Bolted Fault Current Calculator

Fast fault calculations for panels, switchgear, and feeders. Adjust X/R, motors, and cable runs easily. Download CSV or PDF results for your project files.

Calculator Inputs

Choose a method, enter values, then calculate to see results above.

Pick based on available site data.
Example: 0.415 for 415 V systems.
Use nameplate rating for accuracy.
Typical range: 4% to 8%.
Used to estimate peak making current.
Optional: add a simple percentage allowance.
Used for I²t estimate (e.g., 0.1 s).
Cable / Feeder Impedance
Per-phase values

Enter R and X per unit length. The calculator multiplies by length.

Match your cable data source.
Enter in the same basis as selected.
Typical LV cables: 0.03–0.2 ohm/km.
Often 0.06–0.15 ohm/km for LV.
Tip
If you only have cable impedance magnitude, estimate R and X using an X/R value.
Custom Thevenin Impedance
Ohms per phase

Use when a short-circuit study provides R and X directly.

Note
In this mode, transformer and cable fields are ignored.
Clear
Example Data Table

Use this as a starting point and replace values with your project inputs.

Case Method Voltage (kV) kVA %Z X/R Cable Sym. Fault (kA) Peak (kA)
1 Transformer only 0.415 1000 5.75 10 — ≈ 24.1 ≈ 63.3
2 Transformer + cable 0.415 1000 5.75 10 0.05 km, R=0.08, X=0.09 ≈ 22.4 ≈ 59.0

Example outputs are approximate and depend on rounding.

Formula Used
1) Full-load current
IFL = (kVA × 1000) / (√3 × VLL)
VLL in volts, 3-phase systems.
2) Transformer impedance in ohms
Zbase = VLL2 / S
|Ztr| = Zbase × (Z% / 100)
S in VA (3-phase), |Z| per phase.
3) Split |Z| into R and X using X/R
R = |Z| / √(1 + (X/R)2)
X = (X/R) × R
4) Symmetrical bolted fault current
Ik = Vphase / |Ztotal|
Vphase = VLL / √3
Ztotal combines transformer and cable R+jX.
5) Peak making current (IEC-style approximation)
k = 1.02 + 0.98 × e(-3 × R/X)
ip = k × √2 × Ik
This is a simplified method for peak estimation.
How to Use This Calculator
  1. Select a method: transformer only, transformer plus cable, or custom Thevenin.
  2. Enter system voltage, transformer kVA, impedance, and X/R ratio.
  3. If using cable mode, input length and per-length R and X.
  4. Optionally add a motor contribution percentage and a duration for I²t.
  5. Click Calculate, then download CSV or PDF for records.

For equipment selection, always confirm with a qualified short-circuit study.

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Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.