Advanced
Short Circuit Current Formula
Calculate available fault current at transformer secondary using kVA, voltage, and percent impedance.
Formulas
Isc = kVA × 1000 / (V × √3 × %Z/100)
Three-phase fault current at transformer secondary
Isc = kVA × 1000 / (V × %Z/100)
Single-phase fault current at transformer secondary
Variable Definitions
Isc=Short circuit (fault) current(amperes (A))
kVA=Transformer rating(kilovolt-amperes)
V=Secondary voltage (line-to-line for 3Φ)(volts (V))
%Z=Transformer percent impedance(%)
Worked Example
1000 kVA, 480V 3Φ transformer, 5.75% impedance. Find available fault current.
- Isc = (1000 × 1000) / (480 × 1.732 × 0.0575)
- Isc = 1,000,000 / (831.4 × 0.0575)
- Isc = 1,000,000 / 47.8 = 20,920A
Answer: 20,920A (≈21 kAIC required for downstream equipment)
When to Use
- •Determining AIC rating requirements for downstream equipment (NEC 110.9)
- •Arc flash hazard analysis input (IEEE 1584)
- •Selecting appropriately rated fuses and breakers
- •Field-marking available fault current per NEC 110.24
Standards & References
- NEC 110.9 — Interrupting Rating
- NEC 110.24 — Available Fault Current Marking
- IEEE 1584 — Arc Flash Hazard Calculations