Master Guide to Building Wire Types

What it is

Building wire is single-conductor insulated wire installed in raceways, cable tray or as part of a cable assembly. Its type designation, for example THHN or XHHW-2, is a short code that describes the insulation material, temperature rating and the locations it can be used in. NEC Article 310 lists the recognized types and their conditions of use. The product standards are UL 83 for thermoplastic insulation and UL 44 for thermoset insulation.

How to identify

• Read the print legend. Wire is surface-printed at intervals with the size, the type letters (often several, such as THHN/THWN-2), the voltage rating, the conductor material, the manufacturer and the listing mark.
• Look at the insulation. THHN/THWN-2 has a thin, glossy nylon outer layer over PVC. XHHW-2 has a matte cross-linked polyethylene insulation with no nylon jacket. RHW-2 and USE-2 have a thicker, rubbery thermoset insulation.
• Many wires are multi-rated. One wire is commonly listed for several types at once, for example XHHW-2/RHW-2/USE-2 or THHN/THWN-2/MTW. The print line shows which ones.

Letter-code decoder: T = thermoplastic insulation; H = heat-resistant (75 °C); HH = high heat-resistant (90 °C); W = moisture-resistant, suitable for wet locations; N = nylon (or equivalent) outer jacket; X = cross-linked polymer insulation; R = rubber / thermoset insulation; U = underground (direct burial) use; SE = service entrance; -2 = 90 °C rating in both wet and dry locations.

Common families

Key points

• The type letters define the insulation and the locations where it can be used. They do not tell you the circuit size it can carry. Ampacity comes from NEC 310.16 and the related tables. It is then limited by the termination ratings (NEC 110.14(C)) and any correction and adjustment factors.
• The "-2" suffix matters. A THWN wire is rated 75 °C in wet locations. THWN-2 is rated 90 °C wet or dry.
• Check the conductor material on the print line: copper (CU), aluminum (AL) or copper-clad aluminum. It affects which terminals and connectors can be used.

Common mistakes

• Selling THHN-only wire for an underground or outdoor conduit run. The inside of those raceways is a wet location, so the wire needs a W rating.
• Assuming NM-B can carry its 90 °C conductor rating. NEC 334.80 limits it, so check the code.
• Swapping a cable type for building wire, or the reverse, without checking the code article for that wiring method.

Governing standards: NEC (NFPA 70) Article 310 and the cable articles 334, 338 and 340; UL 83; UL 44; UL 854. Use the edition adopted by the authority having jurisdiction.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.