THHN - Thermoplastic High Heat-resistant Nylon-coated
THHN is the default building wire for dry-location branch circuits and feeders across North America. Its PVC insulation is thin, and a nylon outer jacket adds abrasion and gasoline/oil resistance without adding much diameter -- which keeps conduit fill calculations favorable on multi-conductor runs.
On its own, THHN's UL 83 listing rates it for 90°C in dry locations only. Nearly all THHN sold today is manufactured to also meet THWN-2 requirements and carries a combined THHN/THWN-2 print on the jacket -- that dual marking is what makes it usable in wet locations at a 75°C-equivalent rating. Wire printed as plain THHN with no THWN-2 designation should be treated as dry-location-only.
Limitation: even dual-rated THHN/THWN-2 has thinner insulation than XHHW-2 and can be more susceptible to nicking during pulls through conduit with sharp bends or rough fittings.
XHHW-2 - Cross-linked High Heat-resistant Water-resistant
XHHW-2 replaces THHN's PVC insulation with cross-linked polyethylene, a thermoset material that holds its 90°C rating whether the conductor is wet or dry -- with no dual-marking dependency the way THHN needs THWN-2. That single, unconditional rating is the entire reason XHHW-2 exists as a separate product class under UL 44.
XLPE insulation also resists moisture absorption, ozone, and many chemicals better than PVC over a multi-decade service life, which is why XHHW-2 is the default in underground feeders, wet/damp equipment rooms, and outdoor installations exposed to weather without conduit-level protection.
Trade-off: XLPE is slightly stiffer than PVC, which can make XHHW-2 marginally harder to pull and terminate in tight panel enclosures compared to THHN.
THHN vs XHHW-2: Comparison Table
| Characteristic | THHN (dual-rated THHN/THWN-2) | XHHW-2 |
|---|---|---|
| Insulation material | PVC + nylon jacket | Cross-linked polyethylene (XLPE) |
| Dry-location rating | 90°C | 90°C |
| Wet-location rating | 75°C (requires THWN-2 dual mark) | 90°C (unconditional) |
| Voltage rating | 600V | 600V |
| UL standard | UL 83 | UL 44 |
| Relative cost | Baseline | ~10-20% higher |
| Typical use | Dry-location branch circuits, panels, conduit runs indoors | Underground feeders, outdoor/wet locations, direct burial with conduit |
Which One Should You Specify?
Choose THHN/THWN-2 when
- Circuit stays in dry, indoor conduit or raceway
- Conduit fill is tight and the smaller diameter matters
- Budget is the primary driver on a large panel/feeder job
Choose XHHW-2 when
- Installation is underground, direct burial in conduit, or outdoors
- The location sees standing water, high humidity, or repeated wet/dry cycling
- You want one SKU with an unconditional 90°C rating rather than depending on a dual THWN-2 mark
Inspection tip: if a run is wet-location and the wire on-site is printed plain “THHN” with no THWN-2 marking, it will fail inspection under NEC 310.104 regardless of gauge or ampacity -- the location classification is what governs, not the current rating.
Frequently Asked Questions
Can THHN be used in wet locations?
Only if it carries a dual rating. Most building wire sold today is printed THHN/THWN-2 on the jacket, which means it meets THHN's 90°C dry rating and THWN-2's 75°C wet rating simultaneously. Plain THHN with no THWN-2 marking is dry-location only and cannot be used underground, outdoors, or in damp conduit runs.
Is XHHW-2 more expensive than THHN?
Yes, typically 10-20% more per foot at the same gauge. The premium comes from the cross-linked polyethylene (XLPE) insulation process, which requires an additional curing step that PVC-insulated THHN doesn't need. The extra cost buys full 90°C wet-and-dry rating with no dual-marking dependency, plus better chemical and moisture resistance over the cable's service life.
Do THHN and XHHW-2 carry the same ampacity at the same gauge?
Yes, when both are rated at 90°C (dual-marked THHN/THWN-2 versus XHHW-2), NEC Table 310.16 assigns them the same ampacity column. The practical difference isn't current-carrying capacity -- it's where each is permitted to be installed and how the insulation performs under sustained moisture exposure.
Which one should I specify for direct burial or outdoor conduit?
XHHW-2 is the safer default for anything outdoor, underground, or in a location that sees standing water or high humidity -- its 90°C rating holds in both wet and dry conditions without needing a dual THWN-2 marking. THHN/THWN-2 dual-rated wire is also acceptable for wet locations and is common in North American commercial construction, but always confirm the jacket print shows the THWN-2 designation before using it outdoors.
Conclusion
THHN and XHHW-2 deliver the same 90°C ampacity at the same gauge -- the decision comes down to where the wire is installed, not how much current it needs to carry. Dry, indoor, cost-sensitive runs favor dual-rated THHN/THWN-2; anything wet, buried, or outdoor should default to XHHW-2's unconditional rating. When a project mixes both environments, standardizing on XHHW-2 for the whole job removes the risk of an inspector finding plain THHN in a wet-location run.