What THHN Wire Meaning and Most Comprehensive Ampacity Table

THHN or THWN stranded building wire is one of the more commonly used wire types in household, commercial and industrial buildings. What are the benefits compared to other types of wire and what is the difference between them?

What does the THHN meaning?

As with most wiring, these abbreviations specify the material of the outer insulation and some of its properties. In this case, THHN stands for Thermoplastic High Heat Resistant Nylon. Thermoplastic is a polyvinyl chloride PVC material that is much thinner than most other insulating materials. This allows for thinner wires and an easier-to-pull outer cover both made of nylon, which also makes pulling easier as the nylon allows the wire to slide more.

Where can THHN wire be used?

THHN stranded wire should only be used in dry and damp places. If you need to wire in wet locations, you must use THWN wire. It's important to note that many wires on the market today are dual THHN and THWN rated, which means the wire can be used in dry, wet, or wet locations. THHN wire has a high temperature rating: insulation up to 90°C or 194°F.

What is the THHN/THWN-2?

As mentioned, most of the time you ask for a THHN solid wire and they will give you a wire rated THHN/THWN which means it has both THHN and THWN properties i.e. it can be used in dry, wet or wet locations . The limit is 90°C insulation when used in dry or wet locations, but 75°C should be used if used in wet locations.

THHN/THWN-2 wire do not have this limitation. In other words, you can use 90°C as an insulation temperature rating even in wet locations.

Precautions when using THHN building wires

Thermoplastic is the material from which the outer sheath of THHN solid wire is made, which releases toxic fumes when burned. Therefore, placement inside the catheter is always recommended. Also, be sure to check the National Electrical Code or other relevant electrical codes in your jurisdiction before doing electrical work.

Ampacity Chart of CU THHN Wire

THHN wire size table as below;