Short Circuit Current Formula
The "infinite bus" assumption calculates the maximum possible short circuit current available at the secondary terminals of a transformer, assuming an infinite source fault capacity upstream. It represents the absolute worst-case scenario.
Frequently Asked Questions
First calculate the full load current (FLA). Then, divide 100 by the precise percent impedance (%Z) of the transformer. Multiply the FLA by this result to get the theoretical maximum short circuit current (Isc).
Short circuit current (Isc) is the extremely high current that flows when a fault creates a low-impedance path between conductors. It can reach 20,000–100,000 Amperes — compared to normal operating currents of hundreds of amperes. Without rapid fault clearance, this causes: conductor melting, arc flash (temperatures >20,000°C), fire, equipment destruction, and electrocution hazards.
IEC 60909 is the international standard for calculating short circuit currents in three-phase AC power systems up to 550kV. It defines an equivalent voltage source method using a voltage factor c (1.0–1.1 × nominal voltage) and calculates: initial symmetrical short circuit current (Ik''); peak current (ip = κ × √2 × Ik''); breaking current (Ib); and thermal equivalent current (Ith) for cable withstand.
Symmetrical short circuit current (Ik'', RMS value) is the AC component only — used for switchgear breaking capacity. Peak current (ip) includes the DC offset present in the first cycle after fault inception, and can be 1.5–2.5× the symmetrical value (ip = κ × √2 × Ik''). Switchgear making (closing) and busbar ratings must withstand ip; breaking ratings are based on Ik'' or Ib.
Short circuit calculations are required by IEC 60364, NEC, and all electrical installation codes to: select circuit breakers with adequate breaking capacity; verify switchgear and busbar withstand ratings; size protective device settings (overcurrent relay coordination); design earthing/grounding systems; verify cable thermal withstand; and protect personnel through proper arc flash hazard analysis (IEEE 1584).
Transformer impedance (%Z, typically 4–6% for distribution transformers) limits the maximum fault current at the secondary terminals. Higher %Z = lower fault current but also higher voltage drop under load. The prospective short circuit current at transformer secondary: Ipsc = Irated_secondary / (%Z/100). This is the starting point for all downstream short circuit calculations.