THHN and XHHW-2 are both 600 V single-conductor building wire, both are rated 90°C, and both use the same NEC ampacity column. The real difference is the insulation. THHN is a thin PVC wall under a nylon jacket. XHHW-2 is a single, thicker wall of cross-linked polyethylene (XLPE) with no jacket. That one difference drives conduit fill, cold-weather handling, heat behavior and the jobs each wire is best suited for.
At a Glance
| THHN/THWN-2 | XHHW-2 | |
|---|---|---|
| Insulation | PVC (thermoplastic) with a nylon jacket | XLPE (thermoset), no jacket |
| UL standard | UL 83 | UL 44 |
| Voltage | 600 V | 600 V (some constructions also rated 1,000 V) |
| Temperature rating | 90°C dry (THHN), 90°C wet (THWN-2) | 90°C wet or dry |
| Common additional markings | MTW, gasoline and oil resistant, sunlight resistant (black), CT use (larger sizes) | Often multi-rated RHH/RHW-2/USE-2, sunlight resistant (black), CT use |
| 12 AWG area for conduit fill (NEC Ch. 9, Table 5) | 0.0133 in² | 0.0181 in² (about 36% larger) |
| Typical minimum installation temperature | 14°F (−10°C) | −40°F (−40°C) |
| Behavior when overheated | Softens (thermoplastic) | Does not melt or flow (thermoset) |
| Usual conductor | Copper | Copper or aluminum |
| Direct burial | No | Only if also marked USE-2 |
Minimum installation temperatures vary by manufacturer, so confirm them on the spec sheet for the exact product.
Reading the Letters
- THHN: Thermoplastic insulation, HH high heat (90°C), Nylon jacket.
- THWN-2: the same wire rated for Wet locations, with -2 meaning 90°C wet. Nearly all THHN sold today is dual-rated THHN/THWN-2. See THHN vs THWN-2 for details.
- XHHW-2: XLPE insulation, HH high heat, Wet-rated, and -2 for 90°C wet. Older plain XHHW is 90°C dry but only 75°C wet.
Construction: Two Very Different Approaches
THHN uses a thin PVC insulation wall covered by a thin nylon (polyamide) jacket. The nylon gives the wire its toughness, its slick pulling surface and its resistance to gasoline and oil. The result is a small, inexpensive conductor that pulls easily through conduit. The trade-off is that nylon can scuff or split on sharp edges or when pulled in the cold.
XHHW-2 uses one homogeneous wall of XLPE. Cross-linking turns polyethylene into a thermoset: once cured it won't melt or flow, it resists moisture very well, and it has excellent electrical properties. It doesn't need a jacket because the XLPE wall is tough on its own. It is harder to strip, so use sharp, correctly sized strippers.
Ratings and Ampacity: No Difference
THWN-2 and XHHW-2 both use the 90°C column of NEC Table 310.16, so a 12 AWG copper conductor is 30 A in that column either way. In practice, most terminations are rated 75°C (NEC 110.14(C)), and small conductors have overcurrent limits under NEC 240.4(D): 15 A for 14 AWG, 20 A for 12 AWG and 30 A for 10 AWG copper. The 90°C rating matters mainly as the starting point for derating in hot locations or crowded raceways. Choosing XHHW-2 does not get you more amps than THHN/THWN-2.
Conduit Fill: THHN Wins in Small Sizes Only
THHN's thin wall makes it noticeably smaller in branch-circuit sizes. From 4 AWG up, the two are essentially the same and XHHW is slightly smaller in many sizes.
| Size | THHN/THWN-2 | XHHW/XHHW-2 |
|---|---|---|
| 14 AWG | 0.0097 | 0.0139 |
| 12 AWG | 0.0133 | 0.0181 |
| 10 AWG | 0.0211 | 0.0243 |
| 8 AWG | 0.0366 | 0.0437 |
| 6 AWG | 0.0507 | 0.0590 |
| 4 AWG | 0.0824 | 0.0814 |
| 1/0 AWG | 0.1855 | 0.1825 |
| 4/0 AWG | 0.3237 | 0.3197 |
| 250 kcmil | 0.3970 | 0.3904 |
| 500 kcmil | 0.7073 | 0.6984 |
Approximate area per conductor in square inches, from NEC Chapter 9, Table 5.
Example: 3/4-inch EMT has 0.213 in² available at 40% fill. That fits 16 conductors of 12 AWG THHN but only 11 of 12 AWG XHHW-2. If you're substituting XHHW-2 for THHN on a branch-circuit job, recheck fill. On feeders the swap usually doesn't change the conduit size.
Cold Weather and Heat
Many THHN spec sheets list a minimum installation temperature of 14°F (−10°C). Below that, PVC and nylon stiffen and the jacket can crack or split during the pull. Store reels in a heated space before cold-weather installs. XHHW-2 is commonly rated for installation down to −40°F (−40°C), which makes it the easier choice for winter work and unheated spaces.
At the other extreme, XLPE keeps its shape when overheated while PVC softens. Short-circuit design references commonly allow about 250°C for XLPE versus 150°C for PVC. That margin is one reason engineers often specify XHHW-2 for feeders and industrial work.
Wet Locations, Underground and Outdoors
- Wet locations: THWN-2 and XHHW-2 are both rated 90°C wet. A conductor marked only THHN is limited to dry and damp locations, so check the print legend.
- Underground in conduit: the inside of an underground raceway is a wet location. Both THWN-2 and XHHW-2 qualify.
- Direct burial: neither type qualifies on its own. Many XHHW-2 constructions are also marked USE-2, and that marking is what permits direct burial. For direct-burial jobs, look at copper or aluminum RHH/RHW-2/USE-2.
- Sunlight: look for a "sunlight resistant" marking, which is typical on black insulation.
Chemicals, Fire and Special Applications
- Oil and gasoline: THHN/THWN-2 is commonly marked gasoline and oil resistant thanks to the nylon jacket. Some XHHW-2 carries the same marking, so check the legend if oil exposure is expected.
- Smoke and halogens: PVC contains chlorine and gives off corrosive gas when it burns. If a project requires low-smoke, halogen-free cable, specify a product listed and marked for it rather than assuming any insulation type qualifies.
- Health-care isolated power: these systems often call for insulation with a low dielectric constant to limit leakage current (see NEC 517.160). XLPE meets that requirement and PVC does not, so XHHW-2 is the usual choice.
- Machine and panel wiring: THHN is frequently also rated MTW. See What is MTW wire? Switchboard wiring often uses XLPE-insulated SIS wire.
Which Should You Use?
| Situation | Usual choice | Why |
|---|---|---|
| Branch circuits in conduit (14–10 AWG) | THHN/THWN-2 | Smaller diameter, more conductors per raceway, lowest cost |
| Feeders and services | Either; XHHW-2 is very common | No conduit-fill penalty at 4 AWG and up; thermoset heat margin |
| Aluminum building wire | XHHW-2 (or RHW-2/USE-2) | The most widely available insulation for aluminum feeders |
| Cold-weather installs | XHHW-2 | Rated for much lower installation temperatures |
| Industrial or high-ambient locations | XHHW-2 | Thermoset holds up better to heat; often required by spec |
| Direct burial | A USE-2 marked conductor | Neither THHN nor XHHW-2 alone is rated for burial |
Common Questions
Can I substitute XHHW-2 for THHN?
Usually, yes. The voltage and ampacity are the same. Recheck conduit fill in small sizes and confirm the project spec allows it.
Is XHHW-2 better than THHN?
Not across the board. XHHW-2 handles cold, heat and moisture better. THHN is smaller and cheaper in branch-circuit sizes, which is why it dominates branch-circuit work.
Does XHHW-2 need a jacket?
No. The XLPE wall is the complete insulation system.
How much do they weigh?
See our wire weight chart, which lists THHN and XHHW-2 copper plus XHHW-2 aluminum by size.
Code references are to the 2023 NEC. Check the edition and any amendments adopted in your jurisdiction.
Related reading: THHN vs THWN-2 and copper vs aluminum conductors. Browse THHN/THWN-2 building wire.