Three-Phase Fault Current Calculator
Prospective short-circuit current at a transformer's terminals, with the peak current and the fault level.
Results
What this tool does
The transformer's impedance voltage is what limits the fault: a 4 % transformer lets through twenty-five times its rated current, a 6 % one about seventeen. That is why a bigger transformer with the same impedance makes the downstream switchgear more expensive, and why impedance is chosen as much for fault limitation as for voltage regulation.
Formula
Icc = Sn ÷ (√3 × U × ucc)
Variables
| Symbol | Meaning | Unit |
|---|---|---|
sn | Transformer rating | kVA |
uu | Voltage | V |
uk | Impedance voltage | % |
kp | Peak factor | — |
IK | Fault current | kA |
IP | Fault current | kA |
IN | Current | A |
ZT | Impedance | mΩ |
SK | Apparent power | MVA |
Worked example
- Transformer rating630 kVA
- Voltage400 V
- Impedance voltage4 %
- Peak factor2.2
- Fault current22.7332 kA
- Fault current50.0130 kA
- Current909.3 A
- Impedance10.1587 mΩ
- Apparent power15.7500 MVA
Limitations
- Electrical installations are governed by national wiring rules. Cable sizing also depends on installation method, grouping, ambient temperature and protection devices, which this calculator does not evaluate.
- For work that must comply with a standard or be signed off, check the result against the applicable code and have it reviewed by a qualified engineer.
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
Frequently asked questions
Is this the figure my breaker has to be rated for?
It is the starting point, not the answer. This calculation assumes an infinite supply behind the transformer and ignores the impedance of the cable between the transformer and the board, both of which pull the real figure in opposite directions — the network upstream reduces it a little, the cable reduces it a lot. Use it to see the order of magnitude at the transformer terminals; a device selection needs the full study to the applicable standard, with the actual network impedance.