How should you determine the ampacity of a 14 AWG copper conductor terminated in a location with 90°C insulation?

• A. Use the 90°C column (or a 60°C/75°C conversion based on device rating) per NEC ampacity table and apply any necessary adjustment factors.
• B. Use the 60°C column only.
• C. Always use 75°C column regardless of device rating.
• D. Use a fixed 15 A rating.

Answer: (D)

Ampacity is determined by more than just wire size; you must match the conductor’s insulation rating with the rating of the termination and include any temperature adjustments. For 14 AWG copper, the NEC ampacity table shows 60°C, 75°C, and 90°C columns corresponding to 15 A, 20 A, and 25 A, respectively. Since the insulation is rated at 90°C, you would normally be able to use the 90°C column, but you can only do that if the device or termination you’re connecting to is also rated for 90°C. If the device is only rated for 60°C or 75°C, you must use the corresponding column (60°C or 75°C) and then apply any required adjustment factors for ambient temperature or the number of current-carrying conductors.

The upshot is that you don’t fix the ampacity at 15 A unless the device rating forces it; instead, you determine the ampacity by selecting the correct column based on device rating and applying any applicable adjustment factors. This ensures the conductor is not overloaded given both its insulation and the termination it’s connected to.