CSLB C-10 Electrical ContractorWiring Methods and MaterialsMedium
A new industrial motor control center requires a feeder connection. The feeder conductors will be 750 kcmil THHN aluminum. The terminals on the circuit breaker and the motor control center are both rated for 75°C. What is the maximum allowable ampacity for each 750 kcmil THHN aluminum conductor under these conditions, assuming no more than three current-carrying conductors in the raceway?
- A385 A
- B515 A
- C475 A
- D430 A
Show answer & explanationAnswer & explanation
Correct answer: A. 385 A
For 750 kcmil THHN aluminum conductors, NEC Table 310.16 (formerly 310.15(B)(16)) lists ampacities based on different temperature ratings. Since the terminations are rated for 75°C, we must use the 75°C column for aluminum conductors. This yields an ampacity of 385 A. The 90°C rating of THHN cannot be utilized if the termination points are rated for a lower temperature.
Why the other options are wrong
- B. 515 A is the ampacity for 750 kcmil THHN copper conductors at 90°C, not aluminum.
- C. 475 A is the ampacity for 750 kcmil THHN copper conductors at 75°C, not aluminum.
- D. 430 A is the ampacity for 750 kcmil THHN aluminum conductors at 90°C, but this cannot be used due to the 75°C termination rating.
Termination Temperature Limitation
The ampacity of a conductor is limited by the lowest temperature rating of any component in the circuit, including the conductor insulation, the circuit breaker, and the equipment terminals.
- Conductor insulation typically has a 75°C or 90°C rating.
- Circuit breakers and equipment terminals often have 75°C ratings.
- The lowest temperature rating dictates the maximum allowable ampacity to prevent overheating at connection points.
Memory trick: Ampacity's limit, a chain's weakest link, termination temp, makes you think!