.16 that matches the lowest-rated conductor in the raceway, and apply conduit fill calculations per NEC 310.15(C).