When to Use This Calculator
Use this calculator when you know the available fault MVA at a bus and want the corresponding balanced three-phase fault current. It is useful for breaker duty checks, transformer studies, and quick fault-level conversions.
Enter Fault Level
Formula Summary
Isc = Fault MVA × 1000 ÷ (√3 × kVLL)
The result is symmetrical RMS current in amperes for a balanced three-phase fault.
Example
For 500 MVA available at 13.8 kV:
Isc = 500 × 1000 ÷ (1.732 × 13.8) = 20.92 kA
Limitations
This conversion does not determine asymmetrical peak current, X/R ratio, DC offset, interrupting duty, or contributions over time from motors and generators.
Equipment duty evaluation requires the correct study method, fault type, operating configuration, and applicable standard.
Common Mistakes
- Using line-to-neutral voltage instead of line-to-line voltage.
- Mixing kV, volts, kA, and amps without checking units.
- Using peak or asymmetrical current where symmetrical RMS current is intended.
- Treating the result as a complete breaker-duty calculation.
Reference Notes
This is a standard balanced three-phase fault-level conversion. It does not replace a short-circuit study, equipment-duty evaluation, or an adopted standard's required calculation method.