THHN vs THWN Wire Guide: Differences, Ratings, and Applications

Choosing building wire by its gauge alone can create problems when a circuit passes through a damp or wet environment. Two conductors may have the same copper size and similar-looking insulation, yet their printed markings can authorize different installation conditions. For electricians and purchasing teams, understanding THHN vs THWN wire is an important part of selecting the right conductor for the complete wiring method.

The central difference is environmental suitability. THHN is a 90°C dry-location designation, while THWN provides a wet-location rating with a lower permitted temperature in wet conditions. THWN-2 extends wet-location performance to 90°C. Many modern products carry more than one designation, but those ratings must be verified on the actual conductor rather than assumed from its appearance.

This guide explains the markings, compares performance, identifies common conduit applications, and outlines the installation factors that affect conductor selection under the locally adopted National Electrical Code (NEC).

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

THHN and THWN are classifications for individual insulated building conductors, not descriptions of a complete cable assembly. The markings identify insulation characteristics and permitted conditions of use. In common copper building wire, manufacturers frequently use thermoplastic insulation with a protective nylon outer covering.

The letters in the designations provide a useful starting point. T identifies thermoplastic insulation, H identifies a heat-resistant rating, HH identifies the higher 90°C heat-resistant category, W identifies a wet-location designation, and N denotes a nylon jacket. The -2 designation is significant because it indicates a 90°C rating in both wet and dry conditions for Type THWN-2.

These letter codes are not substitutes for checking a specific product listing. The conductor's complete printed legend, material, size, voltage rating, and manufacturer documentation determine its approved applications. For a broader comparison of wiring methods, see Lumera Electric's Electrical Wire Types Guide.

THHN vs THWN vs THWN-2: Comparison Chart

These are insulation-use ratings, not universal operating temperatures for an installed circuit. Termination ratings, ampacity rules, conductor count, ambient conditions, and equipment listings can impose lower usable limits. Southwire's THHN/THWN and THHN/THWN-2 specifications document the differences between the 75°C wet rating and the 90°C wet rating.

THHN Wire: Where Is It Used?

THHN is commonly used for feeders and branch circuits where the individual conductor is installed in an approved raceway in a dry location. Examples may include protected indoor conduit systems in offices, commercial facilities, and industrial buildings, provided the complete wiring method and equipment ratings permit its use.

The defining limitation is that THHN marking alone does not establish wet-location suitability. An installer should not treat an outdoor or underground conduit as dry simply because it has a cover, is sealed, or contains no visible water at the time of installation.

THHN Temperature and Voltage Ratings

The THHN designation allows a maximum conductor insulation temperature of 90°C in dry locations. Many current copper THHN products are marked for 600-volt use, but the actual voltage rating must be confirmed from the selected product. Neither a 90°C insulation rating nor a 600 V marking authorizes ignoring circuit ampacity, conductor protection, or terminal temperature limitations.

Common THHN Applications

THHN is often encountered in indoor raceway systems serving lighting circuits, receptacle circuits, equipment feeders, and control-related installations where the wiring method is permitted. Some conductors marketed under the familiar THHN name are additionally marked THWN or THWN-2, which significantly expands their possible use. The additional marking—not the product's marketing name—is what matters for wet-location eligibility.

THWN Wire: What Changes in Wet Locations?

THWN is intended for wet-location use, but its conventional wet-location temperature rating is 75°C, compared with 90°C in dry locations. This distinction matters when selecting a conductor for circuits routed through outdoor conduit, underground raceways, and other locations classified as wet by the adopted code.

THWN does not mean that every installation containing the wire is automatically compliant. The raceway, connectors, enclosures, conductor insulation, corrosion protection, and termination arrangements still need to suit the actual environment. The permitted wiring method is as important as the insulation marking.

Is THWN the Same as THWN-2?

No. THWN-2 carries a 90°C wet-or-dry insulation rating, while conventional THWN is limited to 75°C in wet conditions. They may look alike, and both may be supplied with similar thermoplastic-and-nylon construction, but a contractor cannot apply the higher THWN-2 wet rating to a conductor marked only THWN.

Many contemporary building-wire products are printed THHN/THWN-2. That dual designation means the conductor has separately established ratings for both conditions. For an actual example, Southwire's THHN/THWN-2 technical specification describes 90°C wet/dry use and shows a sample conductor legend.

Can THHN Wire Be Used Outdoors or Underground?

A critical distinction is between the conductor insulation and the wiring method. Outdoor and underground raceways are commonly treated as wet locations under NEC installation rules, including where moisture can accumulate inside the conduit. THHN-only marking therefore cannot be relied on for these conditions.

A conductor marked THWN or THWN-2 may be appropriate within a suitable outdoor or underground raceway, subject to the full design requirements. That does not mean loose THWN-2 building wire is automatically approved for direct burial without a raceway. Direct burial depends on an independently permitted cable or conductor listing and installation method.

Location-Based Selection Table

If the project also involves selecting a suitable raceway, Lumera Electric's Types of Electrical Conduit Guide explains common conduit choices and environmental considerations.

How to Read THHN/THWN-2 Wire Markings

The most dependable way to identify a conductor is to read the printing on its insulation. Modern products may carry a long legend listing several permitted uses, and each marking can impose its own conditions.

A typical legend may identify conductor size (such as 12 AWG), material (such as CU for copper), the manufacturer, a voltage rating, and multiple insulation designations. A conductor bearing both THHN and THWN-2 can be evaluated under the applicable rating for the installation environment, while a conductor marked only THHN should not be presumed wet-rated.

Look for the manufacturer's certification or listing information as well. Color may indicate intended circuit identification, but color by itself does not establish the insulation class, conductor size, allowable temperature, or suitability for a wet location.

How Temperature Ratings Affect Ampacity

One of the most common mistakes is using a 90°C insulation marking to assume that the circuit can operate at the 90°C ampacity-table value. Insulation rating and final allowable ampacity are not the same thing. Equipment terminal temperature limits, the number of current-carrying conductors, ambient temperature, and additional NEC rules all affect the permitted installation.

For example, a THHN/THWN-2 conductor may be permitted to operate at a 90°C insulation temperature in a wet or dry location, but a connected terminal may be rated for only 75°C or 60°C. The final circuit design must respect the applicable termination and equipment limitations. The 90°C conductor value may still be relevant as the starting point for certain adjustment and correction calculations, where permitted.

The conductor must also be correctly sized for the load, wiring method, and overcurrent protection. Lumera Electric's Wire Gauge and Ampacity Guide explains how conductor size, temperature columns, and breaker selection interact.

Conduit Fill, Pulling, and Installation Considerations

THHN and THWN-2 are frequently installed as individual conductors in raceways. Selecting the correct conductor rating is only one part of the installation: the contractor must also verify conduit fill, bending, permitted fittings, insulation protection, and the ability to pull the wires without damage.

The NEC Chapter 9 conduit-fill calculation uses the actual area of each conductor type and size. Even wires with the same AWG designation can differ in overall insulation dimensions. For that reason, contractors should use the applicable published conductor-area values and the correct raceway tables rather than estimate fill visually.

The separate Conduit Fill Chart and Calculation Guide addresses those calculations in detail. This THHN vs THWN guide focuses on insulation markings and environmental compatibility rather than duplicating the conduit-sizing procedure.

THHN/THWN vs. NM-B and XHHW-2

Not all electrical wiring products solve the same installation problem. THHN/THWN-2 is commonly an individual insulated conductor used with an approved raceway, while NM-B is a factory-made cable assembly generally intended for dry indoor locations. XHHW-2 is another building-wire insulation designation that can offer a wet-and-dry 90°C rating, but uses a different insulation construction.

These are not interchangeable labels. A wiring method may permit a cable assembly in one situation and individual conductors in conduit in another. Conductor material, insulation type, mechanical protection, environmental exposure, and installation labor all affect selection.

Common THHN vs THWN Selection Mistakes

1. Assuming all THHN is wet-rated: Many products have dual markings, but THHN by itself describes a dry-location rating. Read the full printed legend before specifying the conductor for an exterior or underground raceway.
2. Treating THWN and THWN-2 as identical: The 75°C wet rating of THWN is not the same as the 90°C wet rating of THWN-2. Use the actual product marking when reviewing the allowed operating conditions.
3. Using the insulation temperature as the breaker rating: Terminal limitations, load calculations, conductor adjustments, and overcurrent-protection requirements must still be checked. Do not choose a larger breaker solely because the insulation is marked 90°C.
4. Assuming conduit makes every location dry: Raceway systems may contain water or condensation. Confirm the installation's environmental classification and select conductors rated for it.
5. Assuming wet-rated means direct-burial rated: A wet-location insulation marking alone does not establish permission for direct burial without an approved wiring method.
6. Ignoring the complete conductor legend: Oil resistance, voltage rating, size, material, and other markings may matter for the project. Product specifications should be reviewed whenever the installation involves special environments.

Key Takeaways

Choosing between THHN, THWN, and THWN-2 starts with the installation environment. THHN is intended for dry locations, while THWN adds wet-location suitability and THWN-2 extends that rating to 90°C in both wet and dry conditions. Because many modern conductors carry multiple designations, the printed marking on the actual wire should always guide product selection.

The insulation rating is only one part of the decision. Contractors should also confirm terminal temperature limits, ampacity adjustments, overcurrent protection, conduit conditions, and whether the complete wiring method is approved for the location. A wet-rated conductor should never be assumed to be suitable for direct burial unless its listing and installation method specifically allow it.