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:
$$Z_{total} = Z_{source} + Z_{transformer} + Z_{cable}$$
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.
