Overcurrent Protection
Overcurrent devices protect conductors from overheating caused by overloads and short circuits. They are selected on three independent attributes: the rating (trip or melting current), the interrupting rating (the fault current they can safely clear), and the time-current characteristic (how fast they respond).
Concepts to master
- The device rating must not exceed the conductor ampacity, except where the Code permits the next standard size up when the calculated load does not match a standard rating.
- The interrupting rating must equal or exceed the available fault current at the point of installation; series-rated combinations allow a lower-rated breaker downstream only when the combination is listed.
- Continuous loads are taken at 125 percent for sizing the overcurrent device.
- Overload protection and short-circuit protection are different functions; motors have separate overload devices and branch-circuit short-circuit protection.
- Coordination ensures the downstream device opens first, limiting the outage.
- Tap rules allow reduced conductor sizes only under specific length, ampacity, and protection conditions.
Traps
Confusing trip rating with interrupting rating, ignoring the next-size-up conditions, applying a tap rule without checking the length limit, or assuming a fuse and breaker with the same number behave identically. Also watch for panels where the bus rating, not the main breaker, is the limiting value.
Worked example
A load calculates to 68 A continuous: 68 × 1.25 = 85 A, so an 85 A or next standard size device is needed, with conductors rated at least that value.