THHN vs XHHW-2: Insulation Chemistry, Conduit Fill & Temperature Ratings
THHN (Thermoplastic High Heat-resistant Nylon-coated) uses PVC insulation with an outer nylon jacket, while XHHW-2 uses thermoset cross-linked polyethylene (XLPE). XHHW-2 provides superior moisture resistance, retains 90°C ampacity ratings in wet underground conduits, and avoids the cold-weather cracking common with PVC.
Technical Comparison & Conversion Matrix
| Specification | THHN / THWN-2 | XHHW / XHHW-2 |
|---|---|---|
| Insulation Material | Thermoplastic PVC with extruded Nylon protective skin | Thermoset Cross-Linked Polyethylene (XLPE) |
| Temperature Rating (Dry) | 90°C (194°F) | 90°C (194°F) |
| Temperature Rating (Wet) | 75°C (THHN/THWN) / 90°C (THWN-2) | 90°C (194°F) standard for XHHW-2 |
| Outer Nylon Jacket | Yes (susceptible to damage during tight conduit pulls) | No (single-layer solid homogeneous insulation) |
| Dielectric / Moisture Durability | Prone to degradation in submerged wet conduits over decades | Superior resistance to water treeing and underground chemical exposure |
| Cold Weather Flexibility | Becomes brittle below -10°C; risk of jacket fracturing | Maintains flexibility down to -40°C without cracking |
| Conduit Fill Factor | Thinner outer diameter allows marginally easier conduit packing | Slightly thicker jacket requires strict conduit sizing verification |
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Frequently Asked Questions
Can THHN wire be used outdoors in conduit underground?
Only if dual-rated as THWN or THWN-2. Standard THHN is dry-location only. Underground conduit is legally classified by NEC Article 100 as a wet location because condensation and groundwater ingress inevitably fill underground raceways.
Why is XHHW-2 preferred over THHN for commercial and industrial service entrances?
XHHW-2 uses cross-linked polyethylene that does not melt when exposed to high short-circuit fault currents. It produces less toxic halogen gas when burned, resists chemical leakage in wet raceways, and has higher dielectric insulation resistance.