Walk into any supply house and ask for THHN — the spool you get will almost certainly read THHN/THWN-2 (often with MTW and gasoline-resistance markings too). So is there any difference? Yes — and understanding it keeps you out of trouble in wet locations.
Decoding the Letters
| Letter | Meaning |
|---|---|
| T | Thermoplastic insulation (PVC) |
| HH | High heat resistance — 90°C |
| W | Water/moisture resistant — approved for wet locations |
| N | Nylon jacket over the insulation (abrasion + gas/oil resistance) |
| -2 | 90°C rating maintained even in wet locations |
The Practical Difference
THHN alone: 90°C in dry and damp locations. No wet-location rating. In conduit outdoors, underground, or anywhere water can collect, plain THHN doesn't qualify.
THWN-2: 90°C wet or dry. The "-2" is important — original THWN was limited to 75°C when wet, so THWN-2's full 90°C wet rating lets you use the higher ampacity column (where terminations allow).
Dual-rated THHN/THWN-2 gives you both certifications on one conductor: 90°C in every location, wet or dry. That's why manufacturers dual-rate nearly all production today — one product covers every installation.
Where Each Rating Gets Used
- Dry interior conduit (walls, ceilings, panels): either rating works
- Outdoor conduit: the NEC treats it as a wet location — THWN-2 rating required
- Underground conduit: always a wet location — THWN-2 required
- Slab or below-grade raceway: wet location — THWN-2 required
Ampacity Quick Reference (Copper, NEC 310.16)
| Size | 75°C | 90°C |
|---|---|---|
| 12 AWG | 25 A | 30 A |
| 10 AWG | 35 A | 40 A |
| 8 AWG | 50 A | 55 A |
| 6 AWG | 65 A | 75 A |
| 4 AWG | 85 A | 95 A |
| 1/0 AWG | 150 A | 170 A |
| 4/0 AWG | 230 A | 260 A |
Remember: you can only use the 90°C column when every termination and device in the circuit is also rated 90°C — otherwise the 75°C column governs. Our AWG wire gauge chart has the full table, and the wire size calculator checks voltage drop on long runs.
Related Reading
Comparing insulation families? See our guide on THHN vs XHHW-2 — the thermoplastic vs thermoset decision matters in high-density and high-temperature installations.
We stock dual-rated THHN/THWN-2 copper building wire in all standard gauges and colors, cut to length.
