When to Use This Calculator
Use this calculator for a quick transformer-terminal fault estimate when nameplate kVA, voltage, phase, and percent impedance are known.
Enter Transformer Data
Formula Summary
Three-phase rated A = kVA × 1000 ÷ (√3 × VLL)
Single-phase rated A = kVA × 1000 ÷ V
Terminal fault A = Rated A × 100 ÷ %Z
Fault MVA = Transformer MVA × 100 ÷ %Z
Example
A 1500 kVA, 480 V, three-phase transformer with 5.75% impedance has rated current of approximately 1804 A and an ideal terminal fault current of approximately 31.38 kA.
Important Limitations
The estimate assumes an infinite primary source and a fault directly at the transformer terminals. Real available current is reduced by source, conductor, bus, and upstream transformer impedance.
Use a proper short-circuit study for equipment ratings. Motor contribution, X/R ratio, tolerance, tap position, and system configuration can materially change duty.
Common Mistakes
- Assuming the primary source is actually infinite.
- Using voltage from one winding side with current from the other.
- Ignoring transformer impedance tolerance, taps, conductor impedance, and motor contribution.
- Using the result as a final equipment-duty value.
Reference Notes
This estimate uses standard transformer percent-impedance relationships. Equipment-duty decisions require a short-circuit study using the applicable method, source data, and equipment ratings.