Short-Circuit Current Calculator: engineer's guide
Prospective short-circuit current sets the breaking capacity you must select for every protective device downstream. It falls off with distance from the transformer as cable impedance adds up.
Open the Short-Circuit Current calculator →
What this calculator does
The calculator computes the three-phase symmetrical fault current at the point of interest using transformer full-load current, transformer percent impedance, and cumulative cable impedance to the fault point.
Formulas used
- Fault current at transformer secondary:
Isc = IFL × 100 / Z% - Fault current at remote point:
Isc,remote = VLL / (√3 × |Ztx + Zcable|) - Cable impedance:
Zcable = L × √(R2 + X2) per phase
Variables
- IFL = transformer full-load current, amps
- Z% = transformer nameplate percent impedance
- VLL = line-to-line voltage, volts
- R, X = cable resistance and reactance per unit length
Worked example
Scenario: 1000 kVA transformer, 415 V secondary, Z% = 6%. Fault at a panel 60 m away fed by 240 mm² Cu XLPE (R ≈ 0.098 mΩ/m, X ≈ 0.08 mΩ/m).
- IFL = 1,000,000 / (√3 × 415) ≈ 1,391 A
- Isc at transformer secondary = 1,391 × 100 / 6 ≈ 23,190 A
- Cable impedance = 60 × √(0.098² + 0.08²) × 10-3 ≈ 0.0076 Ω
- Transformer impedance referred to secondary ≈ 0.010 Ω. Total ≈ 0.018 Ω.
- Isc,remote ≈ 415 / (√3 × 0.018) ≈ 13,300 A. Select downstream breakers with breaking capacity ≥ 15 kA.
Common mistakes to avoid
- Using primary IFL to compute secondary fault current
- Ignoring source impedance from the utility upstream of the transformer
- Summing R and X arithmetically instead of vectorially
- Using nominal voltage instead of the actual secondary voltage at no load
Frequently asked questions
Do I need to consider motor contribution?
Yes for the first 3 to 5 cycles. Running motors feed the fault at 4 to 6 times their full-load current initially.
What breaking capacity should I specify?
The next standard rating above the calculated prospective fault current: 10, 15, 25, 36, 50, 65, 100 kA are typical steps.
Does the calculation change for earth faults?
Yes. Earth-fault current uses the zero-sequence loop impedance, which includes the earth return path. Loop-impedance testing on site verifies this.
Standards and references
IEC 60909, BS 7671 Appendix 3, IEEE 141 (Red Book)
Related tools on this site
Open the calculator that pairs with this guide: Short-Circuit Current Calculator.
Verification note
This guide is planning support only. Final design must be verified against the applicable project specification, authority requirements, manufacturer data, and the standards cited above. Consult a licensed engineer for construction-issue calculations.