THHN: Understanding THHN Wire

Building wire is generally used to carry electrical current to all external uses of power in a building or dwelling. This product is utilized in the construction of almost every industrial, residential and commercial building. The most popular type is THHN building wire.

THHN Wire stands for Thermoplastic High Heat-resistant Nylon coated. THHN can come in stranded or solid conductors depending on the size. It is either manufactured in copper or aluminum and covered in a PVC (polyvinyl chloride) insulation with a nylon jacket. THHN is UL listed with a rated 90 degrees Celsius in dry locations or 75 degrees Celsius in wet applications with a THWN rating. The vast majority of THHN building wire carries a dual rating on the cable marked THHN / THWN for both the wet and dry temperature rating. THHN building wire may also be used for wiring of machine tools, control circuits or on certain appliances.

THHN building wire has several main distinctions compared to other building wire products. THHN uses a thinner PVC insulation which is a key factor in terms of its electrical properties. This thinner insulation can often lead to a current leakage and even a breakdown during chemical or environmental exposure. The PVC insulation in THHN also creates a toxic smoke when burned therefore making it undesireable in certain applications.

THHN is not a very flexible product due to its nylon coating. This can often be a factor for many contractors or end users since there is usually a preference to use a product that saves energy and time during installation. However, THHN building wire has grown in popularity since it is a cost effective alternative compared to other types of building wire such as XHHW building wire. Despite some of its down falls, many users of THHN building wire have found this products to be sufficient for meeting their projects specifications.

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