Short-Circuit Current Calculator — IEC 60909

Compute the prospective short-circuit currents (3-phase, 1-phase, peak) at the end of a cable run using the impedance method per IEC 60909. Size circuit breakers and check Icu/Icw requirements.

3-phase Isc (kA) (kA) —

1-phase Isc Ph-N (kA) (kA) —

Peak current ip (kA) (kA) —

Total impedance Zk (mΩ) (mΩ) —

Formule

THREE-PHASE FAULT CURRENT

  Ik3  =  c × Un / (√3 × Zk)

  Variables :
    c    1.05  (LV max voltage factor, IEC 60909 Table 1)
    Un   nominal line-to-line voltage (V)
    Zk   total fault loop impedance (Ω)
    √3   ≈ 1.732  (line-line vs line-neutral voltage ratio)

  Rationale : Ik3 is the highest short-circuit current value in LV.
  It dimensions the breaker's ultimate breaking capacity (Icu).
  Required : Icu ≥ Ik3.


SINGLE-PHASE FAULT CURRENT

  Ik1  =  c × (Un / √3) / (Zs/3 + 2 × Rc)

  Variables :
    Un/√3   phase-to-neutral voltage (V)
    Zs/3    source contribution for single-phase fault (Ω)
    Rc      resistance of one cable conductor (Ω)
    2 ×     loop = phase forward + PE return

  Rationale : in a TN system a phase-to-neutral fault loops through
  both the phase conductor AND the return PE — hence the factor 2.
  Assumes : PE cross-section equal to phase cross-section.


PEAK MAKING CURRENT

  ip  =  κ × √2 × Ik3
  κ   =  1.02 + 0.98 × e^(−3 × R/X)

  Variables :
    κ      peak factor (dimensionless)  ≈ 1.45 for resistive LV
    √2     ≈ 1.414  (RMS → peak)
    R/X    resistance to reactance ratio of the fault loop

  Rationale : ip is the instantaneous peak during the first half-cycle
  after the fault. It mechanically stresses busbars and sets the breaker's
  closing capacity requirement.


SOURCE IMPEDANCE

  Zs  =  Un / (√3 × Isc_upstream)

  Variables :
    Isc_upstream   Isc available just before the cable (kA, utility data)

  Use this when you only know the upstream Isc (kA) rather than the
  short-circuit power Scc (MVA).


CABLE RESISTANCE

  Rc  =  ρ × L / S

  Variables :
    ρ     conductor resistivity (Ω·mm²/m)
          ρ_Cu = 0.0175    ρ_Al = 0.029       (at 20°C)
    L     cable length (m)
    S     conductor cross-section (mm²)


REQUIRED CHECKS

  Ik3  ≤  Icu  (breaker ultimate breaking capacity, kA)
  ip   ≤  Ipk  (peak withstand — busbars, breaker closing capacity)
  Ik1  ≥  magnetic trip threshold (to ensure automatic disconnection)

Reference: IEC 60909-0:2016, IEC 60364-4-41

The prospective short-circuit current is the fundamental quantity for selecting circuit breakers (Icu / Icw), fuses, and busbars. IEC 60909-0 defines the c·Un/√3·Zk impedance method — this calculator implements its simplified form for LV networks.

How to use it:

Enter the nominal voltage at the busbar (typically 400 V for LV).
Enter the upstream Isc — this is the short-circuit current before the cable, available from your utility, your transformer datasheet, or a previous busbar calculation.
Set the cable data (material, cross-section, length).
Read Ik3 (three-phase), Ik1 (phase-to-neutral for TN earthing), and ip (peak making current).

Icu vs Icw: Circuit breakers have a rated ultimate breaking capacity (Icu) and a short-time withstand current (Icw). Ik3 must be ≤ Icu. For distribution boards, ip must be ≤ the rated peak withstand current.

Limitations: This tool uses the resistive impedance method (ignores cable reactance for cross-sections below ~95 mm²). For upstream impedances, it assumes a predominantly resistive source — valid for most LV installations. For HV/MV fault calculations, use the full complex impedance method per IEC 60909 directly.

Short-Circuit Current Calculator — IEC 60909

Short-Circuit Current Calculator — IEC 60909

Standards

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