PRELIMINARY ENGINEERING TOOL
Transformer terminal fault-current estimate
A narrow three-phase, transformer-only arithmetic shortcut—not a protection or equipment-selection result.
Inputs
Calculation steps
I_FL = 100000 VA / (√3 × 415 V) = 139.12 A; I_fault = 139.12 A / (5% / 100) = 2782.41 A
Formula ID: TRANSFORMER_3PH_TERMINAL_FAULT_INFINITE_SOURCE_V1
- apparentPowerVA
- 100000
- secondaryVoltageV
- 415
- impedancePu
- 0.05
- fullLoadCurrentA
- 139.12055
- terminalFaultCurrentA
- 2782.4109
Intermediate values are retained without display rounding in the calculation.
Assumptions and references
Assumes a three-phase bolted fault at the low-voltage secondary terminals and an unlimited upstream source. The entered %Z must come from the actual transformer record. Source impedance, voltage variation, impedance tolerance, motors, downstream conductors and other fault types are excluded.
Understand percentage impedance →UNDERSTAND THE TOOL
What this calculator does
Show how an entered transformer impedance changes a simplified symmetrical RMS fault-current estimate at the low-voltage secondary terminals.
How to use it
- Use the actual three-phase transformer nameplate kVA and impedance.
- Enter the low-voltage secondary line-to-line voltage.
- Review the full-load current and transformer-only terminal estimate.
- Obtain a complete project fault study before selecting any equipment.
Fields and meanings
- Transformer rating
- Three-phase apparent-power nameplate rating in kVA.
- Secondary line voltage
- Low-voltage line-to-line basis in volts.
- Nameplate impedance
- Actual transformer percentage impedance; no typical value is assumed.