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What Is THHN Wire?

When designing and building a new facility or machine, engineers need to fully understand the specifications – the strengths and weaknesses of each wire available for the job. The biggest challenge for an engineer is to balance the best wire for the project and the budget.

The most popular type of building wire used in construction is Thermoplastic High Heat-resistant Nylon coated (THHN) wire. THHN wire is also commonly used in machine tools, control circuits and some appliances.


Thermoplastic is a PVC material, which makes it a thinner insulation than most other types. Thermoplastic also emits toxic smoke when it gets hot enough to burn, so the installed wire shouldn't be exposed.

High Heat

It is important to know the temperature at which the insulation will break down. HH means the insulation will withstand a 90 C (194 F) temperature.


Protecting the thermoplastic is a nylon jacket. The nylon also makes installation easier because the wire will slide freely through conduit.

THHN wire comes in a variety of characteristics:

  • Conductors: Depending on the size can be either stranded or solid
  • Wire material: Copper or aluminum
  • Jacket: Polyvinylchloride (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, the jacket protects the wire from hydrocarbons such as oil, gasoline and grease. The tough nylon jacket also protects the wire from being puncture or accidentally cut as it protects the insulation.

THHN wire is approved for up to 600V and is listed by Underwriters Laboratory (UL), as rated for 90° C (194° F) in dry and 75° C (167° F) in wet environments. THHN wire comes with a dual rating: THHN / THWN, which means it’s appropriate for both dry and wet applications.

Post time: Mar-25-2021
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