The 90°C rating can be used for ampacity adjustments for NM cable provided that the final ampacity does not exceed the ampacity of which temperature rating?

• A. 30°C
• B. 60°C
• C. 75°C
• D. 90°C

Answer: (B)

Ampacity calculations are tied to both the insulation rating and the termination rating of the equipment you’re using. NM cable may have insulation rated at 90°C, but the actual allowable current is limited by the temperature rating of the terminations and the conductors’ listing for typical residential equipment. In practice, the final ampacity for NM cable is restricted to the 60°C column. So even if the insulation is 90°C, you size or adjust the current not to exceed what the 60°C rating would allow. This keeps the current within what terminals, lugs, and protection devices in standard installations are designed to handle safely. For example, a 12 AWG NM-B conductor has a 60°C ampacity of about 20 A, and you wouldn’t increase that limit based on the 90°C insulation unless all terminations are explicitly rated for 90°C and the calculation permits using the higher column.