Start with the acronyms
• THHN = Thermoplastic High Heat-resistant Nylon-coated
• THWN = Thermoplastic Heat- and Water-resistant Nylon-coated
• THWN-2 = THWN "second generation"
Temperature and moisture limits (per UL-83)
• THHN: 90°C in dry air, only 75°C when wet.
• THWN: 90°C dry / 75°C wet, plus basic water resistance.
• THWN-2: 90°C in both dry and wet locations, giving the widest safety margin.
Construction differences
All three use a PVC insulation layer plus a thin nylon outer jacket for abrasion resistance. THWN and THWN-2 add extra waterproofing compounds and slightly thicker jackets; THWN-2 is roughly 10% larger in diameter than THHN of the same gauge.
Where each wire shines
• THHN: Indoor conduit, dry commercial or residential branch circuits-lowest cost, highest availability.
• THWN: Interior/exterior conduit that might see occasional moisture (basement, garage).
• THWN-2: Underground conduit, coastal sites, chemical plants, or any project that must pass inspection in both wet and high-heat conditions.
The modern shortcut-dual-rated THHN/THWN-2
Most wire sold today carries both labels. A single reel marked "THHN/THWN-2" meets all three specs, so you no longer need to stock separate spools.
Practical selection checklist
Identify the environment: dry only, damp, or fully wet.
Check the ambient temperature: if >75 °C in any condition, specify THWN-2.
Verify conduit fill tables-THWN-2's larger OD may reduce allowable conductors.
When in doubt, buy dual-rated wire; the extra cost is minimal compared to a re-pull.
