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WIRE·FILL·CHARTNEC 2023 · CH. 9
DOC · DEFINITION

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.

·1 MIN READ·EDITORIAL

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-2
  • THHN/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

Related

FIG. 99

FAQ

Thermoplastic Heat & Water-resistant Nylon-coated, second generation. The -2 distinguishes it from earlier THWN, which was rated only 75°C wet. THWN-2 is rated 90°C wet AND dry — matching THHN's dry-only 90°C.

FIG. 98

RELATED TOOLS

01

Conduit Fill Calculator

02

Thhn Wire Fill Chart

METHODOLOGY & SOURCES

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).

WireFillChart Editorial Team·EDITORIAL·