Transformer fault current is a critical parameter in power system protection and design. It represents the short-circuit current that can flow through the transformer under fault conditions — such as a phase-to-phase or phase-to-ground fault.
The amount of fault current available is influenced by the transformer’s rated power (kVA), the system voltage, and the transformer’s impedance, which is typically expressed as a percentage value from nameplate data.
Base Current (Ibase) = (Transformer kVA × 1000) / (√3 × Voltage)
Fault Current = Ibase × (100 / % Impedance)
This means that a transformer with a lower impedance allows more current to flow under fault conditions, while a higher impedance transformer limits the available fault current.
For example, a 100 kVA transformer at 400 V with 5% impedance would produce a fault current of approximately:
Fault Current = (100,000 / (√3 × 400)) × (100 / 5) = ~2,887 A
Understanding and calculating fault currents is essential for:
Engineers often use fault current calculations to simulate worst-case scenarios and assess whether protective devices will operate correctly and safely under fault conditions.
Impedance also affects voltage drop during normal operation. A higher impedance transformer will have more voltage drop under load. The percentage impedance value is not only crucial for fault current estimation but also affects transformer performance and power quality.
In practical applications, fault current is further influenced by the upstream network impedance, cable lengths, and connection configurations (delta or wye), but transformer impedance remains the primary local determinant.
This calculator provides a reliable estimate of transformer fault current using simplified but widely accepted engineering methods.