Short Circuit Current Calculator Formula
Understand the math behind the short circuit current calculator. Each variable explained with a worked example.
Formulas Used
Full Load Current
fla = full_load_currentProspective Short-Circuit Current
isc = full_load_current / (impedance_pct / 100)Short-Circuit Current
isc_ka = full_load_current / (impedance_pct / 100) / 1000Variables
| Variable | Description | Default |
|---|---|---|
transformer_kva | Transformer Rating(kVA) | 500 |
voltage | Secondary Voltage(V) | 400 |
impedance_pct | Transformer Impedance(%) | 5 |
full_load_current | Derived value= transformer_kva * 1000 / (sqrt(3) * voltage) | calculated |
How It Works
Transformer Short-Circuit Current
The prospective short-circuit current at the transformer secondary is limited by the transformer impedance.
Formulas
I_FL = kVA x 1000 / (sqrt(3) x V) (full load current)
I_SC = I_FL / (Z%/100) (short-circuit current)
where Z% is the transformer impedance expressed as a percentage. This is the maximum fault current available at the transformer terminals, ignoring cable impedance downstream.
Worked Example
A 500 kVA transformer, 400 V secondary, 5% impedance.
transformer_kva = 500voltage = 400impedance_pct = 5
- 01I_FL = 500,000 / (1.732 x 400) = 721.7 A
- 02I_SC = 721.7 / 0.05 = 14,434 A = 14.43 kA
- 03Switchgear must have a breaking capacity above 14.43 kA.
Ready to run the numbers?
Open Short Circuit Current Calculator