Overcurrent Protection and Short-Circuit/Coordination Concepts
Devices and ratings
Overcurrent protective devices include fuses, molded-case circuit breakers, and supplementary protectors. Every device has an ampacity rating, a voltage rating, and an interrupting rating (AIC). The interrupting rating must equal or exceed the available fault current at the terminals. Breakers are thermal-magnetic (inverse-time) or electronic; fuses are time-delay or fast-acting. Short-circuit protection clears faults; overload protection protects conductors and motors from long-term heating.
The next-size-up rule
Section 240.4(B) permits the next standard rating (Table 240.6) when conductor ampacity does not match a standard size, provided the ampacity is 800 A or less, the next size does not exceed 800 A, and the circuit is not a multioutlet receptacle branch circuit. Section 240.4(D) caps small conductors: 14 AWG at 15 A, 12 AWG at 20 A, 10 AWG at 30 A for most applications.
Coordination and series ratings
Selective coordination means only the device nearest the fault opens. Series-rated combinations use a current-limiting device ahead of a lower-rated breaker, allowing the downstream device to have a lower interrupting rating than the available fault current. Motor circuits separate overload (sized to motor nameplate) from branch-circuit short-circuit and ground-fault protection (Table 430.52).
Worked example
A conductor with 55 A ampacity supplies a 60 A load. Table 240.6 includes 60 A, so a 60 A breaker is permitted under 240.4(B) if the ampacity is not over 800 A and the circuit is not a multioutlet receptacle circuit. If the conductor were 14 AWG, 240.4(D) would cap the breaker at 15 A regardless.