Fault Current Calculator
Enter system voltage, transformer MVA, and percent impedance to calculate three-phase and single-line-to-ground fault currents instantly.
System Parameters
Common: 0.48 kV, 4.16 kV, 13.8 kV, 34.5 kV
From nameplate. Typical: 2–6% (distribution), 6–10% (power).
Leave blank for infinite bus (maximum fault current).
Simplified One-Line Diagram
Results
Enter parameters and click Calculate
Three-Phase Fault Current
—
Symmetrical, bolted
SLG Fault Current
—
Single-line-to-ground
Base Current (Ibase)
Base Impedance (Zbase)
Transformer Z (per-unit)
Total System Z (per-unit)
Results represent symmetrical RMS fault current. For asymmetrical (peak) fault current multiply by the asymmetry factor (typically 1.6–2.7 depending on X/R ratio and protective device timing). IEEE Std 551 provides detailed X/R correction factors.
Formula Breakdown
Common Transformer Presets
| Rating | Voltage | %Z | Application |
|---|---|---|---|
| 500 kVA | 480 V | 3.5% | Small commercial |
| 10 MVA | 13.8 kV | 5.75% | Industrial substation |
| 30 MVA | 34.5 kV | 7.0% | Distribution feeder |
| 100 MVA | 115 kV | 9.0% | Transmission substation |
Click a row to load its values.
Summary
Enter system voltage, transformer MVA, and percent impedance to calculate three-phase and single-line-to-ground fault currents instantly.
How it works
- Enter the rated line-to-line voltage at the fault point in kilovolts (kV).
- Enter the transformer MVA (or kVA) rating and select the unit.
- Enter the transformer percent impedance (%Z) from the nameplate data.
- Optionally enter a utility source impedance (in MVA short-circuit capacity) to combine with the transformer impedance.
- Click Calculate to see the three-phase and SLG fault currents along with base values.
- Review the formula breakdown and use the results in breaker or fuse sizing studies.
Use cases
- Sizing circuit breakers and fuses for the correct interrupting rating.
- Protective relay coordination studies in industrial and utility systems.
- Verifying that switchgear interrupting capacity exceeds available fault current.
- Determining conductor short-circuit withstand ratings.
- Teaching power systems fundamentals with per-unit analysis.
- Checking NEC / IEEE 1584 arc flash boundary calculations.
- Validating transformer nameplate data against system design requirements.
- Preliminary fault analysis before full symmetrical component software runs.