Free Online Tool
The Formula and How It Works
This calculator uses a simplified "point-to-point" calculation method, which is a common approach for basic electrical systems. The core principle is Ohm's Law, where the short-circuit current ($I_{sc}$) is found by dividing the system voltage ($V$) by the total system impedance ($Z_{total}$).
$$I_{sc} = \frac{V_{LL}}{\sqrt{3} \times Z_{total}}$$
Where the total impedance is the sum of all impedances in the circuit:
$$Z_{total} = \sqrt{R_{total}^{2} + X_{total}^{2}}$$
Definitions of Key Inputs
- System Voltage (V): The line-to-line voltage of your electrical system.
- Transformer kVA: The transformer's nameplate rating in kilo-Volt-Amperes.
- Transformer % Impedance (%Z): The transformer's nameplate percentage impedance, which is a key factor in limiting fault current.
- Cable Length (ft): The length of the cable run from the transformer to the point of the fault.
- Cable Type/Gauge: The size and type of the conductor, which determines its resistance and reactance.
- Source Fault Current (kA): The available short-circuit current from the utility source, if known. If not provided, the calculator makes a conservative assumption that the source impedance is negligible.
How to Use This Calculator
- Enter the system voltage, transformer kVA, and transformer % impedance from your equipment's nameplate.
- Measure the length of the cable run from the transformer to the point where you need to know the fault current.
- Select the correct cable type and size from the dropdown menu.
- Click the "Calculate" button to see the result in kiloamperes (kA).
Limitations & Assumptions
This simplified calculator is a great tool for a quick estimate, but it has important limitations. It uses the point-to-point method, which is suitable for simple radial systems. It does not account for motor contributions, parallel paths, or complex network topologies. For detailed, code-compliant studies, always consult professional electrical engineering software and a licensed professional.
Calculation scope, worked check, and sources
This calculator estimates the initial symmetrical three-phase RMS short-circuit current at the selected load point. It is a screening calculation, not a protection-coordination or arc-flash study.
The transformer impedance is calculated in ohms as Ztr = (uk/100) × VLL²/(S × 1000). If an upstream fault current is entered, it must be the three-phase symmetrical value referred to the same voltage level.
The cable model uses one-way length and approximate copper-conductor R/X values at 75 °C. Transformer and source impedances are treated as predominantly reactive; actual manufacturer and installation data take priority.
Worked check: 480 V, 500 kVA, 5.75% impedance, 50 ft of 4/0 Cu and an infinite upstream source gives about 9.45 kA at the load point (about 10.46 kA at the transformer terminals before the cable).
It does not implement the complete IEC 60909-0:2026 method: voltage factors, correction factors, motor/generator contributions, unbalanced faults, peak current and minimum-current cases are outside this quick model.
Authoritative references: IEC 60909-0:2026; Schneider Electric – Impedanzmethode; Eaton – point-to-point short-circuit calculations.
