When designing and building a new facility, machine or appliance, engineers need to know the strengths and weaknesses of each wire available for the job.
As an engineer, you have the difficult task of balancing the best wire for the project and your budgets. THHN and XHHW are two common types of wire, and it’s crucial for you to understand their differences in order to choose which is best for your application.
The most popular type of building wire is Thermoplastic High Heat-resistant Nylon coated (THHN) wire. This building wire, as defined by the National Electrical Code, is for use in construction. Engineers also commonly use THHN in machine tools, control circuits and some appliances.
THHN wire comes in a variety of types. The wire comes in two types of conductors depending on the size: stranded or solid. It’s manufactured with either copper or aluminum, and it’s covered in a polyvinyl chloride (PVC) insulation with a nylon jacket.
Aside from the mechanical protection, a nylon jacket protects the wire from abrasion as it’s pulled through conduit. In addition, such jacketed wire is protected from hydrocarbons such as oil, gasoline and grease. Cut-through protection is also increased with nylon jackets while protecting the insulation.
The use of nylon jackets also enables a reduction in the amount of insulation required, allowing a conduit to contain more wires compared to unjacketed wires of the same current carrying capacity. This can be especially advantageous when utilizing built-in or underfloor systems.
The wire is approved for up to 600 volts and is listed by Underwriters Laboratory (UL), an independent product standards company, as rated for 90 degrees Celsius in dry locations and 75 degrees Celsius in wet environments. Much of THHN wire comes with a dual rating: THHN/THWN, which means it’s appropriate for both wet and dry applications.
XHHW and XHHW-2
Cross-linked Polyethylene High Heat-resistant Water-resistant (XHHW-2) wire is the newest generation of XHHW wire. While its main designation is for use in residential, commercial and industrial buildings, it’s also used in raceways and feeder and circuit wiring.
This wire is manufactured with copper or aluminum conductors and is either solid or stranded cable. A larger size wire is stranded to maintain flexibility.
XHHW-2 is approved for use up to 600 volts and rated by UL for 90 degrees Celsius in dry and wet locations. The previous generation of XHHW had a lower temperature rate. It was only approved up to 75 degrees Celsius in wet environments, making the new XHHW-2 generation a more desirable product.
A Comparison Between THHN and XHHW-2
A few factors make THHN wire different than other products.
THHN uses a thinner PVC insulation. This thinner insulation—while saving space—could be a disadvantage depending on the application. The reduced coating can lead to current leakage and can break down from chemical or environmental exposure.
XHHW-2 has XLPE insulation instead of the PVC insulation used in THHN wire.
This makes XHHW the more expensive option, but its coating is more resistant to chemicals, ozone and abrasions. THHN’s coating is thinner and emits a toxic smoke when burned. This is avoided in XHHW-2 with the use of a different insulation.
THHN wire isn’t as flexible as some alternatives because of the nylon coating—what you gain in mechanical strength and sturdiness, you lose in flexibility. XHHW-2’s coating is more flexible and may be advantageous for difficult projects.
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