What is THHN Wire?
THHN is Thermoplastic High Heat-resistant Nylon-coated insulated copper or aluminum building wire, rated 90°C in dry locations per NEC Article 310.
THHN is the most-pulled insulated conductor in US commercial and residential building wiring. The acronym stands for Thermoplastic High Heat-resistant Nylon-coated — describing the insulation system that protects the copper or aluminum conductor inside.
Construction
A THHN conductor consists of:
- Conductor: Solid or stranded copper or aluminum
- Insulation: PVC (polyvinyl chloride) thermoplastic, 15-30 mils thick depending on gauge
- Outer jacket: Nylon (polyamide) film, ~4-5 mils, for abrasion and chemical resistance
The nylon jacket is what gives THHN its slippery feel and makes it easier to pull through conduit than non-nylon-jacketed alternatives like THW or RHH.
Ratings
- Temperature (dry): 90°C / 194°F
- Temperature (wet): Not rated — must be THWN-2 or XHHW-2 for wet locations
- Voltage: 600V nominal
- Use: Building wire in raceways, NEC Article 310
Conductor cross-sectional area (NEC Table 5)
| Gauge | Area in² |
|---|---|
| 14 AWG | 0.0097 |
| 12 AWG | 0.0133 |
| 10 AWG | 0.0211 |
| 8 AWG | 0.0366 |
| 6 AWG | 0.0507 |
| 4 AWG | 0.0824 |
| 2 AWG | 0.1158 |
| 1/0 | 0.1855 |
| 4/0 | 0.3237 |
| 250 kcmil | 0.3970 |
| 500 kcmil | 0.7073 |
| 1000 kcmil | 1.3478 |
These values feed directly into the wire fill chart — multiplied by conductor count, divided by conduit interior area, compared to the NEC 53/31/40% fill rule.
When NOT to use THHN
- Wet locations: Conduit subject to water infiltration (outdoor, underground, even some indoor damp). Use THWN-2 or XHHW-2 instead.
- Direct burial: THHN is not rated for direct burial. Use USE-2 for direct burial.
- Free air without raceway: Not permitted as exposed wiring; must be in a raceway or cable assembly.
Common dual stamps
Most modern THHN sold in the US is stamped with multiple ratings on the same conductor:
- THHN/THWN-2: Adds wet 90°C rating (most common modern building wire)
- THHN/THWN-2/MTW: Adds Machine Tool Wire rating
- THHN/THWN-2/AWM: Adds Appliance Wiring Material
For NEC fill calculations, all five behaviors (THHN, THWN, THWN-2, XHHW, XHHW-2) share identical Table 5 areas — pick whichever matches your wet/dry conditions.
Pricing and availability
THHN is the dominant SKU in electrical supply houses — typically the cheapest insulated copper conductor per foot in 14–500 AWG sizes. Sold in 500-ft and 2500-ft spools, plus cut-lengths at most distributors.
Related
FAQ
RELATED TOOLS
This page's figures are transcribed from NEC 2023 (NFPA 70) Chapter 9, Tables 1, 4 and 5, and checked by a version-controlled automated test suite — they are not signed off by a licensed professional. WireFillChart is an independent reference, not affiliated with or endorsed by NFPA. Always verify against the code edition adopted by your local authority having jurisdiction (AHJ).