THHN vs THWN: What the Letters Mean

Decoding the letters

Each letter is a property:

• T — thermoplastic insulation.
• H — heat resistant, rated 75°C.
• HH — heat resistant to 90°C.
• W — moisture resistant, suitable for wet locations.
• N — nylon or equivalent jacket. Oil and gasoline resistance depend on the specific product markings and listing.

Why THWN-2 is the practical default

THHN is rated 90°C in dry and damp locations, subject to its listing; THHN alone is not rated for wet locations. THWN is a wet-location conductor rated 75°C; that 75°C rating is the designation, not an occasional limit. THWN-2 is wet-location rated and good for 90°C, which is why dual-rated THHN/THWN-2 is so common in conduit.

The 90°C rating does not mean you get to run the conductor at 90°C ampacity. Terminations and equipment ratings usually cap you lower.

Frequently asked

Can I use THHN outdoors?

THHN alone is suitable for dry and damp locations where listed, but not wet locations. In wet locations you need the W rating, which is why dual-rated THHN/THWN-2 is commonly used.

If it is 90°C rated, can I use the 90°C ampacity?

Usually not in full. The termination temperature limit can cap the usable ampacity, and the 90°C column is mainly useful for applying correction factors from a higher base.

Does the insulation type change the diameter?

Yes, different insulation types have different outside diameters, which changes how many conductors fit a raceway.

Related calculators

• Wire Ampacity Chart
• Ampacity Calculator
• Conduit Fill Calculator
• Quick reference (PDF)