Enter your transformer specs — kVA rating, primary and secondary voltages, and impedance (%Z) — and this Short Circuit Current Calculator gives you the secondary fault current broken down into 3-phase, line-to-line, and line-to-ground fault currents, plus the transformer multiplier used in the calculation.
Secondary Fault Current
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3-Phase Fault Current
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Line-to-Line Fault Current
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Line-to-Ground Fault Current
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Transformer Multiplier
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Short circuit current (fault current) is the maximum current that flows through a circuit during a fault condition. It's critical for selecting proper protective devices, ensuring safety, and meeting electrical codes like NEC requirements.
Transformer impedance (%Z) is usually marked on the transformer nameplate. Typical values range from 2-8% for distribution transformers. If not available, consult the manufacturer's specifications or use industry standard values for similar transformers.
3-phase faults typically produce the highest fault currents, while single-phase faults are more common. The calculator accounts for different fault types including line-to-line and line-to-ground faults, each producing different current levels.
The primary fault current is provided by your utility company or can be calculated from the upstream source impedance. Contact your electrical utility for the available fault current at your service point.
The X/R ratio is the relationship between reactance and resistance in the circuit. It affects the asymmetrical fault current and DC component. Typical values are 10-15 for transformers and 15-20 for motors.
These calculations provide good estimates for planning purposes but may vary from actual conditions due to system variables, temperature effects, and equipment tolerances. Always verify with detailed engineering analysis for critical applications.
Circuit breakers and fuses should have interrupting ratings that exceed the calculated fault current. Add a safety margin and consider future system growth. Consult NEC Article 110.9 for specific requirements.
This calculator focuses on transformer fault contributions. Motors also contribute to fault current but with different characteristics and decay rates. For comprehensive analysis including motor contributions, additional calculations are needed.