What Is THHN Wire

By Michael Grossman


Coming into the electrical wiring market today can be daunting. With a plethora of choices, and codes being called for it may seem impossible to know exactly what you need, but with the proper education you too can be a cabling expert. The basic construction of copper based wire and cable is a copper core or conductor surrounded by a jacketing material and an insulation material covering that. Different combinations of these materials create cables rated for differing environments and applications.

Copper: A conductor made to size

Copper is a naturally occurring mineral and is known for its stable chemical state as one of the native metals.  The most useful feature of copper to us will be its high thermal and electrical conductivity. For this reason we commonly mold copper into wires of different diameter. In America the sizes of these diameters are determined by the desired American Wire Gauge (AWG) size, which adhere to the appropriate American Society for Testing and Materials (ASTM) standard described in B 258. Through the process known as wire drawing cables are repeatedly fed through specially designed die in order to reduce the area of the wire with the wire being annealed as needed. The differing sizes proportionately scale the ampacity from as low as 25 amps to as high as 615 amps, with larger sizes rating a larger amperage.

THHN: Thermoplastic High-Heat Resistant Nylon Jacket

When an individual refers to THHN what they are actually referring to is copper wire that is covered in a thermoplastic which is usually Polyvinyl chloride (PVC). THHN is a designation for thermoplastic that is being used in an environment where a high temperature or dampness are a concern.  The THHN designation is closely related to the THWN (Thermoplastic Heat and Water-resistant Nylon) designation, which is also water resistant for wet locations. For many applications that require THHN, THWN may be substituted in its place without loss of rating, as they both have the same temperature ratings.