What is THWN-2 Wire?
THWN-2 is Thermoplastic Heat & Water-resistant Nylon-coated wire, rated 90°C wet AND dry. Same NEC fill area as THHN.
THWN-2 is a thermoplastic insulated, nylon-jacketed conductor rated 90°C in both wet AND dry locations. It's the wet-rated counterpart to THHN — and most modern US building wire is dual-stamped THHN/THWN-2.
Construction (identical to THHN)
- Conductor: Solid or stranded copper or aluminum
- Insulation: PVC, 15–30 mils
- Outer jacket: Nylon (polyamide) film
- Voltage: 600V
- Temperature: 90°C dry AND wet
Why does the "-2" matter?
The original THWN (no -2) was rated 75°C in wet, 90°C in dry. The newer THWN-2 (introduced into NEC editions starting 1999) is rated 90°C in both — giving you higher allowable ampacity for wet-location installations.
For the wire ampacity calculator, the difference matters when conductors run in:
- Underground PVC (always wet per NEC 300.5(B))
- Outdoor risers exposed to weather
- Unconditioned attics where condensation forms
In those locations, THWN gives you the 75°C column ampacity; THWN-2 gives you the 90°C column ampacity.
Same fill area as THHN
NEC Table 5 lists THHN, THWN, THWN-2, XHHW, and XHHW-2 with identical cross-sectional area. So the wire fill chart doesn't change when you switch among them.
Sold as dual-stamped
Virtually all US building wire today is sold with both stamps printed on the jacket:
THHN/THWN-2THHN/THWN-2/MTW(machine tool wire)THHN/THWN-2/MTW/AWM(appliance wiring material)
You get THHN's dry-location compliance plus THWN-2's wet-location compliance at the same price. Single-stamp THHN exists but offers no savings.
When to specify THWN-2 over THHN
- Any application where the conduit might see moisture, condensation, or direct water
- Underground service laterals, even in "sealed" raceways (NEC 300.5(B))
- Outdoor risers above grade
- Exterior building wiring
- Damp / unconditioned spaces