A raceway run is classified as a wet location. Which conductor insulation type carries a 90 degree C rating in both wet and dry locations?
- ATHHN
THHN is a heat-resistant thermoplastic listed for dry and damp locations only. It has no wet-location rating at all, so it cannot be used where water is expected inside the raceway.
- THWN-2
- CXHHW
XHHW is rated 90 degree C only in dry and damp locations. In a wet location its rating drops to 75 degree C, which is why the -2 version exists.
- DRHH
RHH is a thermoset insulation carrying a 90 degree C dry rating. It is not a wet-location insulation; the wet-rated companion designation is RHW or RHW-2.
Why B is correct
Insulation trade names encode both a temperature and an environment. The W means the thermoplastic passed long-term water-immersion testing, and the -2 suffix means it was listed to hold 90 degrees C while wet rather than only while dry. Table 310.4(A) is where that pairing lives, and it is why a single dual-marked building wire can start at the 90 degree C column indoors or in a flooded rooftop run.
What this question is testing
This item tests whether you can read the conductor insulation table and connect an insulation designation to its permitted operating environment and temperature rating, rather than assuming all 90 degree C wire behaves identically.
On the job
Every rooftop conduit, every underground run, and every raceway on the outside of a building is a wet location. Inspectors routinely reject pulls where plain THHN was used because the reel on the truck was single-marked. Before you pull, read the surface print on the conductor: if it says THHN/THWN-2 you are covered in both conditions, and if it only says THHN you cannot use it outdoors or underground. Conductor marking disputes are one of the most common correction notices on commercial jobs.
Memory technique
W is for water, and dash-two doubles down: 90 degree C wet and dry.
Exam tip
The letter W means wet-rated, and the suffix -2 means 90 degree C in wet locations too. No W in the designation means dry and damp only.
Where to look it up
Turn to the conductor applications and insulations table in Article 310 and read across the Maximum Operating Temperature and Application Provisions columns.