What is the meaning of a THHN wire insulation code?

THHN, or thermoplastic high heat-resistant nylon-coated wire, is made with the use of a PVC or polyvinyl chloride-insulated nylon sheath. Most THHN creation is done with the use of dual-rated THHN or THWN. So it is the best option to use in dry and wet conditions, as well as for both indoor and outdoor uses. The UL rating of THHN wires is 90 degrees in dry conditions and 75 degrees in wet conditions. The conductor used can be made of copper or aluminum. There are twelve color combinations for THHN, and it is mostly part of about six hundred voltage uses. So let’s start. What is the meaning of a THHN wire insulation code?

Introduction to Romex Wire

Romex is an electrical cable best used for indoor wiring applications, specifically for homes and small buildings. These wires come in different sizes and designs, so they can be easily pulled through conduit.

What is the meaning of a THHN wire insulation code?

Romex NM-B wire

was created by Southwire, which is a famous cable and wire services provider. The NM-B code defines the wire’s nonmetallic sheath and is used in branch wiring. Romex, also called SIMpull, is used due to its jacketing, which helps to pull wire through conduit easily.

Romex cables normally come with many THHN wires bundled together and sheathed in PVC, making them best for indoor uses like grated inner walls and surface wiring on the ground. These wires are also used for the connection of electrical panels with different lights and appliances.

Features of THHN Wires

  • Its main feature is thermoplastic insulation that comes with materials like PE or PVC. It can easily be stripped and used and has different colors.
  • These wires have ninety-degree temperatures, so they can be used in buildings.
  • It has nylon material wrapped around it to protect it from cuts and abrasion and also from destructive agents like chemicals, oil, and water.
  • These wires come in ten different colors: gray, red, orange, violet, brown, black, blue, yellow, and green with a yellow stripe. These different colors are not to define any certain feature of wire but are used to make a difference between different bundles of wire.

THHN Wires Sizes

  • These wires come in different sizes, and each is used for different functions. The standard AWG wire size is from 14 AWG to 1 AWG. With that, larger sizes ranging from 1/0 to 4/0 can be obtained.
  • THHN 10 AWG, 12 AWG, and 14 AWg come with standard or solid wires and normally come in Romex cables.
  • 1 AWG, 2 AWG, 4 AWG, 6 AWG, and 8 AWG sizes are best for uses that need high power. High power is best used for dryers, stoves, etc.
  • While the largest THHN cables are normally used to provide power from distant appliance connections,

Advantages and Disadvantages of THHN Wire


  • Its main benefit is that it fulfills the different codes. But before using it, make sure your project is configured with these wires.
  • It can handle a high temperature of about ninety degrees.
  • It comes with a nylon coating that helps to protect copper wire from abrasion, oils, and water.


  • These wires are not the best option for outdoor wires; instead, these wires use UF-B cables.

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Uses of THHN Wires

THHN wire is very commonly used in the USA. It is best to use different instruments and circuits. It is preferred by electrical contractors since it is low-cost and mostly used in industries.

The NEC rating of these wires is NFPA 70. It can handle oils, heat, and grease and also passes the flame test. It is used for indoor uses like garbage disposal due to its insulation and voltage capacity. For indoor applications, it is used as a single or multi-conductor design. With that, it can handle electrical current between buildings and outer power sources, different machines, and control wiring.

It is also used for outdoor applications, and for that purpose, it is first installed in conduit and then connected to underground wiring with these features. To prevent any damage, this wire comes with a nylon cover.

Why should THHN wire come in bundles?

It is the best wire for electrical contactors since its features fulfill the needs of electrical circuits at an affordable cost. Its low cost makes it a less costly option than XHHW.

THHN PVC insulation enhances conduction but minimizes the flexibility of the wire. So it helps to reduce the effect of inflexibility. To have safety measures for working people and connected instruments, THHN wire can be configured in a bundle to cover the configuration of the working area. Manual installation of wire increases accuracy and minimizes the workforce. One pull can be employed to bundle THHN wires with each other with a protective UFB covering for saving time in outdoor circuits. The high heat resistance and water resistance make it best for outdoor and indoor uses.

Differences Between THHN and THWN Cable

  1. THHn wire insulation is made with PCV and then the wire is insulated with polyethylene sheath. polyethylene sheath comes with protection from oil, water, dirt, and moisture as compared to PVC. So for outdoor conditions, Thwn is preferred
  2. The conductors of Thhn wire are solid conductors and thwn has stranded conductors. Standard conductors are normally best for high-flex circuits for example wiring motors and fixtures since they are flexible structure and has less chance of breakage. But the conductors of thwn are high cost as compare to thhn. The PVC insulation on thwn with a combination of nylon covering offers high flexibility and easy connection features.
  3. Thhn wire is made following the NEMA standards and thwn is made using UL 83 and UL 1063 standards. According to uses these standards can be best for certain uses.

Differences Between THHN and THWN Cable


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