THHN vs THWN vs THWN-2 Wire: Key Differences Explained

All three are 600V single-conductor building wires that look nearly identical, but each handles heat and water differently. THHN is a dry-location wire. THWN adds water resistance. THWN-2 is the modern upgrade — rated 90°C in both wet and dry locations — and most wire sold today is dual-marked THHN/THWN-2 so it covers every common install.

This guide breaks down what each label means, when to use which, and why one print legend on the jacket can list two or three names at once.

What Do THHN, THWN, and THWN-2 Actually Mean?

All three are 600V single-conductor building wires with PVC insulation and a nylon jacket. The only real differences are their temperature and moisture ratings.

Here’s what each letter stands for, decoded once:

• T — Thermoplastic insulation (PVC)
• H — Heat-resistant (75°C)
• HH — High heat-resistant (90°C)
• W — Water-resistant (suitable for wet locations)
• N — Nylon outer coating
• -2 — Upgraded 90°C rating in wet locations

Now, one line each on what they actually do:

• THHN — High-heat dry wire. Rated 90°C in dry locations only — THHN itself has no wet-location listing, so it’s strictly for indoor conduit.
• THWN — The older water-resistant version. Rated 90°C dry, 75°C wet. Still legal, but largely replaced by THWN-2 in modern stock.
• THWN-2 — The current standard. Rated 90°C in both wet and dry environments, plus better resistance to oil and chemicals.

How Do THHN, THWN, and THWN-2 Compare?

The fastest way to see the difference is in one table. Notice that THWN-2 is the only one rated 90°C both wet and dry — that’s the headline.

Two things matter most here:

• Wet rating drives where you can install it. A wire without a “W” can’t go in any wet location, and per the 2026 National Electrical Code (NEC) Article 300.9, the inside of a raceway above ground in an outdoor environment is itself a wet location.
• The 90°C wet rating on THWN-2 means higher ampacity. When you derate for ambient temperature or conduit fill, starting from a 90°C column instead of 75°C can save you a wire size.

When Should You Use Plain THHN Wire?

Use THHN when the run stays dry — that’s it. It’s the cheapest option and the thinnest, so you can fit more conductors in a given conduit.

Good fits for THHN-only:

• Indoor branch circuits inside conduit
• Wiring inside switchboards, control panels, and machine tools
• Push-button stations, fire alarm controls, and other dry-location control circuits

Where THHN falls short:

• Outdoor conduit runs (treated as wet locations under NEC 300.9)
• Underground conduit, even if “sealed”
• Damp basements, car washes, food-processing washdown areas
• Direct burial — never for THHN/THWN/THWN-2 under any rating

In practice, you’ll rarely buy THHN that isn’t also dual-rated as THWN-2, so this category is shrinking fast.

When Do THWN and THWN-2 Wire Make Sense?

Pick THWN-2 whenever water might touch the wire — even indirectly through humidity or condensation in a conduit. It’s the safer default for almost every modern install.

A few real-world examples where THWN-2 is the right call:

• Rooftop conduit runs — Rooftops can hit 60–70°C ambient on hot days. The 90°C wet rating gives you derating headroom.
• Underground conduit between a panel and a detached garage, well, or outbuilding
• Industrial floors with oil mist, coolant, or washdowns nearby
• Damp utility rooms, pump houses, and outdoor disconnects

Plain THWN (no “-2”) still exists, mainly in smaller AWG sizes, but it caps out at 75°C in wet locations (90°C dry). That lower rating means lower ampacity, which usually pushes you to upsize the conductor — so most spec sheets just call for THWN-2.

Why Is Most Wire Labeled “THHN/THWN-2”?

Manufacturers print multiple ratings on the same jacket because the wire legally meets all of them.

What that print legend really means:

• The wire meets the 90°C dry requirement (THHN)
• It also meets the 90°C wet requirement (THWN-2)
• It usually carries extra ratings too — MTW (machine tool wire), AWM (appliance wire), and “gasoline and oil resistant II”

A common point of confusion: there is no such thing as THHN-2. The “-2” suffix exists only for wet-location ratings, so THWN-2 is real but THHN-2 is not, despite occasionally showing up on older spec sheets or in discussions on Mike Holt’s electrical forum.

If you want a deeper look at the build of a single common size, our 6 AWG THHN/THWN copper building wire guide walks through the strand count, insulation thickness, and ampacity numbers.

Which Wire Should You Pick for Your Job?

Here’s a simple decision checklist. Match your install to the closest scenario, then pick the wire on the right.

A few cautions worth flagging:

• THHN/THWN-2 is not waterproof. THHN/THWN-2 is rated for wet locations inside a raceway, but it must not be installed outside a conduit underground or in direct contact with the earth.
• Direct burial requires a different wire. Even THWN-2 must be inside conduit to go underground in most jurisdictions.
• Sunlight-resistance markings vary by manufacturer — typically only larger sizes (often 6 or 8 AWG and up) carry the listing. Always check the print legend before exposing the wire to UV.

For higher-temperature or harsher chemical environments, XHHW-2 with cross-linked polyethylene insulation is often a better choice than THWN-2.

Conclusion

For most modern conduit jobs, dual-rated THHN/THWN-2 wire is the simple, safe answer — it carries every relevant rating (90°C dry, 90°C wet, oil/gas resistant) on a single jacket. Plain THHN is fine when you know the run will stay dry. In larger sizes (#8 AWG and up), THWN-2 has largely replaced plain THWN; in smaller sizes (#10 and below), much of the dual-rated stock is still THHN/THWN with a 75°C wet rating. If you’re spec’ing wire for an outdoor, underground, or high-heat install, default to THWN-2 unless you have a reason to specify XHHW-2 or a PV-rated cable.

Need help matching a wire spec to your project? Browse our building wire range guide.

FAQs

Is THHN wire the same as THWN-2 wire?

No, but they are close cousins. THHN is rated 90°C in dry locations only. THWN-2 is rated 90°C in both wet and dry locations. Most wire today carries both labels on the same jacket, which is why they’re often treated as one product.

Can I use THHN wire outdoors or in underground conduit?

Not on its own. Outdoor and underground conduit are both treated as wet locations under NEC 300.9, and THHN is not wet-rated. If your wire is dual-marked “THHN/THWN-2,” then yes — the THWN-2 rating qualifies it for those installs.

What does the “-2” mean in THWN-2?

The “-2” means a 90°C wet-location rating instead of the older 75°C wet rating. It’s the second-generation insulation formula. Note that “THHN-2” is not a real UL designation — only the wet-rated types get the “-2” suffix.

Is wire inside outdoor conduit considered a wet location?

Yes. NEC 300.9 specifies that the interior of raceways and enclosures installed in wet locations above grade is itself a wet location. That’s why outdoor conduit runs need wire with a “W” in the rating, ideally THWN-2.

Why is most modern building wire labeled “THHN/THWN-2”?

Because the same physical wire meets both standards. Manufacturers print every applicable rating to give installers maximum flexibility and to consolidate inventory. One reel covers indoor, outdoor, wet, and dry installs.

Sources & References

• National Electrical Code (NEC), Article 310 — Conductors for general wiring
• NEC Article 300.9 — Raceways in wet locations above grade
• UL 83 — Standard for Thermoplastic-Insulated Wires and Cables
• UL Product iQ — Wire and Cable Categories
• Mike Holt Forums — Practicing Electrician Discussions