THHN vs THWN Wire: What's the Difference?
Both are common building wire types, but they're rated for different conditions. Here's what electricians need to know about choosing the right wire.
Decoding Wire Names
Wire designation letters tell you exactly what you're getting. Here's what each letter means:
THHN Breakdown
- T - Thermoplastic insulation
- H - Heat resistant (75°C)
- H - High heat resistant (90°C)
- N - Nylon jacket (abrasion resistant)
THWN Breakdown
- T - Thermoplastic insulation
- H - Heat resistant (75°C)
- W - Wet location rated
- N - Nylon jacket (abrasion resistant)
Quick Comparison
| Feature | THHN | THWN | THWN-2 |
|---|---|---|---|
| Dry Location Rating | 90°C | 75°C | 90°C |
| Wet Location Rating | Not rated | 75°C | 90°C |
| Wet Location Use | No | Yes | Yes |
| Conduit Use | Yes | Yes | Yes |
| Nylon Jacket | Yes | Yes | Yes |
The Real-World Answer: THHN/THWN-2
Here's what most electricians should know: modern wire is typically dual-rated THHN/THWN-2. Look at the wire jacket - you'll usually see both designations printed.
Dual-Rated Wire: Best of Both Worlds
THHN/THWN-2 wire is rated 90°C in dry locations AND 90°C in wet locations. This makes it suitable for virtually any building wire application. Most major manufacturers produce their building wire this way.
When you buy "THHN" from a supply house today, you're almost certainly getting THHN/THWN-2 dual-rated wire. But always verify by checking the markings on the wire itself.
What is a "Wet Location"?
Per NEC Article 100 definitions:
Wet Locations Include:
- Underground installations
- Concrete slabs in direct contact with earth
- Locations subject to saturation with water
- Unprotected outdoor locations
- Below-grade conduit (even if above water table)
Damp Locations Include:
- Protected outdoor locations (under canopies)
- Interior locations subject to moderate moisture
- Some basements
Important: Any conduit run underground, through a slab, or outdoors is considered a wet location - even if the conduit appears dry. Always use wet-rated wire in these applications.
Temperature Rating and Ampacity
The wire's temperature rating affects its ampacity from NEC Table 310.16:
| Wire Size | 75°C Column | 90°C Column |
|---|---|---|
| 12 AWG | 25A | 30A |
| 10 AWG | 35A | 40A |
| 8 AWG | 50A | 55A |
| 6 AWG | 65A | 75A |
Remember the Termination Rule
Even with 90°C wire, you typically use the 75°C ampacity column because most terminations (breakers, devices) are rated 75°C. The 90°C rating is mainly useful for derating calculations.
When to Use What
Use THHN/THWN-2 (Dual-Rated) For:
- All general conduit work
- Underground conduit runs
- Outdoor conduit
- Anywhere you need flexibility for future changes
Other Wire Types to Know:
- XHHW-2 - Cross-linked polyethylene, excellent for wet locations
- USE-2 - Underground service entrance (direct burial rated)
- NM-B - Romex, for dry indoor locations only
- UF-B - Underground feeder, direct burial rated
Bottom Line
For most electricians doing conduit work:
- Buy THHN/THWN-2 dual-rated wire (most common at supply houses)
- Verify the wire jacket shows both ratings
- You'll be covered for both dry and wet locations
- Use the 90°C rating for derating calculations, 75°C for final ampacity
Size Wire Correctly Every Time
Ampora's wire sizing calculator handles temperature ratings, derating factors, and NEC ampacity tables automatically.