Short Circuit Current Calculator Formula
Understand the math behind the short circuit current calculator. Each variable explained with a worked example.
Formulas Used
Short Circuit Current
isc = fla_secondary / (transformer_z_pct / 100)Short Circuit Current
isc_ka = fla_secondary / (transformer_z_pct / 100) / 1000Full Load Amps
fla = fla_secondaryAvailable Short Circuit kVA
short_circuit_kva = transformer_kva / (transformer_z_pct / 100)Variables
| Variable | Description | Default |
|---|---|---|
transformer_kva | Transformer Rating(kVA) | 500 |
transformer_z_pct | Transformer Impedance(%) | 5 |
secondary_voltage | Secondary Voltage(V) | 480 |
fla_secondary | Derived value= transformer_kva * 1000 / (sqrt(3) * secondary_voltage) | calculated |
How It Works
Estimating Short Circuit Current
The maximum short-circuit current at a transformer secondary determines the required interrupting capacity of protective devices.
Formula
I_sc = I_fla / (Z% / 100)
Where I_fla is the transformer full-load secondary current and Z% is the transformer impedance.
I_fla (3-phase) = kVA x 1000 / (sqrt(3) x V_secondary)
This gives the maximum available fault current at the transformer terminals. Downstream cable impedance reduces the fault current at the load.
Worked Example
500 kVA transformer, 5% impedance, 480 V secondary.
- 01FLA: 500,000 / (1.732 x 480) = 601.4 A
- 02I_sc: 601.4 / 0.05 = 12,029 A = 12.0 kA
- 03Available SC kVA: 500 / 0.05 = 10,000 kVA
Frequently Asked Questions
Why is short-circuit current important?
All protective devices (breakers, fuses, switches) must be rated to safely interrupt the available fault current. Under-rated devices can explode.
Does this include utility contribution?
This calculation assumes the utility source has infinite capacity (worst case at transformer). In practice, utility impedance slightly reduces the fault current.
How does cable length reduce fault current?
Cable impedance adds to the transformer impedance, reducing the fault current at downstream points. Longer cables mean lower fault currents.
Ready to run the numbers?
Open Short Circuit Current Calculator