THHN vs XHHW: Temperature, Wet Rating & When to Use Each | NEC 2023
NEC Quick Answer
THHN and XHHW are both rated 90C in dry locations with a 600V maximum. The key difference is wet-location performance: THHN is not wet-rated (THWN companion is 75C wet), while XHHW is 75C wet. The dual-rated versions (THHN/THWN-2 and XHHW-2) are both 90C wet, making them functionally equivalent. THHN has a thinner profile for conduit fill; XHHW has better chemical resistance.
NEC 2023: Table 310.4(1), Table 310.16
THHN/THWN-2 vs XHHW/XHHW-2
| Feature | THHN/THWN-2 | XHHW/XHHW-2 |
|---|---|---|
| Insulation material | PVC (thermoplastic) + nylon jacket | XLPE (cross-linked polyethylene) |
| Dry temp rating | 90°C | 90°C |
| Wet temp rating | Not rated (THHN alone is dry/damp only) | 75°C |
| Dual-rated version | THHN/THWN-2 (most common today) | XHHW-2 |
| Dual-rated wet temp | 90°C wet (THWN-2) | 90°C wet (XHHW-2) |
| Voltage rating | 600V | 600V |
| Conduit fill | Smaller OD per gauge - more conductors per conduit | Slightly larger OD (XLPE thicker than PVC+nylon) |
| Chemical resistance | Moderate (PVC) | Higher (cross-linked PE) |
| Flexibility | Stiffer due to nylon jacket | More flexible, easier long pulls |
| Cost | Lower - highest production volume | Slightly higher (10-20% more) |
| Availability | Everywhere - every supply house | Less common at hardware stores |
| Typical applications | General commercial/residential conduit | Industrial, underground (USE-2 dual rating) |
| NEC ampacity table | Table 310.16, 90°C column | Table 310.16, 90°C column (identical) |
| NEC insulation listing | Table 310.4(1) | Table 310.4(1) |
When to use THHN
THHN (Thermoplastic High Heat-resistant Nylon-coated) is the most commonly installed building wire in the US. Every electrical supply house and hardware store stocks it. The dual-rated THHN/THWN-2 version is rated 90°C in both dry and wet locations, making it suitable for nearly any conduit installation.
It is the default for branch circuits, feeders, and most commercial work. The nylon jacket provides abrasion resistance during pulling and gives THHN a thinner overall profile than XHHW, which means more conductors fit in a given conduit size per NEC Chapter 9 Table 5. This conduit fill advantage is the main practical reason THHN dominates the market.
When to use XHHW
XHHW (Cross-Linked High Heat-resistant Water-resistant) is preferred in industrial and underground applications. The XLPE insulation provides superior chemical resistance compared to PVC, making it the better choice in manufacturing plants, chemical facilities, and environments with oil or solvent exposure. XHHW-2 maintains its full 90°C rating in wet locations.
Many XHHW-2 products carry a dual USE-2 rating, making them suitable for direct burial and underground service entrance conductors without conduit. For large feeders in industrial settings where chemical exposure is a concern, engineers often specify XHHW-2 over THHN/THWN-2.
InterchangeabilityAre THHN and XHHW interchangeable?
In dry locations at 90°C, THHN and XHHW have identical ampacity from Table 310.16 and are fully interchangeable. In wet locations, only the -2 rated versions of each (THHN/THWN-2 and XHHW-2) can use the 90°C column; plain THHN is not wet-rated at all, and plain XHHW drops to 75°C wet. For conduit fill calculations, they are NOT interchangeable because XHHW has a slightly larger outer diameter per NEC Chapter 9 Table 5.
Using XHHW dimensions in a fill calc designed around THHN could push you over the 40% fill limit. Always use the correct wire OD from Table 5 for the insulation type you are actually installing.
On the job
You are pulling four 6 AWG conductors (3 phase + ground) through 1" EMT for a commercial HVAC disconnect. With THHN, the smaller OD keeps conduit fill comfortably under the 40% limit per NEC Chapter 9, Table 1. With XHHW, the slightly larger diameter per gauge means the same four conductors take up more space.
On this particular pull, both fit, but THHN gives you more margin if you later need to add a conductor. The reverse scenario also exists: in a chemical plant, the engineer specifies XHHW-2 for chemical resistance even though it means upsizing from 1" to 1-1/4" EMT on some runs.
Cost comparison
THHN typically runs $0.15-0.30/ft for 12 AWG and $0.50-1.00/ft for 6 AWG. XHHW costs about 10-20% more due to lower production volume and the more expensive XLPE insulation. On a typical residential or small commercial job, the cost difference is negligible.
On a large industrial project with thousands of feet of wire, the premium adds up but is usually justified by the chemical resistance requirements that drove the XHHW specification in the first place.
InstallationInstallation difficulty
Pull through conduit. Universally stocked.
Same pull technique. Slightly more flexible.
Both wire types install identically - they pull through conduit the same way. XHHW is slightly more flexible due to the lack of a nylon jacket, which can make pulling easier on longer runs or runs with multiple bends. Both require proper wire-pulling lubricant on runs over 100 feet or with more than two 90° bends.
The pulling tension limits are similar for both types at the same gauge.
Related guides
External resourcesTrusted references
Frequently asked questions
Which is better, thhn/thwn-2 or xhhw/xhhw-2?
Neither is universally better - each has specific strengths for different situations. THHN/THWN-2 is the default for most commercial and residential conduit work. It is cheaper, universally stocked, and its thinner profile fits more conductors per conduit size. XHHW-2 is preferred in industrial environments with chemical exposure, underground installations (often dual-rated USE-2/XHHW-2), and when specified by an engineer.
Can I use thhn/thwn-2 and xhhw/xhhw-2 in the same installation?
Yes, many installations use both where each is best suited. Transition points must be properly connected per NEC requirements. Always ensure the transition is accessible and properly identified.
What does the NEC say about thhn/thwn-2 vs xhhw/xhhw-2?
The NEC specifies where each is permitted and prohibited. Check the specific NEC articles referenced in the comparison table above. Your local jurisdiction may have amendments that further restrict or modify these requirements.
Do I need a permit for this electrical work?
In most jurisdictions, any new circuit or wiring change requires a permit and inspection. Check our state-by-state permit guide for your specific requirements. Working without a permit can void insurance and create liability issues.
Is XHHW better than THHN?
Neither is universally better. THHN wins on conduit fill (thinner profile fits more conductors) and cost (cheaper, higher production volume). XHHW wins on moisture and chemical resistance (XLPE insulation outperforms PVC in harsh environments). For most residential and commercial work, THHN/THWN-2 is the standard. For industrial or underground work, XHHW-2 is often preferred or specified.
Can I use THHN in wet locations?
Only if it is dual-rated THHN/THWN-2, which most modern THHN is. Plain THHN (without the THWN-2 designation) is rated for dry and damp locations only per NEC Table 310.4(1). Check the wire jacket markings - it should be stamped THHN/THWN-2 for wet location use. THWN alone is 75°C wet; THWN-2 is 90°C wet.
What is the voltage rating of THHN and XHHW?
Both THHN and XHHW are rated 600V maximum per NEC Table 310.4(1). This covers standard 120V, 208V, 240V, 277V, and 480V systems. For voltages above 600V, medium-voltage cables with different insulation systems are required.
Does THHN or XHHW have higher ampacity?
They have identical ampacity at the same temperature column. Both use the 90°C column of NEC Table 310.16 in dry locations. Ampacity is determined by conductor gauge, material (copper or aluminum), and temperature column - not by insulation type. The practical usable ampacity is further limited by the 75°C or 60°C termination temperature per NEC 110.14(C).
NEC 2023 references verified April 2026