💥 ELECTRICAL

Free Online Short Circuit Calculator

Calculate three-phase and single-phase fault current, fault MVA and switchgear breaking capacity from transformer rating and impedance. IEC 60909 basis. Free, no sign-up.

📐 Standard: IS 13234 / IEC 60909
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Short Circuit Calculator Calculator
Reference: IS 13234 / IEC 60909
💥 ELECTRICAL
Calculate three-phase and single-phase fault current, fault MVA and switchgear breaking capacity from transformer rating and impedance. IEC 60909 basis. Free, no sign-up.
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ℹ️ About This Calculator

Switchgear and cables must withstand and safely interrupt the maximum fault current that can flow during a short circuit. This calculator estimates the three-phase and single-phase fault current, the fault level in MVA and the required switchgear breaking capacity from the transformer rating, voltage and impedance, per IS 13234 / IEC 60909.

Fault current is governed mainly by the transformer's percentage impedance (%Z) - a lower %Z gives a higher fault current. The result sets the breaking capacity (kA) of breakers and the short-circuit withstand of busbars and cables. Motor contribution and source impedance raise the real value, so switchgear is chosen with margin above the calculated figure.

📐 Short Circuit Formula (IEC 60909)

IS 13234 / IEC 60909

Isc = In × 100 / %Z
In = kVA / (√3 × V)
Fault MVA = kVA × 100 / %Z

Isc = symmetrical fault current, A
In = transformer full-load current, A
%Z = transformer percentage impedance
kVA = transformer rating
V = line voltage, V

🧮 Worked Example

Example: A 1000 kVA, 415 V transformer with 5% impedance has a full-load current In = 1,000,000 / (1.732 × 415) ≈ 1391 A. Fault current Isc = 1391 × 100 / 5 ≈ 27.8 kA, and fault level = 1000 × 100 / 5 = 20 MVA. Switchgear is then selected at the next standard rating (e.g. 36 kA) to cover margin and motor contribution.

📊 Transformer Impedance & Fault Levels (IEC 60909)

Fault current scales inversely with transformer percentage impedance. Typical distribution-transformer impedances:

Transformer (kVA)Typical %ZApprox. fault current at 415 V
5004.5%~15 kA
10005.0%~28 kA
16006.25%~36 kA
20006.25%~45 kA

Selecting Switchgear

Choose the breaker breaking capacity (Icu) above the calculated fault current, and the busbar and cable short-time withstand (Icw) for the fault duration. Add motor contribution (motors feed the fault for the first few cycles) and, for the utility source impedance, use the network operator's fault level. Standard MCCB/ACB ratings are 25, 36, 50, 65 kA.

Simplified estimate ignoring source and cable impedance - a full study per IEC 60909 gives the exact figure.

❓ Frequently Asked Questions

How do I calculate short circuit current from a transformer? +
Divide the transformer full-load current by its percentage impedance (as a fraction): Isc = In × 100 / %Z. A 1000 kVA, 415 V transformer at 5% impedance gives about 28 kA. Add source and motor contribution for the full value.
What is fault MVA? +
Fault MVA (or fault level) is the apparent power that would flow into a bolted three-phase fault: transformer kVA × 100 / %Z. It is a convenient way to express system strength and to select switchgear at a busbar.
How does transformer impedance affect fault current? +
Fault current is inversely proportional to impedance - halving %Z roughly doubles the fault current. A lower-impedance transformer gives better voltage regulation but a higher prospective fault current, demanding higher-rated switchgear.
How do I select switchgear breaking capacity? +
Pick a breaking capacity (Icu) above the calculated fault current with margin for motor and source contribution, then confirm the busbar and cable short-time withstand for the fault duration. Standard ratings are 25, 36, 50 and 65 kA.
What is the difference between Icu and Ics? +
Icu is the ultimate breaking capacity - the maximum fault the breaker can interrupt once. Ics is the service breaking capacity - the fault it can interrupt repeatedly and remain in service. Both must exceed the prospective fault current.
Do motors add to fault current? +
Yes. During the first few cycles, running motors act as generators and feed current into the fault, typically adding 4-6 times their full-load current. Large motor loads must be included when selecting switchgear.
What standard covers short circuit calculations? +
IEC 60909 (and IS 13234) is the international method for calculating short-circuit currents in three-phase AC systems. It defines symmetrical and peak values and the treatment of source, transformer, cable and motor contributions.
Why must cables withstand short circuit current? +
During a fault, cables carry the full fault current until the protection clears it, heating rapidly. The conductor must survive that energy (I²t) without damage, which sets a minimum size - checked with the adiabatic equation alongside normal current rating.

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⚠️ Disclaimer: For preliminary engineering design only. Verify all results with a licensed engineer before use. Full disclaimer →