Swipe the diagram to see the switch loop
- Two temperature ratings printed together (THHN/THWN-2) means the wire meets both: 90 degrees C dry and 90 degrees C wet.
- THWN with no -2 is 75 degrees C in wet locations. Same wire family, lower wet rating, and it changes your ampacity column.
What each type actually means
The letters are not a brand. Each one is a property, and the combination is what the wire is listed for.
THHN90 degrees C, dry and damp locations Thermoplastic, High Heat-resistant, Nylon-coated. Table 310.4(A) lists it for dry and damp. Not wet. |
THWN75 degrees C in wet locations Thermoplastic, Heat and Water-resistant, Nylon-coated. |
THWN-290 degrees C wet and dry The -2 is the part that matters. It raises the wet rating to 90 degrees C. |
THHN/THWN-2What you almost certainly have Dual-marked wire meets both listings. This is the standard product on US shelves. |
Why the argument keeps happening
Both sides are quoting something true. THHN really is limited to dry and damp locations, and the wire in the van really is fine underground. The reason both hold is that the wire is dual-marked, so people argue about the label while holding the answer.
What counts as a wet location
This is where people get caught, because a wet location does not mean standing water.
Article 100 defines it to include installations underground, in concrete slabs in direct contact with the earth, and locations exposed to weather and unprotected. 300.5(B) is explicit that the interior of a raceway installed underground is a wet location.
Conduit undergroundWet Including the inside of the raceway, even where no water is visible. |
Conduit outdoors, above groundWet Exposed to weather and unprotected. |
Inside a heated buildingDry |
Unheated basement, damp but not wetDamp A separate classification. Check the listing covers it. |
Check the wire, not the chart
Manufacturers print the listings on the insulation because that is the authority. If the jacket says THHN only, it is a dry-location conductor whatever anyone tells you it is normally sold as.
Insulation types and their temperature ratings come from NEC Table 310.4(A). Your AHJ may enforce an older edition, and product markings vary by manufacturer. The jacket print on the wire in your hand is the authority, not any chart.
Common questions
Is THHN rated for wet locations?
THHN on its own covers dry and damp locations at 90 degrees C, not wet, so on the strict reading no. In practice nearly all US building wire is dual-marked THHN/THWN-2, and the THWN-2 half is rated 90 degrees C wet. Read the jacket rather than assuming either way.
Is the inside of underground conduit really a wet location?
Yes. 300.5(B) says the interior of a raceway installed underground is a wet location. Water gets in through condensation and through fittings, so the code treats it as wet regardless of what you can see.
What does the -2 in THWN-2 mean?
It raises the wet-location temperature rating to 90 degrees C. THWN without it is 75 degrees C wet. That difference can change which ampacity column you are allowed to start from.
Does the printing wear off?
It can on older or heavily pulled conductors. Where you cannot read the print you cannot confirm the listing, and an inspector is entitled to treat it as unidentified.