Short-Circuit Current Calculator

This is a simplified radial impedance implementation of IEC 60909-0:2016, with no copied standard tables. It estimates initial symmetrical RMS currents; it does not implement the entire standard. The method basis is stated as of 25 October 2026. IEC 60909-0:2026 has superseded the 2016 edition: check against the standard in force.

Utility, transformer and cable

Keep nominal system voltage $U_n$ separate from transformer rated secondary voltage $U_{rT}$. The equivalent phase source is $E=cU_n/\sqrt{3}$; transformer impedance uses its nameplate voltage $U_{rT}$. For example, a 400 V system may have a transformer rated 420 V on its secondary. ABB Technical Application Paper No.15 (2017), printed pp.67–68 (PDF pp.69–70), distinguishes these quantities explicitly.

Enter the utility nominal primary-network voltage $U_{nQ}$ and the transformer rated primary voltage $U_{rH}$ for the selected tap. The rated transformation ratio is $t_r=U_{rH}/U_{rT}$. With utility initial fault power $S''_{kQ}$, the primary-side magnitude is $|Z_{QH}|=c_Q U_{nQ}^2/S''_{kQ}$ and the LV-referred magnitude is $|Z_Q|=|Z_{QH}|/t_r^2$. $R_Q/X_Q$ separates its components. Thus, when utility nominal and transformer rated primary voltages coincide, $|Z_Q|=c_Q U_{rT}^2/S''_{kQ}$. The independent factor $c_Q$ must match the utility study; use $c_Q=1$ for a nominal-voltage impedance-base power $U_{nQ}^2/|Z_{QH}|$. An infinite supply sets $Z_Q=0$.

The transformer has $|Z_T|=(u_k/100)U_{rT}^2/S_T$, $R_T=P_kU_{rT}^2/S_T^2$, and $X_T=\sqrt{|Z_T|^2-R_T^2}$. These are the same impedance relationships used in Schneider's Electrical Installation Guide, sections on networks and transformers. Optional correction uses $x_T=X_TS_T/U_{rT}^2$ and $K_T=0.95c_{max}/(1+0.6x_T)$, as described by DIgSILENT for IEC 60909-0:2016 §6.3.3. $c_{max}$ remains independent of the study's $c$.

Cable impedance is $Z_C=(r_C+jx_C)L$ with length converted to kilometres. Enter effective installed-circuit data; resistance and reactance are not both automatically divided by the number of parallel cables. The utility branch is $Z_g=Z_Q+K_TZ_T+Z_C$.

Motor and sequence networks

ABB handbook §6.5.4, printed pp.224–225 (attached PDF p.115), gives the locked-rotor motor equivalent. The local motors are assumed rated at the nominal LV system voltage $U_n$, independently of the transformer's secondary rating. With $k_{LR}=I_{LR}/I_r$, $|Z_M|=U_n/(\sqrt{3}k_{LR}I_r)$; entered R/X separates its components. A connected local motor contributes $I''_M=c\,k_{LR}I_r$ to a bolted three-phase fault. The utility and motor equivalent sources share the voltage $E=cU_n/\sqrt{3}$ and its phase. Their complex admittances combine to give $Z_1=Z_g\parallel Z_M$; with motors off $Z_1=Z_g$. The approximation takes $Z_2=Z_1$ and motor zero-sequence admittance as zero.

For delta/grounded-star with a metallic return, $Z_0=k_0K_TZ_T+(z_C+3z_{return})L$. The delta blocks upstream utility zero-sequence current. $k_0$ and the effective return-path impedance are user data. ABB §6.5.4, attached PDF p.114, explains the sequence paths. Alternatively, enter the complete, already-corrected source/transformer zero sequence and the complete cable zero sequence directly.

The symmetrical-component connection gives

The single-phase current is for a bolted line-to-neutral or line-to-PE fault with the stated return path, not a generic earth-electrode fault. These equations follow the sequence treatment in ABB §6.5, attached PDF pp.113–117.

Peak and rating comparison

For each contributing branch use $\kappa=1.02+0.98\exp(-3R/X)$, then $i_{p,branch}=\sqrt{2}\kappa I''_{branch}$ (ABB §6.6, printed p.229, attached PDF p.117). The reported three-phase peak envelope sums the utility and motor branch peaks. It is a conservative branch-sum estimate within this model, not a timed waveform or a complete multi-infeed peak calculation.

Initial RMS duty is $\max(I''_{k3},I''_{k1})$. Reserve is entered breaking capacity minus this duty. The comparison assumes an outgoing breaker immediately downstream of the modelled bus, carrying every included contribution. It does not infer current through an incomer or a motor branch.

• One balanced radial LV source, one two-winding transformer and one effective feeder. Nominal LV system voltage Un sets the equivalent source, while transformer nameplate voltages UrH and UrT set impedances and the rated transformation ratio. Utility nominal voltage UnQ is entered independently. Cable and custom zero-sequence impedances are physical ohms on the LV side, with no nominal-voltage rescaling.
• Bolted faults, negligible arc and breaker impedance, equal positive and negative sequence impedances, and no prefault load-voltage calculation.
• The default 400 V nominal system, 20 kV/400 V rated 1000 kVA, 6% transformer, 20 kV utility and 100 m feeder are illustrative study inputs, not a manufacturer catalogue. Replace them with site data. Cable resistance is entered at the chosen study temperature; reactance is entered at the actual 50 or 60 Hz frequency.
• In the delta/grounded-star mode the secondary neutral is solidly grounded. The default transformer Z0/Z1 ratio of 1 and the uncoupled metallic-return approximation must be confirmed for the transformer and installed conductor arrangement. Custom Z0 data include neutral earthing, mutual coupling and all zero-sequence paths.
• The local bank consists of running induction motors at the bus before the checked outgoing breaker, with equal source-voltage phases and motor nameplate voltage equal to nominal system voltage Un. Changing the transformer rated secondary voltage does not change the motor rated voltage. Motor Z1 and Z2 are approximated by the same locked-rotor impedance; motor Z0 is open. Remote motor cables, transient decay and converter controls are omitted. The default motor R/X of 0.42 is ABB's LV approximation (attached PDF p.115).
• For a maximum-duty study use the appropriate maximum voltage factor, highest utility fault level, cold cable resistance and all relevant running motors. For a separate minimum-current study enter the appropriate lower factor and hot conductor resistances and omit motor assistance; a single default run does not establish both extremes.
• The peak is an initial three-phase envelope. Motor current decay, contact-parting time, DC interruption capability and the single-phase peak are not calculated.
• The feeder-length chart changes only the feeder and its return path. The local motor bank stays at the receiving bus; it is not moved to the source end.

For preliminary radial LV studies at 50 or 60 Hz, using credible positive/negative/zero-sequence equivalents. Supported faults are balanced three-phase and bolted single-phase with a defined return. The custom sequence mode requires an engineer's network equivalent; it does not automatically resolve vector groups or earthing arrangements.

Excluded: IT faults, TT earth-electrode studies, meshed supplies, multiple transformers, synchronous generators, converter-fed motors, photovoltaic or battery inverter contributions, transformer saturation, remote motor feeders, arc flash, protection sensitivity and trip time, selectivity, cable I²t withstand, and current-limiting/cascading device combinations.

An adequate entered Icu/Icn reserve addresses only the modelled initial symmetrical RMS duty at the entered voltage. It is not a declaration of installation compliance or a full breaker selection. Check rated voltage, service breaking capacity Ics where required, making capacity Icm, asymmetric interruption duty, thermal/dynamic withstand and manufacturer coordination data separately. Do not compare the instantaneous peak directly with an RMS breaking rating.

This tool documents a simplified IEC 60909-0:2016 method; IEC 60909-0:2026 is the current edition at the stated review date. Check against the standard in force, applicable local rules and the equipment manufacturer's data. Installation work belongs to a qualified person.