Thinkstock/Comstock/Getty Images
A circuit breaker’s breaking capacity is the maximum fault current it can interrupt safely. Below this interrupting rating, the breaker will clear a fault without damage, protecting the rest of the system. Above it, the current may damage components or create hazardous arcs. Residential breakers typically list their breaking capacity on the label.
Multiply the number of phase sets (or electrical windings) by the voltage each offers. For instance, a three‑phase 520‑volt transformer has a total system voltage of 3 × 520 V = 1,560 V.
Divide the transformer's VA rating by the total system voltage. In the example, 78,000 VA ÷ 1,560 V = 50 A.
Divide the current by the device’s impedance (expressed as a decimal). With a 5% impedance, 50 A ÷ 0.05 = 1,000 A. This value is the breaker’s breaking capacity in amperes.
By following these steps, you can accurately determine the breaking capacity of any circuit breaker based on the system’s electrical parameters.
