THHN vs THWN-2 — What's the Difference?
THHN is rated 90°C dry-only; THWN-2 adds 90°C wet rating. Most modern building wire is dual-stamped THHN/THWN-2 — identical Table-5 area, only labeling differs.
THHN and THWN-2 are two NEC insulation classifications that appear on US-manufactured building wire — usually printed on the same piece of cable in a dual stamp like THHN/THWN-2. The differences are subtle but matter for code compliance in wet locations.
Side-by-side comparison
| Property | THHN | THWN-2 |
|---|---|---|
| Acronym | Thermoplastic High Heat-resistant Nylon | Thermoplastic Heat & Water-resistant Nylon, 2nd-generation |
| Dry temperature | 90°C | 90°C |
| Wet temperature | Not rated | 90°C |
| Voltage | 600V | 600V |
| Insulation | PVC | PVC |
| Outer jacket | Nylon | Nylon |
| NEC Table 5 area | Same | Same |
| Common stocking | Dual-stamped with THWN-2 | Dual-stamped with THHN |
What's "wet location" mean per NEC
NEC Article 100 defines:
- Dry location: Not normally subject to dampness or wetness. Interior, conditioned spaces.
- Damp location: Subject to moderate degrees of moisture. Basements, partially protected exteriors.
- Wet location: Subject to saturation with water. Underground (per NEC 300.5(B)), direct contact with earth, exposed to weather.
If your conduit might ever see water — outdoor riser, underground (yes, even with sealed conduit per NEC 300.5(B)), unconditioned attic, swimming pool equipment — you need wet-rated conductors. THHN alone fails this test. THWN-2 passes.
Why "THWN-2" not just "THWN"?
THWN (without the -2) was the original wet-rated thermoplastic conductor, rated 75°C wet / 90°C dry. THWN-2 is the upgraded version rated 90°C in both wet AND dry, matching THHN's temperature in dry.
If you find THWN (no -2), it's the older stock. For new installations, THWN-2 is preferred — the 90°C wet rating gives you more ampacity headroom.
What's the area difference?
There is none. NEC Chapter 9 Table 5 lists THHN, THWN, THWN-2, MTW, and XHHW all with identical cross-sectional areas:
| Gauge | All five insulations |
|---|---|
| 14 AWG | 0.0097 in² |
| 12 AWG | 0.0133 in² |
| 10 AWG | 0.0211 in² |
| 8 AWG | 0.0366 in² |
| 6 AWG | 0.0507 in² |
So if you switch from THHN to THWN-2 on the same gauge, the wire fill chart count doesn't change.
Practical choice
For 99% of US installations today, just use THHN/THWN-2 dual-stamped wire. You get:
- Dry-location performance of THHN
- Wet-location compliance of THWN-2
- Same price (virtually all suppliers stock dual-stamped)
- No risk of accidentally using dry-only wire in a wet location
The only reason to specify single-stamp THHN is if you're absolutely certain the application is dry AND you found a price discount — rarely worth the risk.
What about THHN vs XHHW?
XHHW (cross-linked polyethylene insulation) is another 90°C option:
- THHN/THWN-2: PVC + nylon, slightly thinner overall, easier to pull, cheaper
- XHHW / XHHW-2: XLPE, slightly thicker, better high-temperature ageing, common for solar PV
NEC Table 5 lists XHHW with identical area to THHN. See THHN vs XHHW.
Quick reference
- Indoor dry conduit: THHN or THHN/THWN-2 — same fill numbers
- Outdoor / underground / wet: THWN-2 (not THHN alone)
- Solar PV: Often XHHW-2 (UV resistance, longer outdoor life)
- High temperature ambient: XHHW-2 has slightly better long-term ageing than THHN
- Cheapest building wire: Dual-stamped THHN/THWN-2 — buy this for general use
Verify your specific conduit fill in the conduit fill calculator.