THHN vs. THWN vs. XHHW: Insulation Types and Where to Use Them
Walk into any electrical supply house and the wire rack will show you spools labeled THHN, THWN, THHN/THWN-2, XHHW, and a few others. These letter codes are not marketing names, they are UL and NEC designation codes that describe the insulation material, temperature rating, and moisture resistance of the conductor. Knowing what each letter stands for makes choosing the right wire straightforward.
Decoding the Letter Codes
Every wire designation follows a loose system. Breaking it down letter by letter removes most of the confusion.
T, Thermoplastic insulation. This is the outer jacket or insulation material. PVC (polyvinyl chloride) is the most common thermoplastic used in building wire.
H, Heat-resistant to 75°C. A single H means the insulation is rated for continuous use up to 75°C in dry locations.
HH, High-heat resistant to 90°C. Two H's bump the dry temperature rating to 90°C.
W, Wet or damp location rated. The W signals the insulation can handle moisture exposure, conduit running outdoors, underground, or through areas subject to condensation.
N, Nylon jacket over the thermoplastic insulation. The nylon layer adds oil resistance and makes the wire easier to pull through conduit because it reduces friction.
X, Cross-linked polyethylene (XLPE) insulation instead of thermoplastic. Cross-linking changes the polymer structure at the molecular level, giving XLPE better heat resistance, chemical resistance, and flexibility at low temperatures compared to PVC.
2, Suffix indicating a 90°C wet rating. THWN-2 and XHHW-2 both carry the "2" to distinguish their higher wet-location temperature rating from older designations rated at 60°C or 75°C wet.
THHN: The Standard Building Wire
THHN stands for Thermoplastic High-Heat resistant Nylon-coated. Its temperature ratings are 90°C in dry locations and 75°C in wet locations (when dual-rated as THHN/THWN-2, which almost all modern THHN is).
You will find THHN in virtually every residential and commercial conduit run. It is inexpensive, widely stocked, and the nylon jacket makes it pull through PVC or EMT conduit with relatively little effort. Common applications include branch circuits, panel feeders, and motor circuits installed in conduit indoors.
One thing worth noting: bare THHN without a W rating is not approved for wet locations. Most wire sold today is dual-stamped THHN/THWN-2, but checking the print on the jacket before buying is good practice, especially with off-brand or surplus stock.
THWN and THWN-2: Wet Location Thermoplastic
THWN originally carried a 75°C wet rating. The updated designation THWN-2 carries 90°C in both dry and wet locations. For most practical purposes, the wire sold on spools today is dual-rated and stamped with both THHN and THWN-2 on the jacket.
The W rating matters any time the conductor will be exposed to water or high humidity. Underground conduit, conduit runs through unheated crawl spaces, exterior conduit exposed to weather, and conduit entering a meter base all qualify as wet locations under the NEC. Using a dry-only rated conductor in those spots is a code violation regardless of whether any water actually gets in.
See AWG wire size chart for a quick reference to conductor sizing before selecting insulation type.
XHHW and XHHW-2: Cross-Linked Polyethylene Insulation
XHHW uses cross-linked polyethylene (XLPE) rather than PVC thermoplastic. The temperature ratings are 90°C dry and 75°C wet for XHHW, and 90°C dry and 90°C wet for XHHW-2.
XLPE insulation holds up better than PVC in a few specific situations. It resists certain chemicals and oils that can degrade thermoplastic over time. It stays flexible at lower ambient temperatures, which matters in unheated outdoor runs during winter installation. And because XLPE does not soften at elevated temperatures the way PVC does, XHHW-2 is frequently specified for service entrance conductors, feeder runs in industrial environments, and applications where the conductor may see higher operating temperatures.
The tradeoff: XHHW is typically more expensive than THHN of the same gauge, and the insulation is stiffer, which can make pulling through tight conduit bends slightly harder.
For ampacity differences between these insulation types, ampacity explained covers how temperature correction factors interact with insulation rating.
Side-by-Side Comparison
| Designation | Insulation | Dry Temp | Wet Temp | Wet Rated | Common Use |
|---|---|---|---|---|---|
| THHN | PVC + Nylon | 90°C | 75°C | Yes (if dual-rated) | Indoor conduit, branch circuits |
| THWN-2 | PVC + Nylon | 90°C | 90°C | Yes | Wet/outdoor conduit, underground |
| XHHW | XLPE | 90°C | 75°C | Yes | Service entrance, industrial feeders |
| XHHW-2 | XLPE | 90°C | 90°C | Yes | High-temp wet runs, service entrance |
Most residential and light commercial work uses THHN/THWN-2 dual-rated wire for everything. XHHW-2 becomes the better call when the application involves service entrance, locations with chemical exposure, or runs where ambient temperature pushes the conductor's operating temperature near its limits.
Choosing by Location and Application
Dry indoor conduit: THHN is the standard choice. It is widely available, easy to pull, and priced competitively at every gauge.
Wet or damp locations: Use THWN-2 or XHHW-2. Check the jacket print to confirm the W2 rating. Conduit running through an attached garage, under a deck, or buried underground counts as a wet location.
Service entrance conductors: XHHW-2 is commonly specified. The 90°C wet rating and the durability of XLPE insulation suit the demands of the service entry point where weather exposure and higher fault current levels are realities.
Underground feeder in conduit: Either THWN-2 or XHHW-2 works. The conductor runs inside conduit, so the insulation needs to handle moisture but not direct soil burial (that's a different product, USE or URD cable). For more on how NEC tables apply to these runs, how to read NEC ampacity tables walks through the process.
Motor circuits and industrial panels: XHHW-2 is often preferred where chemical splash, oil mist, or elevated ambient temperatures are present. The 90°C rating also allows higher ampacity on a given conductor size compared to a 75°C-rated installation, which can matter in tight equipment enclosures.
For conductor material choices that interact with insulation selection, copper vs aluminum wire covers the tradeoffs in detail.
Always verify your conductor selection against the current edition of the NEC and confirm the installation with a licensed electrician. Code editions and local amendments vary by jurisdiction, and table values change between NEC cycles.
Frequently Asked Questions
Is THHN the same as THWN?
Not exactly, though most wire sold today is dual-rated as THHN/THWN-2 and carries both designations printed on the jacket. Original THHN (without the W) is approved only for dry locations. Original THWN carries a 75°C wet rating. The modern dual-rated product covers both applications and is what you will typically find at supply houses.
Can I use THHN in outdoor conduit?
Only if the wire is also rated THWN or THWN-2, which dual-stamped wire is. Outdoor conduit is considered a wet location even if the conduit is watertight, so the conductor must carry a W rating. Check the jacket print before assuming.
What does the "2" mean in XHHW-2 vs XHHW?
The "2" designates a 90°C wet temperature rating rather than 75°C. XHHW without the 2 is rated 75°C in wet locations. XHHW-2 holds its 90°C rating in both dry and wet environments, which allows the use of 90°C ampacity tables for that conductor in qualifying installations.
When should I choose XHHW-2 over THWN-2?
For most residential and light commercial conduit runs, THWN-2 performs well and costs less. XHHW-2 earns its place in service entrance applications, industrial environments with chemical or oil exposure, outdoor runs in very cold climates where PVC stiffens noticeably, and situations where the 90°C wet ampacity is needed to size a conductor correctly without stepping up to a larger gauge.