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.
transformer_kva = 500transformer_z_pct = 5secondary_voltage = 480
- 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
Ready to run the numbers?
Open Short Circuit Current Calculator