Can I use THHN outdoors?

THHN alone is a dry-location conductor. 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.

THHN vs THWN: What the Letters Mean

The letters describe the insulation: heat resistance, moisture resistance and where the conductor may be installed. The designation decides which temperature column you are allowed to use.

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 jacket, resistant to gasoline and oil.

Why THWN-2 is the practical default

THHN is dry-location only. THWN is wet-location rated but sometimes limited to 75°C. THWN-2 is both wet-location rated and good for 90°C, which is why it 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 a dry-location conductor. 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

Educational information only — not professional engineering advice. Reference table values are pending review; see the methodology page.