About the Short Circuit Current Estimator
What is the Short Circuit Current Estimator?
Circuit breakers must be able to interrupt the largest current a fault can cause. This short circuit current estimator works it out from the transformer's kVA rating and percentage impedance, and adds the resistance of the cable to the fault point.
The result guides the breaking capacity (kA rating) of breakers and switchgear.
How to use the Short Circuit Current Estimator
- Transformer Rating (kVA) (sample: 500)
- Transformer Impedance (%) (sample: 4.5)
- Secondary Line Voltage (V) (sample: 415)
- Cable Length to the Fault Point (m, 0 at the transformer) (sample: 0)
- Cable Size (mm² per phase) (sample: 95 mm²)
- Conductor (sample: Copper)
- Press Estimate Fault Current (results also refresh as you type). The result card shows the Prospective Short-Circuit Current, along with At the transformer terminals, Transformer impedance, Cable resistance and Breaking capacity needed.
Formula used
Isc = V ÷ (√3 × Z); Z_transformer = V² ÷ kVA × Z%
- Z = √(Z_transformer² + R_cable²)
- At the terminals = Isc = kVA × 1,000 ÷ (√3 × V × Z%)
Assumes an infinite upstream fault level, so the estimate errs on the safe (higher) side at the transformer.
Worked example
These are the sample values already filled in the calculator above, and the result it returns for them:
| Input | Sample value |
|---|---|
| Transformer Rating (kVA) | 500 |
| Transformer Impedance (%) | 4.5 |
| Secondary Line Voltage (V) | 415 |
| Cable Length to the Fault Point (m, 0 at the transformer) | 0 |
| Cable Size (mm² per phase) | 95 mm² |
| Conductor | Copper |
| Result | Value |
|---|---|
| Prospective Short-Circuit Current | 15.46 kA |
| At the transformer terminals | 15.46 kA |
| Transformer impedance | 15.5 mΩ |
| Cable resistance | 0 mΩ |
| Breaking capacity needed | Choose breakers rated above 16 kA |
| Assumption | Infinite upstream fault level – the real value will be slightly lower |
Tips for using the Short Circuit Current Estimator
- Choose breakers with a breaking capacity above the prospective fault current.
- Fault current falls quickly along small cables – downstream breakers can have lower kA ratings.