Given a 3 AWG THHN copper conductor with two current-carrying conductors in the raceway, ambient temperature 35 C, terminations rated at 60 C, the ampacity is ___ amps.

Given a 3 AWG THHN copper conductor with two current-carrying conductors in the raceway, ambient temperature 35 C, terminations rated at 60 C, the ampacity is ___ amps.

• A. 105.6
• B. 85
• C. 77.35
• D. 110

Answer: (C)

Ampacity is the maximum current a conductor can carry safely given the insulation rating, ambient temperature, and how many current-carrying conductors share the raceway. Since the terminations are rated at 60°C, you use the 60°C ampacity from NEC table 310.16 for a copper THHN, which for a 3 AWG conductor is 85 A. With two current-carrying conductors in the raceway, no adjustment for the number of conductors is required (the adjustment applies only when there are more than three). The actual ambient is 35°C, so apply the ambient temperature correction factor for 35°C. The correction factor for 35°C is 0.91, so you multiply 85 A by 0.91: 85 × 0.91 = 77.35 A. Therefore, the ampacity is 77.35 amps.

Ampacity is the maximum current a conductor can carry safely given the insulation rating, ambient temperature, and how many current-carrying conductors share the raceway. Since the terminations are rated at 60°C, you use the 60°C ampacity from NEC table 310.16 for a copper THHN, which for a 3 AWG conductor is 85 A. With two current-carrying conductors in the raceway, no adjustment for the number of conductors is required (the adjustment applies only when there are more than three).

The actual ambient is 35°C, so apply the ambient temperature correction factor for 35°C. The correction factor for 35°C is 0.91, so you multiply 85 A by 0.91:

85 × 0.91 = 77.35 A.

Therefore, the ampacity is 77.35 amps.