THHN vs THWN vs THHN-2: What the Letters Mean

Decoding the print

Each Letter Is a Rating

Building wire type designations are not brand names. Every letter is a property the insulation was tested and listed for.

Letter	What it means
T	Thermoplastic insulation
H	Heat resistant
HH	High heat resistant — 90°C in dry locations
W	Rated for wet locations
N	Nylon (or equivalent) outer jacket
-2	90°C in both wet and dry locations

Assemble them and the names explain themselves. THHN is thermoplastic, high-heat-resistant, nylon-jacketed, and rated 90°C in dry and damp locations only. THWN adds the wet-location rating. THWN-2 carries 90°C in wet and dry locations both, which is the rating most feeders and underground runs actually need. The insulation types and their ratings are listed in the conductor tables in NEC Article 310 — the table number moved in recent cycles, so look for the table of conductor applications and insulations rather than a remembered reference.

The practical part

Most Copper Building Wire Is Dual-Rated

Walk the aisle and you will find very little wire printed with a single type. Common copper building wire is surface-printed with a string of ratings — typically THHN and THWN-2 together, often with additional listings such as MTW, AWM, or a gasoline-and-oil-resistant designation. That single conductor is legal as THHN in a dry location and as THWN-2 in a wet one.

Which means the real skill is reading the print on the jacket, not shopping by name. If a spool is marked THHN only, with no W, it does not belong in a wet location. If it is marked THWN without the -2, its wet-location rating is 75°C, and you cannot start a derating calculation from 90°C in that wet run.

Underground conduit is a wet location. NEC 300.5(B) treats the interior of an underground raceway as a wet location, whether or not you ever see water in it.
Above-grade raceway outdoors is also wet. Under NEC 300.9, the interior of a raceway or enclosure installed in a wet location above grade is considered a wet location. A dry-only conductor in that EMT run down an exterior wall is a violation even if the pipe looks sealed.
A slab or masonry wall in contact with earth falls under the wet-location definition in Article 100. Read the definition — it is short and it settles most arguments.
The 90°C rating rarely sets the ampacity. NEC 110.14(C) limits you to the termination rating, which on most equipment means the 75°C column of Table 310.16. The 90°C column is for starting derating calculations, not for squeezing a smaller conductor onto a lug.

Neighbors on the shelf

XHHW-2, and Why Aluminum Is Usually Not THHN

XHHW-2 is cross-linked polyethylene insulation, 90°C wet and dry, with no nylon jacket. It is the standard insulation on aluminum building wire and a common choice for large copper feeders. Compared with THHN/THWN-2 it has a thicker wall and a larger outside diameter at the same conductor size, so a raceway sized around THHN conductors may not take XHHW-2 — recheck fill from Chapter 9 when you substitute.

Why the nylon matters

The N jacket is a hard, slick abrasion barrier. It is what lets THHN pull through a long conduit run with less lubricant and less skinning, and it adds resistance to oil and gasoline that plain thermoplastic does not have.

Diameter and fill

THHN's thinner wall means more conductors fit a given raceway than with XHHW-2 at the same AWG. That is a real advantage on a full pipe, and a real trap when you swap types late in a job.

Stranded vs. solid

Both exist in the smaller sizes. Stranded pulls easier and terminates better on lugs; solid is easier on back-wired devices and stiffer in a box. Neither changes the temperature or wet-location rating.

Colors are not universal law

The NEC fixes white or gray for grounded conductors and green for equipment grounding, and requires ungrounded conductors of different systems in the same premises to be identified. Beyond that, ungrounded conductor colors in most sizes are convention, not code.

At the counter

What to Tell Us When You Order

Wet or dry

Underground, outdoors, or in a slab means you want the -2 rating. Say so and the question of THHN versus THWN never comes up.

Size, metal, and stranding

AWG or kcmil, copper or aluminum, solid or stranded. Aluminum in feeder sizes will normally come as XHHW-2.

Footage per color

We cut wire to length, so give us the run plus your slack per color rather than buying a full reel you will not finish.

Everything above is on the shelf — browse our THHN/THWN-2, XHHW-2, Romex, MC cable, and conduit, or pick up the connectors, lugs, bushings, and pulling supplies that go with the pull. For a cut-to-length price on a specific run, send us your footages and we will quote it the same day.

Reference only. This page is general reference information, not a substitute for the currently adopted code or the ruling of the authority having jurisdiction. California adopts the NEC as the California Electrical Code (Title 24, Part 3) with state amendments, and your city or county may amend it further. Requirements change from one code cycle to the next — verify every value against the edition in force for your permit, and confirm the details with your inspector before you buy material or energize anything. Ora Electric Lighting & Supply is a wholesale distributor. We sell materials. We do not perform electrical work, and we do not provide design or engineering services.

Common questions

Frequently Asked Questions

What is the difference between THHN and THWN?

THHN is thermoplastic, high-heat-resistant, nylon-jacketed wire rated 90°C in dry and damp locations. THWN adds a wet-location rating. THWN-2 carries 90°C in both wet and dry locations. Most copper building wire sold today is printed with more than one type designation, so read the surface print on the conductor.

Can THHN be used in wet locations?

Not on its own. THHN alone is a dry and damp location rating. In practice most building wire is dual-rated THHN/THWN-2 and is fine in a wet location — but the rating has to be printed on the conductor. Note that the interior of an underground raceway is a wet location under NEC 300.5(B), and the interior of a raceway installed above grade in a wet location is a wet location under 300.9.

What does the -2 mean on THWN-2 or XHHW-2?

It means the conductor is rated 90°C in wet locations as well as dry. Without the -2, the wet-location rating is 75°C, which limits where you can start a derating calculation.

If my wire is rated 90°C, can I use the 90°C column for ampacity?

Generally no. NEC 110.14(C) limits the circuit to the temperature rating of the lowest-rated termination, which on most equipment means the 75°C column of Table 310.16. The 90°C rating is used as the starting point for adjustment and correction factors, with the final result held to the termination rating.

Why is aluminum wire usually XHHW-2 instead of THHN?

XHHW-2 is the standard insulation on aluminum building wire. It is cross-linked polyethylene, rated 90°C wet and dry, with no nylon jacket. It has a larger outside diameter than THHN at the same size, so recheck conduit fill from Chapter 9 when you substitute one for the other.

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