Circuit Breaker Sizing Calculator (Fault)

The Circuit Breaker Sizing Calculator (Fault) determines the minimum circuit breaker trip rating and required interrupting capacity (AIC — Ampere Interrupting Capacity) based on load current and available fault current. Selecting a circuit breaker with inadequate trip rating or insufficient fault interrupting capacity is a serious safety violation that can result in catastrophic failure during a fault event.

Circuit breaker selection involves two independent but equally critical parameters. The trip rating (continuous current rating) must be equal to or greater than the maximum expected load current, adjusted for load type. For continuous loads (loads that operate for 3 hours or more continuously), NEC 210.20(A) requires the breaker be rated at 125% of the continuous current. The interrupting rating must equal or exceed the available fault current at the installation point — this is a non-negotiable safety requirement per NEC 110.9.

Standard MCCB (Molded Case Circuit Breaker) interrupting ratings in North America include: 10 kA, 14 kA, 18 kA, 22 kA, 25 kA, 35 kA, 42 kA, 65 kA, 85 kA, and 100 kA. The required AIC must be met or exceeded — using a 22 kA-rated breaker in a location with 25 kA available fault current is a code violation and safety hazard. The breaker may fail to interrupt the fault, resulting in a sustained arc that can cause fire, explosion, and severe injury.

When available fault current exceeds the breaker's interrupting rating, engineers have several options: series rating (placing a current-limiting fuse upstream to limit let-through to within the breaker's rating — permitted by UL 489 and NEC 240.86 under specific conditions), replacing the breaker with a higher-rated model, adding impedance upstream (transformer or cable) to reduce available fault current, or using current-limiting circuit breakers that limit fault current in the first half-cycle.

The arc flash hazard analysis (IEEE 1584) depends on available fault current — higher fault current at a given location results in higher arc flash incident energy. Selecting appropriate arc flash PPE categories and establishing flash protection boundaries requires accurate fault current data at every bus in the system. Undersized or improperly selected breakers compromise both personnel safety and arc flash analysis validity.

For motor circuits, NEC Article 430 permits breaker trip ratings up to 250% of motor FLA (for instantaneous trip breakers) to allow for starting current. The maximum trip setting is governed by the motor's service factor and the type of breaker (thermal-magnetic MCCB versus electronic trip or motor circuit protector). This calculator uses a simpler load-type multiplier for general guidance — always verify against NEC Article 430 tables for motor applications.