XHHW vs THHN Wire: What's the Difference?

XHHW and THHN wires are very similar. So similar, they can often be used interchangeably. But there are differences which make XHHW a little more robust (and slightly more expensive), than it's more cost-effective THHN counterpart. Their primary differences stem from their insulating materials.

First, let's clear up any confusion about acronyms. XHHW is now typically known as XHHW-2 or XHHW/XHHW-2-- a higher standard that increases it's temperature rating in wet locations. The same is true of THHN, which you'll now see labelled THHN/THWN-2, also a higher rating for wet locations. In both cases, these dual-rated wires are backward-compatible, meaning any application that calls for XHHW can use XHHW-2, and any that requires THHN can also use THHN/THWN-2.

Now, about those insulations.

XHHW-2 Wire: Thermoset Insulation

XHHW stands for XLPE (crosslinked polyethylene) insulation, High Heat-resistant and Water-resistant.

XLPE is a thermoset compound, meaning it undergoes a curing process, enabling polymers to cross-link. Post curing, they maintain their shape and don't remelt under heat, enhancing safety in high-temperature settings. XLPE is tough enough that no jacket is required.

THHN/THWN-2 Wire: Thermoplastic Insulation

THWN-2 stands for Thermoplastic Insulation, Heat and Water-Resistant, Nylon-Coated. The thermoplastic compounds in use here are PVC (polyvinyl chloride) and can be reheated, reshaped, and cooled to retain the desired shape. However, this re-melting property can result in potential safety issues in certain high-temperature applications.

PVC insulation, though lighter and cheaper, could lead to current leakage or dielectric breakdown in demanding circuits or from chemical or environmental exposure. Moreover, PVC emits toxic smoke when burned. Nylon jacketing provides added abrasion, fatigue and UV resistance to the underlying PVC, but that doesn't make it quite as tough as XLPE.

XLPE outperforms PVC in chemical, ozone, and abrasion resistance, as reduced electrical leakage. Modern XLPE yields less toxic smoke than PVC when burned, and typically, thermoset compounds like XLPE provide more flexibility than thermoplastics, making installation easier in projects involving complex, closely-spaced bends.

XHHW-2 and THHN/THWN-2 Conductors

Both wire types are made with copper and aluminum conductors that can be solid or stranded. Copper offers greater flexibility while aluminum is lighter and costs less. Likewise, solid conductors have lower manufacturing costs, hold their shape, and can carry higher currents, while stranded conductors are more flexible and less likely to break if subjected to repeated bending.

Approved Applications for XHHW-2 and THHN/THWN-2 Wire

Ratings for each type of wire are very similar. They are both considered general use building wires with a maximum temperature of 90°C in dry or wet locations, and they are also both resistant to chemicals.

However, XHHW-2's thermoset XLPE insulation makes it a more robust, flexible and durable wire that will out-perform THWN-2 over the long term, and where low current leakage is critical. While its temperature rating matches THWN-2, it has a higher failure tolerance at both ends of the temperature spectrum. These factors make it better-suited for more challenging industrial environments, or in hospitals, where electrical leakage must be tightly controlled. Of course, when these factors aren't needed, THHN/THWN-2 is a cost-effective alternative.

We carry both XHHW-2 and THHN/THWN-2 Wire n a wide range of colors and sizes. As always, be sure to consult an expert to ensure the proper selection for your project.