THHN vs. THWN vs. XHHW: The Three Wires Electricians Actually Pull
On the reel they all look like the same black wire. The difference is in the insulation wrapped around the copper — and it decides where the wire is allowed to go.
Part of The Wire Guide — every wire topic, explained plainly.
It's a location rating, not a quality ranking
Pulling wire through dry indoor conduit? THHN is the standard answer. If any part of the run can see water — underground conduit, outdoor raceways, damp basements — you need the W: THWN-2 or XHHW-2. And if the job calls for tougher insulation on feeders and service runs, that's where XHHW earns its keep.
One wire isn't "better" than the others. Each letter combination is a promise about temperature and moisture. Buy the promise your location requires — nothing more, nothing less.
The difference is literally in the layers
Strip these three wires down and the difference is physical. Here's what each layer is doing:
THHN — PVC insulation plus a nylon jacket
Thermoplastic, Heat resistant, High heat (90°C), Nylon jacket. The conductor gets a coat of PVC insulation, then a thin, slick nylon skin on top. That nylon skin is the part you feel — it's hard, smooth, and it's what makes THHN pull easily through conduit. It also resists oil, gasoline, and abrasion. Temperature rating: 90°C in dry and damp locations. No W means no wet locations. Full stop.
THWN and THWN-2 — same build, plus a moisture rating
Same PVC insulation, same nylon jacket — but tested and listed for wet locations. Plain THWN is rated 90°C dry and 75°C wet. THWN-2 carries the full 90°C in dry and wet locations. The -2 suffix is doing real work: it means the wet rating equals the dry rating. In practice, nearly every spool sold today is dual-marked THHN/THWN-2 — one wire that covers both jobs, which is why electricians stock one reel instead of two.
XHHW and XHHW-2 — crosslinked insulation, no nylon needed
The X is crosslinked polyethylene (XLPE) — a thermoset insulation, not thermoplastic. Instead of melting when overheated, it's chemically tougher: better heat resistance, better abrasion resistance, and it doesn't need the nylon skin at all. The trade-off is a thicker insulation wall, so XHHW runs slightly larger in diameter than the same gauge of THHN. Ratings: plain XHHW is 90°C dry/damp and 75°C wet; XHHW-2 is 90°C dry and wet.
Want the full letter-by-letter decoder? Here's how to read every marking on a wire jacket.
Three cross-sections, side by side
Same copper conductor, three different insulation systems. The layers are the whole story:
Temperature and wet-location ratings at a glance
| Type | Dry / damp | Wet | Insulation |
|---|---|---|---|
| THHN | 90°C | Not rated | PVC + nylon jacket |
| THWN | 90°C | 75°C | PVC + nylon jacket |
| THWN-2 | 90°C | 90°C | PVC + nylon jacket |
| XHHW | 90°C | 75°C | XLPE, no nylon |
| XHHW-2 | 90°C | 90°C | XLPE, no nylon |
One detail that trips people up: underground conduit counts as a wet location under the NEC. Conduit buried in the earth will have water in it — that's assumed, not debated. So plain THHN has no business in buried conduit, even though the conduit looks like protection. That's the single most common misapplication of these three wires.
Matching the wire to the job
THHN — dry indoor conduit
The workhorse of commercial and industrial wiring: branch circuits and feeders pulled through EMT or rigid conduit inside dry buildings. In the Chicago area, where commercial work runs in conduit rather than Romex, THHN (usually dual-marked THHN/THWN-2) is the wire you'll see on nearly every pull.
THWN-2 — anywhere moisture can reach
Underground conduit, outdoor raceways, damp basements, parking structures. Since most THHN sold today carries the dual THHN/THWN-2 marking, this is often the same spool — but the marking is what the inspector reads. If the jacket only says THHN with no W, it stays dry.
XHHW-2 — tougher insulation for feeders and service work
Service-entrance conductors, feeders, and anywhere the spec calls for a tougher insulation wall. XHHW-2 handles abrasion and heat better than PVC, which is why engineers write it into service and feeder specs. It costs more than THHN and takes up slightly more room in the conduit — on a big pull, that diameter difference affects conduit fill, so it gets planned, not improvised.
Cost, size, and what the inspector reads
Cost: THHN/THWN-2 is the commodity wire — cheapest of the three, stocked everywhere. XHHW-2 costs more per foot, which is why you see it specified on feeders and service work rather than general branch circuits.
Size: XHHW's XLPE wall is thicker than THHN's PVC-plus-nylon, so the same gauge of XHHW-2 has a slightly larger outside diameter. That matters twice: once for conduit fill calculations, and once when you're buying — a "smaller-looking" spool of XHHW can hold fewer feet than you expect.
What the inspector reads: the jacket marking. 12 AWG CU TYPE THHN/THWN-2 600V tells the whole story — gauge, copper, dual rating, voltage. If you're holding a roll and want to decode every letter, here's the full jacket-marking guide. And when you're sizing the circuit itself, the wire gauge chart is the master reference.
Pulling out old conduit runs or sitting on partial spools of THHN? We buy usable leftover wire — anywhere in the country. Fast, free quote.
THHN, THWN, and XHHW questions, answered.
Is THHN waterproof?
No. THHN is rated for dry and damp locations only — it has no wet-location rating. For anywhere water can reach the wire, including underground conduit, you need THWN-2 or XHHW-2.
What's the difference between THWN and THWN-2?
The -2 suffix means the wire carries its 90°C rating in wet locations too. Plain THWN is 90°C dry but only 75°C wet. THWN-2 is 90°C dry and wet.
Can I use XHHW instead of THHN?
Yes — XHHW-2 meets or exceeds THHN's ratings in every location, so it's an acceptable substitute wherever THHN is allowed. It just costs more and runs slightly larger in diameter, so it's usually specified rather than substituted casually.
Why does my wire say THHN/THWN-2 — both?
It's dual rated: it meets the THHN standard (90°C dry, nylon jacket) and the THWN-2 standard (90°C wet). One product covers both applications, so distributors and electricians stock a single reel.
Can THHN go in underground conduit?
No — underground conduit is classified as a wet location, and THHN has no wet rating. Pull THWN-2 or XHHW-2 for buried conduit.
Which of the three is cheapest?
THHN/THWN-2, by a clear margin. It's the commodity building wire. XHHW-2 costs more per foot, which is why it's specified for feeders and service work rather than general branch circuits.
I know wire. That's how I know what it's worth.
The letters on the jacket tell us the insulation, the rating, and the location it's listed for — that's how we sort resale inventory from scrap the minute a roll comes in. Know your wire, and you know what it's worth.