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Understanding Electrical Wire Lettering

What do the Letters on Electrical Wire Mean?

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An image of electrical wire lettering.

Electrical Wire Lettering

Timothy Thiele

Understanding electrical wire lettering will help you decide which type wire is best for the installation that you are using. There are many different types of insulation used for residential use. Some of the most commonly used are THHN, THWN, THW and XHHN.

THWN is flame-retardant and heat resistant. Its insulation can be used in dry, and damp locations. This is not to be used in wet locations however. THHN wire is commonly used in conduit installations in residential and commercial installations. Due to its nylon insulation, it is smaller and is more flexible than the more rigid plastic coated wire.

THWN is flame retardant, heat resistant, moisture resistant, gasoline resistant, and oil resistant. It can be used in dry, damp, or wet locations. This cable is often used in underground conduit feeds that will be subject to moisture and other liquids.

THW is heat resistant, flame retardant, and moisture resistant. It can be used for dry, damp, and wet locations. It too can be used for underground feeds that are subject to moisture.

XHHN is used for service entrance and is designated type SE cable.

So exactly what do the letters on the wire mean? Her's a breakdown to help make some sense of the lettering.

  1. The "T" stands for thermoplastic insulated cable.
  2. A single "H" means the wire is heat resistant.
  3. "HH" means that the wire is heat resistant and can withstand a higher temperature. This wire can withstand heat up to 194 degrees Fahrenheit.
  4. A "W" means that the wire is approved for damp and wet locations. This wire is also suitable for dry locations.
  5. The "X" means the cable is made of a synthetic polymer that is flame-retardant.
  6. The "N" is for the nylon coating that covers the wire insulation.

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