THHN is Thermoplastic High Heat-resistant Nylon-coated: a single insulated conductor rated 600 V and 90°C in dry locations. Nearly everything sold today is dual-marked THHN/THWN-2, which adds a 90°C wet rating. It is the default wire for anything pulled through conduit, and it is a conductor rather than a cable — it always needs a raceway.
| Property | Value |
|---|---|
| Voltage rating | 600 V |
| Temperature rating (dry) | 90°C (194°F) |
| Temperature rating (wet) | 75°C for THWN, 90°C for THWN-2 |
| Insulation | PVC (polyvinyl chloride) with an extruded nylon jacket |
| Conductor | Copper or aluminum, solid through 10 AWG, stranded above |
| Sizes | 14 AWG through 1000 kcmil |
| NEC article | Article 310, Table 310.4(A) |
| Ampacity column | 90°C for derating, but limited to 75°C at terminations |
| Direct burial | Not permitted as a bare conductor — requires a raceway |
| Sunlight resistance | Only where the marking says "sunlight resistant" |
| Size | Ampacity |
|---|---|
| 14 AWG | 20 A |
| 12 AWG | 25 A |
| 10 AWG | 35 A |
| 8 AWG | 50 A |
| 6 AWG | 65 A |
| 4 AWG | 85 A |
| 3 AWG | 100 A |
| 2 AWG | 115 A |
| 1 AWG | 130 A |
| 1/0 AWG | 150 A |
| 2/0 AWG | 175 A |
| 3/0 AWG | 200 A |
| 4/0 AWG | 230 A |
T — thermoplastic insulation. H — heat resistant, good for 75°C. HH — high heat resistant, 90°C. W — suitable for wet locations. N — nylon outer jacket, which adds abrasion, oil and gas resistance and makes the wire slippery enough to pull easily.
The -2 suffix on THWN-2 means 90°C in both wet and dry locations. Plain THWN is only 75°C wet. Since almost all stock is dual-rated THHN/THWN-2, you get the 90°C wet rating whether you asked for it or not — but check the printing on the jacket rather than assuming.
THHN is a single conductor, not a cable assembly. It must be installed in a raceway — EMT, PVC, rigid metal, IMC, flexible metal conduit, or a wireway. It may not be run exposed, stapled to framing, or fished through walls the way NM-B cable can.
In wet locations, including any underground raceway, you need the THWN-2 rating. NEC 300.5(B) defines the interior of an underground raceway as a wet location, so plain THHN is not legal in buried PVC even though the pipe looks dry when you install it. Direct burial without a raceway is never permitted — that is what USE-2 and UF-B are for.
The most common THHN mistake is loading a conductor to its 90°C ampacity. NEC 110.14(C) limits the circuit to the lowest temperature rating of any termination, and virtually every breaker and lug is listed at 75°C. So 12 AWG THHN is a 30 A conductor on paper, a 25 A conductor at the terminals, and a 20 A circuit once NEC 240.4(D) is applied.
What the 90°C rating actually buys you is headroom for derating. Nine conductors in a conduit derate to 70%: from the 90°C column that is 30 × 0.70 = 21 A, which still supports a 20 A circuit. From the 75°C column it would be 17.5 A and you would be forced up to 10 AWG.