THHN vs THWN-2 — What's the Difference?

THHN and THWN-2 are two NEC insulation classifications that appear on US-manufactured building wire — usually printed on the same piece of cable in a dual stamp like THHN/THWN-2. The differences are subtle but matter for code compliance in wet locations.

Side-by-side comparison

What's "wet location" mean per NEC

NEC Article 100 defines:

• Dry location: Not normally subject to dampness or wetness. Interior, conditioned spaces.
• Damp location: Subject to moderate degrees of moisture. Basements, partially protected exteriors.
• Wet location: Subject to saturation with water. Underground (per NEC 300.5(B)), direct contact with earth, exposed to weather.

If your conduit might ever see water — outdoor riser, underground (yes, even with sealed conduit per NEC 300.5(B)), unconditioned attic, swimming pool equipment — you need wet-rated conductors. THHN alone fails this test. THWN-2 passes.

Why "THWN-2" not just "THWN"?

THWN (without the -2) was the original wet-rated thermoplastic conductor, rated 75°C wet / 90°C dry. THWN-2 is the upgraded version rated 90°C in both wet AND dry, matching THHN's temperature in dry.

If you find THWN (no -2), it's the older stock. For new installations, THWN-2 is preferred — the 90°C wet rating gives you more ampacity headroom.

What's the area difference?

There is none. NEC Chapter 9 Table 5 lists THHN, THWN, THWN-2, MTW, and XHHW all with identical cross-sectional areas:

So if you switch from THHN to THWN-2 on the same gauge, the wire fill chart count doesn't change.

Practical choice

For 99% of US installations today, just use THHN/THWN-2 dual-stamped wire. You get:

• Dry-location performance of THHN
• Wet-location compliance of THWN-2
• Same price (virtually all suppliers stock dual-stamped)
• No risk of accidentally using dry-only wire in a wet location

The only reason to specify single-stamp THHN is if you're absolutely certain the application is dry AND you found a price discount — rarely worth the risk.

What about THHN vs XHHW?

XHHW (cross-linked polyethylene insulation) is another 90°C option:

• THHN/THWN-2: PVC + nylon, slightly thinner overall, easier to pull, cheaper
• XHHW / XHHW-2: XLPE, slightly thicker, better high-temperature ageing, common for solar PV

NEC Table 5 lists XHHW with identical area to THHN. See THHN vs XHHW.

Quick reference

• Indoor dry conduit: THHN or THHN/THWN-2 — same fill numbers
• Outdoor / underground / wet: THWN-2 (not THHN alone)
• Solar PV: Often XHHW-2 (UV resistance, longer outdoor life)
• High temperature ambient: XHHW-2 has slightly better long-term ageing than THHN
• Cheapest building wire: Dual-stamped THHN/THWN-2 — buy this for general use

Verify your specific conduit fill in the conduit fill calculator.