Overcurrent Protection and Short-Circuit Coordination
Device Types and Ratings
Know the difference between fuses and circuit breakers, standard ampere ratings, and the rule that the next higher standard rating may be used when the calculated load does not exceed it (with limits for certain loads). A frequent trap is selecting a breaker larger than the conductor ampacity allows.
Conductor Protection
Conductors must be protected against overcurrent in accordance with their ampacity after adjustment and correction. Remember to apply ambient temperature correction and conductor-count adjustment before comparing to the breaker. Another trap: using the 90°C column for the final ampacity when terminations are rated 75°C.
Short-Circuit Coordination
Coordination ensures the device nearest the fault opens first. Series-rated systems and current-limiting devices appear in questions. Understand interrupting rating-the device must have an interrupting rating at least equal to the available fault current.
Motor and HVAC Exceptions
Motor circuits permit overload protection separate from branch-circuit short-circuit protection, and the branch-circuit device may be sized larger than the conductor ampacity per the motor tables. Do not apply general rules to motor circuits without checking the motor article.
Worked Example
A conductor with adjusted ampacity of 68 A on a 75°C termination may be protected by a 70 A breaker if the load does not exceed 68 A. If the answer offers 80 A, verify the load first-80 A is only acceptable if the load does not exceed the next standard rating and no other limit applies.