Principle: Sizing a circuit breaker (or fuse/overcurrent protective device) involves determining the appropriate current rating for normal operation and ensuring the device can interrupt fault currents. The breaker must be chosen to carry the load safely (without nuisance tripping) and to trip quickly under overload or short-circuit conditions before damage occurs.
There are two primary aspects:
In) should be at or above the circuit's design current (Ib) but not so high as to leave the circuit unprotected. Typically, Ib ≤ In ≤ Iz, where Iz is the cable’s continuous capacity. This ensures the breaker will carry the load but trip if current exceeds cable capacity.
IC or AIC in kA) must exceed the maximum prospective fault current at its installation point. If the fault current is higher than what the breaker can handle, it could fail to clear the fault, which is dangerous. In some cases, a series combination (backup protection) is used, but generally each device must stand alone at its location.
Standards give rules of thumb. For instance, in BS 7671 (UK): Design Current (Ib) ≤ Device Rating (In) ≤ Cable Capacity (Iz). For example, if a circuit draws Ib = 22 A, one might choose a 25 A breaker (In = 25 A) and ensure the cable can handle at least 25 A (with an Iz of, say, 30 A to provide margin). The device's time-current curve should protect the cable from overheating – often checked by ensuring I2 ≤ 1.45 Iz (where I2 is the current that causes the breaker to trip in a specified time, e.g., 1 hour).
In the NEC (USA), branch circuit breakers are often sized at 125% of the continuous load plus 100% of any non-continuous load. “Continuous” means operation for 3 hours or more. Thus, a 20 A continuous load would require a breaker rated for about 25 A (or the next standard size, such as 30 A, since 25 A is not common in NEC standards). This accounts for heat buildup – while a breaker can carry 100% continuously, the code adds a 25% margin for continuous loads to prevent nuisance tripping due to thermal accumulation. Similarly, conductors are sized at 125% for continuous loads, aligning both cable and breaker sizing.
After ensuring proper overload sizing, the breaker's interrupting rating must be at least equal to the available fault current at its point of installation. For example, a typical residential breaker might have a 10 kA IC. In a home where the service fault current is about 5 kA, that is acceptable. However, in an industrial plant with fault currents of 50 kA, high-rupturing-capacity breakers (often rated at 65 kA or 100 kA) are required. If the fault level exceeds what a single breaker can handle, designers may use a current-limiting fuse or an upstream breaker to reduce the fault seen by downstream devices (this is known as series rating or let-through energy coordination).
Additionally, breakers must be selected for the correct voltage rating. For instance, a breaker used on a 480 V system must be rated for 480 V; using a 240 V-rated device on a 480 V system could lead to arc flashover even if the current remains within range.
In, while D trips at around 10×In) to accommodate varying inrush loads.
Ib ≤ In ≤ Iz and require that the device's breaking capacity meets the prospective fault current as specified by standards such as BS EN 60898 or IEC 60947-2. They also ensure that the let-through energy does not exceed what the cable can handle.
In, Icu (ultimate breaking capacity), and Ics (service breaking capacity). When designing per IEC, the breaker’s short-circuit capacity must meet or exceed the fault level, with Annex A of IEC 60947-2 detailing discrimination techniques for selectivity and backup.
In summary, circuit breaker sizing marries the normal operating current of a circuit with its abnormal fault current conditions. The breaker must carry the load during normal operation while reliably interrupting fault currents. By following code formulas and utilizing available fault data, engineers ensure that In is appropriate and that the breaker's interrupting capacity is adequate—thereby protecting both equipment and personnel from overcurrent conditions.