⚡ Short-Circuit Current Calculator
Calculate the prospective short-circuit current from transformer kVA and impedance, then get the breaker rated current and breaking capacity — in IEC or NEC standard sizes.
⚙️ Transformer & System
📊 Result
For designers and contractors sizing LV switchgear and breakers against fault level.
How it is calculated
· Rated current: In = kVA ÷ (√3 × U).
· Short-circuit current: Isc = In ÷ (Z%/100).
· Peak current: ip ≈ 2.5 × Isc; breaker rating ≥ 1.25 × In and breaking capacity ≥ Isc.
Referenced standards
| Standard | Scope |
|---|---|
| IEC 60909 | Calculation of short-circuit currents in three-phase AC systems |
| IEC 60947-2 | Low-voltage circuit-breakers — rated breaking capacity |
| NEC 110.9 | Interrupting rating of overcurrent protective devices |
Worked example — 1000 kVA transformer → 24 kA fault
1. 1000 kVA, 0.4 kV, Z = 6% → In = 1000 ÷ (√3 × 0.4) = 1443 A.
2. Isc = 1443 ÷ 0.06 = 24.1 kA; peak ip ≈ 2.5 × 24.1 = 60 kA.
3. Breaker rating = 1600 A (IEC) with breaking capacity 25 kA.
4. Recommended LV cabinet: GGD-1600.
FAQ
How do I calculate short-circuit current?
Use IEC 60909: short-circuit current = voltage / (sqrt(3) x total impedance). For a 0.4 kV system, the Transformer impedance dominates. A 1000 kVA, 6% impedance transformer contributes about 24 kA at the LV bus (1000 / (sqrt3 x 0.4 x 0.06) ~ 24 kA). The calculator computes three-phase and single-phase fault currents.
What breaker breaking capacity do I need?
The breaker breaking capacity must exceed the prospective Fault Current at its location. For 24 kA at the bus, select a 36 kA breaker (standard IEC 60947 ratings are 16/25/36/50 kA). Undersizing risks a failure to interrupt a fault, so use a 1.25-1.5 safety margin. The calculator matches the breaker to the fault level.
What is the difference between Icu and Ics?
Icu is the ultimate breaking capacity — the breaker can interrupt it once but may need replacement. Ics is the service breaking capacity — the breaker can interrupt it repeatedly and remain usable. Ics is typically 50-75% of Icu. Select breakers whose Icu exceeds the fault level; the calculator checks against standard ratings.
How does cable length reduce short-circuit current?
Long cables add impedance that reduces fault current downstream. A 50 m run of 35 mm2 copper cable can cut a 24 kA fault to roughly 15-18 kA at the far end. This is why downstream breakers can be lower-rated. The calculator accounts for cable impedance when computing the fault level at the load.
What is the difference between symmetrical and asymmetrical fault current?
Symmetrical fault current is the steady AC component; asymmetrical adds a decaying DC offset, peaking in the first half-cycle. The peak (making) current is 1.5-2.5 times the RMS symmetrical value, so Switchgear has both a breaking (Isc) and a making (peak) rating. IEC 60909 computes both; the calculator reports the symmetrical level for breaker selection.
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