What's the difference between these very similar wires?
In short, THWN wire is more water resistant than THHN. Both are single conductor building wires with PVC insulation and nylon jackets, rated to 600V. Both can have stranded or solid copper or aluminum conductors. THHN's advantage is that it carries a higher temperature rating in dry environments. However, THWN-2 is rated for BOTH higher heat and wet environments.
Even better: THHN/THWN-2 cable is dual-rated, so you don't even have to choose. Many manufacturers supply only the dual-rated option for simplicity and cost-effectiveness in manufacturing.
What is THHN Wire?
THHN is an extremely common, inexpensive building wire suitable for use in conduit and raceways, as defined by the National Electrical Code (NEC). While intended for residential or commercial construction, THHN wire is also considered a general purpose cable for a variety of industrial applications including motors, control circuits and other machine tools.
THHN stands for Thermoplastic, High Heat-Resistant, Nylon-Coated. "Thermoplastic" means a plastic polymer material, in this case, polyvinyl chloride (PVC), is heated to become pliable, extruded onto the conductor, then solidified as it cools.
PVC insulation came into common use in the 1930's and '40s. It's both cost-effective and can be made to serve a variety of uses, including THHN's flame retardant, moisture resistant, high heat standard.
Nylon jacketing is applied for added abrasion, fatigue and UV resistance.
THHN wire is suitable for use in temperatures up to 90°C or 194°F in dry conditions, and 75°C or 167°F, when wet. Although it is considered moisture resistant, THHN is not recommended for use outdoors or in other environments where exposure to moisture, including high humidity is likely. Generally speaking, it should only be used within conduit or other protective enclosure.
What is THWN and THWN-2 Wire?
THWN and THWN-2 are simply improved variants of THHN.
THWN stands for Thermoplastic Heat and Water-Resistant, Nylon-Coated. Tracing back to the early 1960's, it was an upgrade of the older TW (Thermoplastic Water-Resistant) wire.
THWN-2 was developed to combine the best of THHN and THWN, with thicker PVC insulation and nylon jacket for added heat resistance and improved resistance to moisture and chemicals. THWN-2 wires are roughly 10% larger in diameter than THHN wires of the same gauge.
THWN-2 (and therefore THHN/THWN-2 dual-rated) wires are suitable for use in temperatures up to 90°C or 194°F in wet or dry environments. However, when used as machine tool wire, it is rated up to 90°C or 194°F when dry, but only 60°C or 140°F if exposed to oil or coolants.
Manufactured in the US by Southwire, Nexans and Alan Wire among others, all THHN/THWN-2 is UL-rated and considered generic. While slight differences between manufacturers may exist, they are completely interchangeable.
Consider THHN/PVC Tray Cable for Multi-Conductor or Exposed Runs
While THWN-2 is more versatile than THHN in single conductor applications, THHN doesn't require the added toughness when used as an internal component in THHN/PVC cables, also known as Vinyl Nylon Tray Cables (VNTC).
Combining two or more THHN wires within a tough PVC jacket makes for a more robust and convenient solution than individual THWN-2 wires where exposed runs are needed, including outdoors and in direct burial without conduit.
Click here for our full selection of THHN/THWN-2 wires. We carry a wide variety of sizes and colors, in stock and cut to order at over a dozen warehouses across the country. If you don't find the option you need, or are looking for something custom, let us know. If it can be made, chances are, we can get it for you.