THHN vs XHHW-2 building wire: nylon jacket or XLPE

Two wires, two standards

THHN (and its dual-rated form THHN/THWN-2) is a single copper or aluminium conductor with heat- and moisture-resistant PVC insulation and a thin nylon jacket over it. It is made to UL 83, the standard for thermoplastic-insulated wires. XHHW-2 is a single conductor with cross-linked polyethylene (XLPE) insulation and no separate jacket, made to UL 44, the standard for thermoset-insulated wires. The letters tell the story: T for thermoplastic, X for cross-linked; HH for high heat; W for wet locations; N for the nylon jacket; the suffix 2 for the higher wet-location temperature rating.

Both are building wire rather than cable: each conductor is pulled individually into conduit or raceway, as explained in wire vs cable. Neither is a sheathed assembly, and neither should be confused with appliance wiring material. AWM made to UL 758 is not permitted as field wiring, a point worth stating on any RFQ that goes to a plant used to export markets outside North America.

Construction compared

The table sets the two side by side for a common size. THHN figures are taken from a public datasheet for 12 AWG; XHHW-2 dimensions vary more between makers and are confirmed per datasheet.

The practical construction difference is the outer surface. THHN relies on nylon to resist abrasion during the pull and to protect a relatively thin PVC wall. XHHW-2 has a thicker, tougher cross-linked wall without a jacket, so it does not depend on a second layer staying intact. Because cross-linked insulation does not soften the way PVC does, XHHW-2 is often chosen where the insulation will see heat, rough pulls or wet conduit for long periods. That is a selection preference in the trade, not a rule; the project engineer and the specification decide.

Weight is another point buyers notice when comparing quotes. Both wires are almost all conductor by mass, so the copper or aluminium content per kilometre drives price far more than the insulation choice does. Two offers for the same AWG size should show similar weight per kilometre; a large gap usually points to a short conductor, a different stranding or a thinner wall, and it is worth asking for the resistance record before comparing price.

Where each one is usually specified

THHN/THWN-2 is the everyday choice for branch circuits and feeders in commercial and residential conduit. It is thin, which helps conduit fill, it slides easily thanks to the nylon, and distributors stock it in every colour. XHHW-2 shows up more in industrial work, service entrances, underground conduit and larger feeder sizes, and in aluminium conductors for feeders where weight and cost per metre matter. Many distributors carry both, and buyers who sell into US electrical wholesale often need a colour range in THHN and a size range in XHHW-2.

• Conduit fill: the thinner THHN wall usually allows more conductors in the same raceway; check against the fill tables your engineer uses.
• Abrasion: THHN depends on the nylon jacket staying intact; damaged nylon is a common complaint after a hard pull.
• Heat and moisture: both carry wet ratings when listed with the -2 suffix; confirm the rating on the listing and the print.
• Minimum sizes: NEC 310.3 sets 14 AWG as the minimum copper size and 12 AWG for aluminium and copper-clad aluminium.

Current rating is not a property you can read off this comparison. It depends on conductor size, temperature column, installation method and correction factors, and it is set under NEC Article 310 by the project engineer, not by the wire supplier.

Listing, AHJ and what to verify

US projects usually require UL (or other NRTL) listed products to satisfy the authority having jurisdiction (AHJ). That means the listing has to belong to the partner plant that actually extrudes the wire, and it has to cover the exact type, size range and conductor material on your order. A UL listing held by the partner plant for that construction is checked in the certifier's public database per order, and the file number printed on the insulation should match it. A listing for THHN does not cover XHHW-2, and a listing for copper does not automatically cover aluminium.

The UL listing guide for US building wire covers the database check step by step, and verifying cable certificates shows how to read a certificate scope. Beyond the listing, the desk checks conductor resistance against the declared size, insulation and jacket thickness, and spark or voltage test records for each production lot.

Sourcing either wire from China

Only a subset of Chinese partner plants hold US listings for building wire, so the shortlist is narrower than for IEC products. Plants with a THHN listing do not always hold an XHHW-2 listing, and the reverse is also true. For a mixed order the desk usually splits the order between two plants or confirms that one plant's listings cover both types before samples are made. Typical points to settle early:

1. Type and size list: THHN/THWN-2 or XHHW-2, AWG sizes, solid or stranded, copper or aluminium.
2. Colours: the colour set per size, including any striping.
3. Print: listing mark, file number, your private label text if allowed under the listing.
4. Packing: reel lengths such as 500 ft or 2,500 ft, reel type and carton or pallet plan.
5. Quantities: total footage per size and colour, which drives minimums and line changeovers.

Minimum quantities and lead times depend on the plant, colour count and copper booking; they are typical ranges confirmed in writing for each order, as set out on the MOQ and lead time page. The building wire line lists the construction options the desk sources, and the quote form takes your type, size and colour list directly.