Short Circuit Current & Fault Level Calculator
COMPUTE 3-PHASE SHORT CIRCUIT CURRENT (KA) & RECOMMENDED BREAKER ICU RATINGShort Circuit Fault Level Results
Short Circuit Formulas
Isc = In / (%Z / 100) where In = S_trafo / (√3 × V_sec)
Electrical circuit breakers and switchgear must have a ultimate breaking capacity (Icu) higher than the maximum prospective short-circuit current to safely extinguish arcs during a fault.
Frequently Asked Questions
What is %Z (uk) impedance voltage of a transformer?
%Z (or uk) is the percentage of nominal primary voltage required to circulate nominal current through a short-circuited secondary winding. Standard distribution transformers (250kVA – 2500kVA) typically have uk values between 4% and 6%.
Why does short circuit current decrease over cable length?
Cables add electrical resistance (R) and reactance (X) to the fault loop impedance. Longer or thinner cables increase total loop impedance, reducing the fault current at the far end of the circuit.
What is the difference between Icu and Ics for circuit breakers?
Icu (Ultimate Breaking Capacity) is the maximum short-circuit current the breaker can interrupt twice without causing danger. Ics (Service Breaking Capacity) is the current it can interrupt three times and still remain fully operational without performance degradation.