Overcurrent Protection and Short-Circuit Coordination
Protection basics
Overcurrent protective devices protect conductors and equipment from overloads and short circuits. Distinguish overload protection, which is often thermal, from short-circuit and ground-fault protection, which is magnetic or current-limiting. Know breaker and fuse trip curves, interrupting rating, series ratings, current limitation, and selective coordination. Conductors must have an ampacity at least equal to the load and be protected at or below their ampacity, with the next-standard-size rule permitted only under stated conditions.
Motor and tap rules
Motor branch circuits separate overload protection from short-circuit and ground-fault protection. Tap conductors have specific length and ampacity limits, and transformer secondary conductors follow their own rules. Series-rated combinations must be listed and marked; a breaker's interrupting rating can never be assumed from its frame size.
Traps
Questions often apply the next-size-up rule to small conductors, where it is restricted, or use the wrong standard size. Another trap is confusing a motor overload device with a branch-circuit short-circuit protective device. Do not assume that a higher interrupting rating is present just because a panel is labeled with a high available fault current.
Worked example
A conductor with 68 A ampacity and a 68 A load: the next standard breaker is 70 A, permitted only if the load does not exceed the conductor ampacity and the conditions for the next-size-up rule are met. If the load is continuous, first take 125%: 68 × 1.25 = 85 A, so the conductor must be larger. Rule of thumb: calculate load, then ampacity, then protection.