CSLB C-10 Electrical ContractorWiring Methods and MaterialsMedium
A new industrial motor control center requires a feeder connection. The feeder conductors are 3/0 AWG THHN copper. The circuit breaker protecting the feeder is rated for 200A, and the terminals on the motor control center are listed for 75°C. What is the maximum allowable ampacity for these conductors, considering the termination temperature limitation?
- A200 amperes
- B175 amperes
- C225 amperes
- D180 amperes
Show answer & explanationAnswer & explanation
Correct answer: B. 175 amperes
According to NEC 110.14(C)(1), the ampacity of the conductor must not exceed the lowest temperature rating of any connected termination. THHN conductors are rated for 90°C, but the terminals are rated for 75°C. Therefore, we must use the 75°C column of Table 310.16. For 3/0 AWG copper in the 75°C column, the ampacity is 175 amperes.
Why the other options are wrong
- A. This is the breaker rating, but the conductor ampacity must be limited by the termination temperature, which is lower.
- C. This is likely the 90°C ampacity for 3/0 AWG THHN, which cannot be used due to the 75°C terminals.
- D. This value is plausible but incorrect; it might be the 75°C ampacity for a different conductor size or material.
Termination Temperature Limitation
Conductor ampacity must not exceed the lowest temperature rating of any termination point (e.g., circuit breaker, lug, equipment terminal) in the circuit, even if the conductor itself has a higher temperature rating.
- NEC 110.14(C) governs this rule
- Conductor ampacity is limited by the lowest rated component
- Common termination ratings are 60°C or 75°C
Memory trick: Terminals are the Temperature Takers; they set the true limit.