A Practical Guide for Calculations at Transformer Terminals and Feeder Ends
Short-circuit current calculation is an essential part of electrical system design.
Selecting a circuit breaker based only on the normal load current is not sufficient. The breaker must also be able to safely interrupt the maximum prospective short-circuit current at its installation point.
This guide presents two practical methods for estimating three-phase short-circuit current in simple low-voltage installations:
The quick calculation at transformer secondary terminals
The Schneider table method for estimating short-circuit current at the receiving end of a feeder
These methods are suitable for preliminary assessment and simple radial installations. They are not a replacement for a complete IEC 60909 study in systems containing multiple power sources.
Scope of This Guide
This guide applies to a symmetrical three-phase short circuit in a 230/400 V low-voltage installation supplied by one distribution transformer.
It may be used for:
Preliminary circuit-breaker selection
Estimating short-circuit current at transformer terminals
Estimating fault current after a feeder cable
Checking software calculations or technical proposals
Systems containing generators, UPS systems, solar inverters, parallel transformers, bus couplers, or large motors require a separate detailed study.
Short-circuit current must never be measured by intentionally creating a fault on site. It must be determined using calculations, utility data, or approved engineering software.
Required Data
Before starting the calculation, collect the following information:
| Required Data | Source |
|---|---|
| Transformer rating in kVA | Transformer nameplate |
| No-load secondary voltage U20 | Nameplate or data sheet |
| Transformer short-circuit impedance Usc% or uk% | Nameplate or test report |
| Short-circuit current at the feeder sending end | Transformer calculation or switchboard data |
| Conductor material and cross-sectional area | Electrical drawing or cable catalogue |
| Feeder length | Drawing or actual site measurement |
The actual transformer impedance shown on the nameplate must always be used when available.
Method One: Quick Calculation at Transformer Terminals
The Schneider Electrical Installation Guide provides a simplified method for estimating the three-phase short-circuit current at the transformer secondary terminals.
This method assumes that the impedance of the upstream medium-voltage network is small enough to be neglected.
The result is therefore suitable for preliminary design but should not be considered a complete short-circuit study.
Step 1: Calculate the Transformer Rated Current
In = S × 1000 ÷ (√3 × U20)
Where:
S = transformer rating in kVA
U20 = no-load secondary line-to-line voltage in volts
In = transformer rated current in amperes
Step 2: Calculate the Short-Circuit Current
Isc = In × 100 ÷ Usc%
Where:
Usc% = transformer short-circuit impedance percentage
Isc = approximate three-phase short-circuit current at the transformer terminals
Worked Example
Transformer data:
Transformer rating = 400 kVA
No-load secondary voltage = 420 V
Transformer impedance = 4%
Transformer Rated Current
In = 400 × 1000 ÷ (√3 × 420)
In = 550 A
Short-Circuit Current
Isc = 550 × 100 ÷ 4
Isc = 13.7 kA
The estimated three-phase short-circuit current at the transformer secondary terminals is:
13.7 kA
This result neglects the impedance of the medium-voltage network, cables, and busbars.
Typical Transformer Impedance Values
The following typical values are presented in Figure G33 of the Schneider Electrical Installation Guide for transformers with medium-voltage windings up to 20 kV.
| Transformer Rating | Oil-Immersed Transformer | Dry-Type Cast Resin Transformer |
|---|---|---|
| 50 to 750 kVA | 4% | 6% |
| 800 to 3200 kVA | 6% | 6% |
These values should only be used for preliminary estimation when the actual transformer impedance is unavailable.
The transformer nameplate value always takes priority.
Quick Calculation Table for a 420 V System
The following values are calculated using the Schneider simplified equation and a no-load secondary voltage of 420 V.
| Transformer Rating | Rated Current | Isc at 4% | Isc at 6% |
|---|---|---|---|
| 250 kVA | 344 A | 8.6 kA | 5.7 kA |
| 400 kVA | 550 A | 13.7 kA | 9.2 kA |
| 500 kVA | 687 A | 17.2 kA | 11.5 kA |
| 630 kVA | 866 A | 21.7 kA | 14.4 kA |
| 800 kVA | 1100 A | 27.5 kA | 18.3 kA |
| 1000 kVA | 1375 A | 34.4 kA | 22.9 kA |
| 1250 kVA | 1718 A | 43.0 kA | 28.6 kA |
| 1600 kVA | 2199 A | 55.0 kA | 36.7 kA |
| 2000 kVA | 2749 A | 68.7 kA | 45.8 kA |
This table was calculated by Power Wadi and is not a reproduction of a Schneider table.
Method Two: Schneider Table Method at the Feeder End
The short-circuit current at the transformer terminals should not automatically be used for every downstream switchboard.
The cable between the transformer and the switchboard adds impedance and reduces the available short-circuit current.
Figure G42 in the Schneider Electrical Installation Guide provides a quick method for estimating short-circuit current at the receiving end of a 230/400 V feeder.
The method requires:
Short-circuit current at the sending end
Conductor material
Phase-conductor cross-sectional area
Cable length
How to Use Schneider Table G42
Determine the short-circuit current at the sending end of the feeder.
Select the conductor material and phase-conductor cross-sectional area.
Determine the actual circuit length in metres.
Select the next higher upstream short-circuit current shown in the table.
Select the next lower cable length shown in the table.
Read the estimated short-circuit current at the receiving end.
Using a higher sending-end current and a lower cable length provides a conservative result for checking circuit-breaker breaking capacity.
Schneider Worked Example
The Schneider example uses the following data:
| Item | Value |
|---|---|
| Short-circuit current at the feeder sending end | Approximately 28 kA |
| Conductor | Copper, 50 mm² |
| Cable length | 20 m |
| Upstream value selected in the table | 30 kA |
| Short-circuit current at the feeder end | 14.7 kA |
The cable reduces the short-circuit current from approximately 28 kA at the sending end to:
14.7 kA at the receiving end
This demonstrates why the transformer-terminal short-circuit current must not be applied to every downstream switchboard.
For the complete table, refer directly to Figure G42 of the Schneider Electrical Installation Guide.
Selecting the Circuit-Breaker Breaking Capacity
At the operating voltage, the circuit-breaker breaking capacity must be equal to or greater than the prospective short-circuit current at its installation point.
Icu ≥ Isc
For circuit breakers rated using Icn:
Icn ≥ Isc
Using the previous example:
Isc = 14.7 kA
A circuit breaker with a breaking capacity of 10 kA is therefore insufficient.
A circuit breaker with a 25 kA breaking capacity at the required operating voltage may be selected, subject to checking:
Ics service breaking capacity
Switchboard short-circuit rating
Protection coordination
Selectivity
Manufacturer-approved cascading tables
Cascading must only be applied when the exact combination of upstream and downstream protective devices is verified by the manufacturer.
Do Not Confuse Rated Current and Breaking Capacity
The breaker rated current In determines how much normal load current the circuit breaker can continuously carry.
The breaking capacity Icu or Icn determines the maximum short-circuit current the breaker can safely interrupt.
A breaker may have the correct rated current for the load but still have insufficient short-circuit breaking capacity.
Practical Calculation Workflow
| Step | Action | Result |
|---|---|---|
| 1 | Read S, U20, and Usc% from the transformer nameplate | Basic transformer data |
| 2 | Calculate In and Isc at the transformer terminals | Sending-end short-circuit current |
| 3 | Identify the cable material, size, and length | Schneider table inputs |
| 4 | Use Figure G42 to estimate Isc at the feeder end | Switchboard fault current |
| 5 | Select Icu or Icn above the calculated Isc | Breaking-capacity check |
| 6 | Check Ics, switchboard rating, selectivity, and cascading | Final protection verification |
Common Calculation Errors
Using transformer rating without checking the actual impedance percentage
Using 400 V when the transformer or reference specifies a 420 V no-load secondary voltage
Applying the transformer-terminal fault current to all downstream switchboards
Selecting the circuit breaker only from its rated current and ignoring its breaking capacity
Using typical transformer impedance values when actual nameplate data is available
Using Schneider Table G42 outside its specified 230/400 V system conditions
Applying the simplified method to systems with generators, UPS systems, or parallel transformers
Using cascading without referring to manufacturer-tested coordination tables
When Is a Full IEC 60909 Study Required?
A detailed short-circuit study should be performed when the installation includes:
Parallel transformers
A closed bus-coupler circuit breaker
Generators
Multiple power sources
UPS systems
Solar or battery inverters
Large motors that may contribute to the fault current
Peak short-circuit current calculations
Minimum fault-current calculations
Detailed protection selectivity
Critical switchboards with ratings close to the calculated short-circuit level
IEC 60909-0:2026 is the general reference for calculating short-circuit currents in low- and high-voltage three-phase AC systems operating at 50 Hz or 60 Hz.
Conclusion
The simplest practical short-circuit calculation starts with the transformer nameplate.
Calculate the transformer rated current, then use the transformer impedance percentage to estimate the three-phase short-circuit current at the secondary terminals.
For downstream switchboards, the feeder cable impedance must be considered. Schneider Table G42 provides a quick method for estimating the short-circuit current at the receiving end of a 230/400 V feeder.
These simplified methods are useful for preliminary assessment of simple radial installations, provided their limitations are understood.
They should not replace a complete engineering study in systems containing multiple sources or complex operating scenarios.