Which of the following listed conductors has a greater ampacity when used in a dry location compared to when used in a wet location?

• A. THHN
• B. RHW
• C. THWN
• D. THHW

Answer: (D)

Ampacity depends on how heat is generated and dissipated, which is affected by the environment. In dry locations, heat can escape more easily from the conductor, so it can carry more current safely. In wet locations, moisture interacts with insulation and heat transfer, typically requiring a lower allowable current to avoid insulation damage or overheating.

THHW uses a thermoplastic insulation that is water-resistant. Its current-carrying capacity is designed to be higher when it’s used in a dry location and lower when used in a wet location, so it shows a greater difference between dry and wet ampacities than the other listed types. That’s why this one is the best answer for having a greater ampacity in dry conditions compared to wet.