Q. According to 110.14(C) of the 2002 NEC, what would the maximum allowable ampacity be for a 250 kcmil THHN conductor that feeds a piece of equipment protected by a 250A inverse-time circuit breaker at the point where the conductor receives its supply?
The terminals at the equipment aren't marked with temperature ratings. The terminals at the circuit breaker are marked 75° (You'll need to determine the conductor temperature rating).
For the purposes of this question only, disregard the load being served. It's non-continuous and is properly protected by the 250A circuit breaker. The conductor is already sized properly for the load served. Our concern is the maximum allowable ampacity of the 250 kcmil THHN conductor, as connected in this circuit.
Note: In next issue's question, we'll add information related to the load requirements and the sizing of the overcurrent device and the conductor, including the temperature limitations at terminations.
Explanation: First go to Table 310.16. The ampacity of a 250 kcmil THHN conductor from the 90°C column under ordinary conditions (with no adjustments of any kind being made) is 290A.
A 1993 Code change to Section 110-14(c), Temperature Limitations at Terminations, requires that when determining the (allowable) ampacity of a conductor, “the temperature rating associated with the ampacity of a conductor shall be selected and coordinated so as not to exceed the lowest temperature rating of any connected termination, conductor or device. Conductors with temperature ratings higher than specified for terminations shall be permitted for ampacity adjustment, correction, or both.”
In the example above, this means that the 250 kcmil THHN conductor really is rated at 290A, per Table 310.16. The THHN conductor has a rating of 90° C (Table 310.13). The terminals at the circuit breaker are marked 75° C. The terminals at the equipment aren’t marked with temperature ratings. Section 110.14 (C)(1)(b)(2) permits a 75°C rating under specified conditions. Because the lowest rating is 75° C, the ampacity from the 75°C column of Table 310.16 shall not be exceeded. The ampacity from the 75°C column is 255A. That is the maximum value that this conductor could be used for in this question.
Note: Sec. 110.14(C) has been revised in four consecutive Code cycles (1993, 1996, 1999, and 2002) and will be again in 2005.
Owen is the owner and president of National Code Seminars and the holder of master electrician certifications in 46 states.