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THHN/THWN vs XHHW-2: which building wire to pull
Both give a 90C wet rating when you buy the -2 version; the choice comes down to fill, pull conditions, and rooftop sun.
Short answer
For most conduit runs, pull dual-rated THHN/THWN-2: it takes less conduit fill, the nylon jacket slides through bends, and it usually costs less. Reach for XHHW-2 when the run sits in the sun on a roof (XHHW-2 is exempt from the rooftop temperature adder that hits THHN), on large feeders, on aluminum, or on rough pulls where the tougher cross-linked insulation earns its slightly larger footprint. The single biggest deciding factor is the run environment, roof and pull conditions, because on temperature both give you the 90C wet column once you buy the -2. When the spec names one, install that one.
THHN/THWN vs XHHW-2: side by side
| Factor | THHN/THWN | XHHW-2 |
|---|---|---|
| Insulation material | Thermoplastic (PVC type) with a nylon outer jacket | Cross-linked polyethylene, a thermoset, no nylon jacket |
| Temperature / wet rating | THHN 90C dry only; THWN 75C wet; dual THHN/THWN-2 is 90C wet and dry | Plain XHHW 90C dry but 75C wet; XHHW-2 is 90C wet and dry |
| Conduit fill | Thinner insulation, less fill, more conductors per pipe | Thicker insulation, more fill; sizing set for THHN can fall short |
| Install / pull | Slick nylon cuts friction, pulls clean through long bent runs | Tougher, handles a harder pull, but takes up more room |
| Rooftop sun runs | Subject to the rooftop temperature adder near the roof surface | Exempt from the rooftop adder, so a roof run escapes it |
| Environment / durability | Nylon resists abrasion, oil, and gasoline | Thermoset holds up to heat and rougher environments on its own |
| Code / standard | 600V building wire, NEC Article 310, listed to the UL thermoplastic wire standard | 600V building wire, NEC Article 310, listed to the UL thermoset wire standard |
| Termination cap | Ampacity capped at the lowest-rated lug under 110.14(C) | Same 110.14(C) cap applies; insulation does not raise the lug |
| Typical use | Branch and feeder wire in EMT, rigid, and PVC; commercial default | Feeders, larger conductors, aluminum, wet and rooftop runs |
Which should you pick?
Choose THHN/THWN when
- General conduit runs where easy pulling and lower conduit fill matter
- You need oil and gasoline resistance, such as a service bay or fuel station
- The run is in EMT, rigid, or PVC on typical commercial branch and feeder work
- Cost is a factor and the run is not a rooftop sun exposure
Choose XHHW-2 when
- The raceway sits in direct sun on or near a roof (exempt from the rooftop adder)
- Large feeders, service conductors, or long runs, especially on aluminum
- Rough or hard pulls where thermoset durability beats a nylon jacket
- The project spec calls out XHHW-2
Bottom line
It depends on the run environment, because on temperature the two tie once you buy the -2 version: dual-rated THHN/THWN-2 and XHHW-2 both give you the 90C wet column to start derating from. Default to THHN/THWN-2 for typical conduit work, where it pulls easier, fills less pipe, and costs less. Switch to XHHW-2 where its advantages are concrete: rooftop sun (it dodges the temperature adder that punishes THHN), large feeders, aluminum, and rough pulls. Either way the termination rating, not the insulation, usually caps the ampacity under 110.14(C), and the adopted code edition and the spec control. Read the jacket print to confirm the actual rating before you pull.
FAQ
Is XHHW-2 better than THHN?
Neither is better across the board. On temperature they match: dual-rated THHN/THWN-2 and XHHW-2 are both 90C wet and dry. THHN's nylon jacket pulls cleaner and takes less conduit fill; XHHW-2's cross-linked insulation is tougher, handles rough pulls, and is exempt from the rooftop temperature adder. Pick by the run environment and the spec.
Can I substitute XHHW-2 for THHN in the same conduit?
Watch the fill. XHHW-2 carries no nylon jacket and its cross-linked insulation is thicker, so the same size takes up more room in the pipe than THHN. A conduit sized for THHN can be over-filled if you field-change to XHHW-2. Recheck conduit fill before swapping, and confirm the spec allows the substitution.
Do THHN/THWN-2 and XHHW-2 both work in wet locations?
Yes, as long as you buy the wet-rated versions. THWN, THWN-2, and XHHW-2 all carry the W and are rated for wet locations, which the code treats as including underground raceway, fillable conduit, and outdoor runs. Plain THHN with no W and no -2 is dry only. Read the jacket print to confirm the wire in hand is wet rated.