Power transformers determine more than the operating voltage and load current of an industrial substation. Their rated capacity, secondary voltage, percentage impedance, quantity, and operating arrangement also affect the short-circuit current that downstream switchgear may need to withstand and interrupt.
This relationship is particularly important on the low-voltage side of the transformer. A power transformer may carry only a few thousand amperes during normal operation but supply a prospective short-circuit current many times higher when a fault occurs.
The basic relationship can be summarized as:
Transformer capacity and secondary voltage
↓
Full-load current
↓
Transformer impedance and source conditions
↓
Prospective short-circuit current
↓
Switchgear breaking and withstand ratings
If the calculated fault current exceeds the rating of the circuit breaker, busbar, internal conductors, or complete switchgear assembly, the equipment may not safely interrupt or withstand the fault.
For this reason, power transformers and switchgear should be selected as parts of the same electrical distribution system rather than specified independently.
Why Does Transformer Data Affect Switchgear Ratings?
During normal operation, the low-voltage switchgear carries the load current supplied by the transformer. During a downstream short circuit, the transformer also becomes one of the main sources feeding current into the fault.
Four transformer-related parameters have a particularly strong influence on the result.
| Transformer factor | Effect on the electrical system |
| Rated capacity | A larger transformer normally supplies more rated current and may supply more fault current |
| Secondary voltage | A lower secondary voltage produces a higher current at the same transformer capacity |
| Percentage impedance | Lower impedance generally allows a higher short-circuit current |
| Quantity and operating arrangement | Transformers operating in parallel may combine their fault-current contributions |
The downstream switchgear must therefore perform two different duties:
- Carry the normal operating current continuously.
- Withstand and interrupt fault current for the required duration.
These duties involve different equipment ratings. A switchboard may have sufficient continuous-current capacity but still be unsuitable for the available short-circuit current.
IEC 60076-5 addresses the ability of power transformers to withstand the thermal and dynamic effects of external short circuits. The applicable IEC 61439 requirements cover the construction and verification of low-voltage switchgear assemblies. Project specifications may also introduce additional requirements for fault duration, protection selectivity, service continuity, and safety margins.
How Is Transformer Full-Load Current Determined?
For a three-phase power transformer, the approximate secondary full-load current can be calculated as:
Rated current = Transformer capacity ÷ (√3 × secondary voltage)
This current is the starting point for selecting several parts of the downstream distribution system, including:
- Transformer secondary terminals;
- Main circuit breaker;
- Low-voltage busbar;
- Cables or bus duct;
- Current transformers;
- Neutral conductor;
- Internal switchgear connections.
For example, a 1000 kVA power transformer with a 400 V secondary has an approximate full-load current of:
1443 A
The main low-voltage circuit breaker and busbar must be able to carry this current under the actual operating conditions. Ambient temperature, enclosure design, ventilation, installation altitude, harmonic content, load profile, and future expansion may all affect the final continuous-current selection.
However, 1,443 A is not the required switchgear short-circuit rating. It represents normal rated current. The available fault current must be calculated separately by considering transformer impedance and the wider electrical system.
How Does Transformer Impedance Limit Short-Circuit Current?
Percentage impedance represents the voltage required to circulate rated current through the transformer under specified short-circuit test conditions.
For a simplified fault at the transformer secondary terminals, the approximate symmetrical short-circuit current can be expressed as:
Short-circuit current ≈ Rated current × 100 ÷ Impedance %
This simplified relationship shows why transformer impedance is an important design parameter.
For transformers with the same capacity and secondary voltage:
- Lower impedance allows more short-circuit current to flow.
- Higher impedance limits the short-circuit current.
- Lower impedance may improve voltage regulation under load.
- Higher impedance may increase voltage drop and motor-starting voltage dip.
Percentage impedance therefore affects several parts of the system at the same time, including fault-current level, voltage regulation, protection coordination, motor starting, and parallel transformer operation.
Simplified example
Consider a 1000 kVA, 400 V power transformer with an approximate rated current of 1443 A.
| Transformer impedance | Approximate terminal fault current |
| 4% | 36.1 kA |
| 6% | 24.1 kA |
| 8% | 18.0 kA |
The comparison clearly shows the influence of impedance. However, this formula should only be used for early screening or preliminary equipment selection.
The actual fault current may also depend on:
- Upstream utility fault level;
- Transformer primary-system impedance;
- Cable and bus-duct impedance;
- Generator contribution;
- Motor contribution;
- Parallel transformers;
- Fault location;
- Earthing arrangement;
- System operating mode.
Final switchgear selection should therefore be based on a project-specific short-circuit study rather than the transformer-terminal approximation alone.
Does a Larger Power Transformer Require Higher-Rated Switchgear?
Not automatically, but a larger power transformer commonly increases both the normal operating current and the potential fault current.
For example, a 1600 kVA, 400 V power transformer has an approximate rated secondary current of 2309 A. At 6% impedance, its simplified short-circuit current at the secondary terminals is approximately 38.5 kA.
Compared with a 1000 kVA transformer operating at the same voltage and impedance, the larger unit may require:
- A higher-current main circuit breaker;
- Larger busbars;
- Higher-capacity cables or bus duct;
- Greater short-circuit breaking capacity;
- Higher short-time withstand capability;
- Stronger busbar supports and internal bracing.
Transformer capacity must still be considered together with impedance. A larger power transformer with higher impedance may produce a similar fault-current level to a smaller power transformer with lower impedance.
| System change | Typical result |
| Transformer capacity increases | Rated current and fault current usually increase |
| Secondary voltage decreases | Current increases |
| Transformer impedance decreases | Fault current increases |
| Transformer impedance increases | Fault current decreases, but voltage drop may increase |
| Transformers operate in parallel | Fault-current contributions may combine |
This is why switchgear should not be selected from transformer kVA alone.
Which Switchgear Ratings Must Be Checked?
A short-circuit review should cover more than the interrupting capacity printed on the main circuit breaker.
Several breaker and switchgear ratings describe different electrical and mechanical duties. They should not be treated as interchangeable.
Circuit-breaker breaking capacity
The circuit breaker must be capable of interrupting the prospective short-circuit current at its installation point.
The selected breaking capacity should be equal to or greater than the calculated fault current, subject to the applicable standard, system voltage, project specification, operating sequence, and required safety margin.
Depending on the breaker type and applicable standard, the review may include:
- Ultimate short-circuit breaking capacity;
- Service short-circuit breaking capacity;
- Short-circuit making capacity;
- Short-time withstand current.
A breaker may be able to interrupt a fault once under specified conditions but have a different rating for continued service after the interruption. The project should therefore confirm which rating is required rather than comparing only one kA value.
Switchgear short-time withstand current
The complete switchgear assembly may need to carry fault current for a specified period while a protective device operates.
This becomes especially important when an intentional protection delay is introduced to improve selectivity between upstream and downstream breakers.
During this delay, the following components must withstand the thermal and mechanical effects of the fault:
- Main busbars;
- Busbar supports;
- Internal conductors;
- Connections;
- Breaker terminals;
- Neutral system;
- Earthing conductors.
For example, switchgear rated at 50 kA for one second does not automatically provide the same duty as equipment rated at 50 kA for three seconds. Both the current value and the duration must match the protection study.
Peak withstand or making current
The first peak of short-circuit current creates strong electrodynamic forces between conductors.
These forces can affect:
- Busbar supports;
- Busbar bracing;
- Bolted joints;
- Breaker contacts;
- Internal connections;
- Transformer terminals;
- Bus-duct connections.
The peak withstand or making rating must therefore be checked in addition to the symmetrical RMS short-circuit current.
Main busbar rating
The main busbar must satisfy two separate requirements:
- Continuous-current capacity under normal operating conditions.
- Short-circuit withstand capacity during a fault.
A busbar may have enough continuous ampacity for the transformer load while still lacking the mechanical or thermal strength required for the available fault current.
How Does Fault Location Change the Required Rating?
Short-circuit current is normally highest close to the power transformer secondary terminals because there is less cable or busbar impedance between the transformer and the fault.
As the fault location moves farther downstream, the additional impedance of cables, bus duct, connections, and distribution equipment generally reduces the available current.
A simplified distribution path is:
Power transformer terminals
Highest available transformer-fed fault current
↓
Main low-voltage switchboard
Slightly reduced by transformer connections and bus duct
↓
Distribution board
Further reduced by feeder impedance
↓
Final load circuit
Further reduction from cable length and conductor impedance
Different points in the same electrical system may therefore have different prospective short-circuit currents.
The main switchgear installed closest to the power transformer often requires the highest short-circuit rating. Downstream panels may use lower ratings only when calculations confirm that the available fault current at those locations is lower.
The calculation should include the actual:
- Cable length;
- Cable cross-sectional area;
- Conductor material;
- Cable configuration;
- Bus-duct impedance;
- Connection arrangement;
- Fault type;
- Earthing arrangement.
Using only the transformer-terminal fault current for every downstream panel may result in unnecessary equipment cost. Conversely, assuming that distance always reduces the current sufficiently can lead to under-rated equipment.
What Happens When Two Power Transformers Operate in Parallel?
Parallel power transformer operation can substantially increase the fault current on a common busbar.
A typical arrangement may include:
Transformer A
↘
Common low-voltage busbar
↗
Transformer B
During a fault on the common busbar, both power transformers may contribute short-circuit current.
For preliminary assessment, the individual transformer contributions may be added when the system configuration allows both sources to feed the same fault. The final result should still account for transformer impedance, upstream source impedance, connection impedance, and the actual system arrangement.
Parallel operation can affect:
- Main breaker breaking capacity;
- Incomer breaker ratings;
- Bus-coupler rating;
- Main busbar withstand rating;
- Feeder breaker ratings;
- Protection settings;
- Arc-flash energy;
- Cable and bus-duct withstand capability.
The short-circuit study should cover every permitted operating mode.
| Operating mode | Fault-current implication |
| One transformer in service | Only one main transformer contribution |
| Two transformers on separate bus sections | Each section receives its own transformer contribution |
| Bus coupler open | Fault contributions normally remain separated |
| Bus coupler closed | Contribution depends on the transformer and interlocking arrangement |
| Transformers operating in parallel | Contributions may combine on the common bus |
| Power Transformer and generator operating together | Both sources may feed the fault |
| Power Transformer with renewable or storage source | Converter or generator contribution must be evaluated |
Switchgear should not be rated only for the normal operating arrangement when the control system permits a higher-fault-current emergency, maintenance, transfer, or parallel mode.
How Does Transformer Replacement Affect Existing Switchgear?
Replacing a power transformer with a larger unit can create a switchgear problem even when the primary and secondary voltages remain unchanged.
The replacement may change:
- Full-load current;
- Percentage impedance;
- Available short-circuit current;
- Main breaker requirements;
- Busbar loading;
- Current-transformer ratios;
- Protection settings;
- Cable or bus-duct current;
- Neutral loading;
- Arc-flash conditions.
For example, replacing a 1,000 kVA power transformer with a 1,600 kVA power transformer may significantly increase the secondary fault current.
The existing switchgear could still have the correct voltage rating and enough continuous-current capacity while lacking sufficient breaking or short-time withstand capability.
Before approving a transformer replacement or capacity increase, the project should verify:
- Existing switchgear nameplate ratings;
- Original short-circuit study;
- New transformer capacity;
- New transformer impedance;
- Actual cable or bus-duct impedance;
- Permitted operating modes;
- Main breaker condition;
- Protection settings;
- Busbar and internal connection ratings.
The new transformer rating should not be finalized until the downstream system has been checked.
Can Higher Transformer Impedance Solve a Switchgear Rating Problem?
Increasing transformer impedance can reduce prospective short-circuit current, but it is not a universal solution.
A higher impedance may also cause:
- Greater voltage drop under load;
- More noticeable voltage dip during motor starting;
- Changes in reactive power behavior;
- More difficult load sharing during parallel operation;
- Reduced sensitivity for some protection functions;
- Different voltage-regulation performance.
Lower impedance may improve load-side voltage performance but increase the fault-current duty imposed on the switchgear.
The selected transformer impedance should therefore balance several requirements:
Voltage performance
↔
Fault-current limitation
↔
Motor starting
↔
Protection coordination
↔
Parallel operation
The power transformer must also be designed to withstand the specified thermal and mechanical effects of external short circuits. However, transformer short-circuit withstand capability does not remove the need for adequately rated downstream switchgear.
Both sides of the connection must be suitable for the same fault conditions.
What Transformer Data Should Be Confirmed Before Selecting Switchgear?
The transformer supplier, switchgear supplier, electrical consultant, and project owner should work from the same approved system data.
Table 1: Transformer Data Required for a Switchgear Short-Circuit Review
| Required data | Why it matters |
| Transformer capacity | Determines rated current |
| Primary and secondary voltage | Determines operating current and equipment voltage class |
| Percentage impedance | Major parameter limiting transformer-fed fault current |
| Impedance tolerance | The final tested value may differ from the nominal design value |
| Transformer quantity | Determines the number of possible source contributions |
| Operating arrangement | Defines whether transformers can feed a common bus |
| Vector group | Affects phase relationships, fault paths, and protection calculations |
| Neutral grounding | Influences earth-fault current |
| Upstream fault level | Affects the source contribution through the transformer |
| Cable or bus-duct data | Adds impedance between the transformer and switchgear |
| Motor or generator contribution | May increase the total fault current |
| Protection clearing time | Determines the required short-time withstand duration |
| Future expansion | May increase load current and fault duty later |
The approved power transformer impedance should be communicated before the switchgear short-circuit rating and protection settings are finalized.
Where possible, the project should also define whether the short-circuit calculation uses nominal impedance, minimum impedance within tolerance, or another value required by the applicable specification.
What Switchgear Information Should Be Checked?
Switchgear data should be compared directly with the maximum calculated fault current and the expected protection clearing time at the installation point.
Table 2: Switchgear Ratings to Compare with the Study
| Switchgear data | What to verify |
| Rated voltage | Matches the transformer secondary system |
| Rated current | Carries the transformer load under site conditions |
| Breaker breaking capacity | Exceeds the prospective short-circuit current |
| Service breaking capacity | Meets the required post-fault service duty |
| Short-time withstand current | Withstands the fault for the protection delay |
| Peak withstand or making capacity | Withstands the first current peak |
| Main busbar rating | Suitable for continuous and short-circuit current |
| Neutral rating | Suitable for grounding and load characteristics |
| Bus-coupler rating | Suitable for every permitted operating mode |
| Protection setting range | Supports transformer protection and selectivity |
| Internal connections | Have adequate thermal and mechanical strength |
| Earthing system | Withstands the calculated earth-fault duty |
The circuit breaker, busbar, enclosure, supports, and internal conductors should be evaluated as one verified assembly.
Selecting a high-breaking-capacity breaker does not automatically prove that the complete switchboard has the same withstand capability.
What Are the Most Common Selection Mistakes?
Using only transformer rated current
Rated current determines the normal loading requirement. It does not define the complete short-circuit duty.
Assuming a standard impedance
Two power transformers with the same capacity and voltage ratio may have different impedance values. Their fault-current contributions may therefore be different.
Ignoring impedance tolerance
The final tested impedance may differ from the nominal design value. The short-circuit study should use the value required by the applicable standard and project specification, particularly when calculating the maximum possible fault current.
Checking only the circuit breaker
The breaker may have sufficient breaking capacity while the busbars, supports, internal connections, or complete switchgear assembly have a lower withstand rating.
Ignoring bus-coupler operation
A normally open bus coupler may still be closed during transfer, emergency operation, maintenance, or testing. That operating mode may allow two sources to contribute to the same fault.
Forgetting motor and generator contribution
Large motors may feed current back into the system during the initial stage of a fault. Generators and other connected sources can also increase the total available current.
Reusing the original study after transformer replacement
A different transformer capacity, voltage, or impedance changes the electrical conditions. The previous study may no longer represent the upgraded system.
Treating a simplified formula as the final calculation
The transformer-terminal formula is useful for preliminary screening, but final equipment selection requires a complete short-circuit study covering the actual network.
How Should the Transformer and Switchgear Be Coordinated Before Production?
The coordination process can be organized into six main steps.
1. Confirm the system diagram
Identify all power transformers, bus sections, bus couplers, generators, motors, renewable sources, energy-storage systems, and alternative supply paths.
2. Finalize the main transformer data
Confirm the rated capacity, voltage ratio, percentage impedance, vector group, tapping range, neutral arrangement, and grounding method.
3. Calculate maximum and minimum fault currents
Review all permitted operating arrangements, including normal, emergency, maintenance, transfer, and parallel modes.
The maximum fault current is used to verify equipment withstand and breaking ratings. The minimum fault current may be needed to confirm that protection devices remain sensitive enough to detect and clear faults.
4. Compare the results with equipment ratings
Check:
- Main and feeder breakers;
- Busbars;
- Complete switchgear assemblies;
- Cables;
- Bus ducts;
- Transformer terminals;
- Earthing conductors;
- Bus couplers.
5. Review protection clearing time
Confirm that the equipment can withstand the calculated current until the relevant protective device clears the fault.
6. Recheck after design changes
Repeat the review if the transformer capacity, impedance, source arrangement, cable length, generator connection, or bus-coupler logic changes.
When HONLE supplies power transformers together with medium- or low-voltage switchgear, the key electrical parameters can be coordinated before production. Transformer rated current, percentage impedance, terminal arrangement, switchgear current rating, short-circuit level, and permitted operating modes can then be checked against the same project information.
Short-circuit impedance should also be confirmed during transformer testing because the measured value directly affects the expected fault-current level and the suitability of the connected switchgear.
Frequently Asked Questions
Does a larger power transformer always produce more short-circuit current?
A larger power transformer normally produces more rated current and may produce more fault current. However, the result also depends on transformer impedance, secondary voltage, upstream source strength, conductor impedance, and system arrangement.
Is transformer impedance the same as short-circuit withstand strength?
No. Transformer impedance influences how much fault current can flow through the transformer. Short-circuit withstand capability describes whether the transformer can withstand the resulting thermal and mechanical effects without unacceptable damage.
Can switchgear breaking capacity equal the calculated fault current?
The selected breaking capacity should not be lower than the calculated prospective short-circuit current. The required margin depends on the applicable standard, project specification, calculation assumptions, and equipment duty.
Does cable length reduce short-circuit current?
Yes. Cable resistance and reactance add impedance between the transformer and the fault location. This normally reduces the available short-circuit current farther downstream.
Must the calculation be repeated when transformers operate in parallel?
Yes. Parallel operation may combine the fault-current contributions of multiple transformers and can increase the required breaker, busbar, switchgear, and bus-coupler ratings.
Conclusion
Power transformers directly affect switchgear short-circuit ratings through their capacity, secondary voltage, percentage impedance, quantity, and operating arrangement.
Transformer rated current determines the continuous-current requirement of the downstream system. Transformer impedance helps determine the prospective short-circuit current. Parallel transformers, generators, motors, cables, bus ducts, and bus couplers can then increase or reduce the final fault level at different points in the network.
A complete switchgear review should cover:
- Circuit-breaker breaking capacity;
- Service breaking capacity where applicable;
- Short-time withstand current;
- Peak withstand or making capacity;
- Main busbar strength;
- Internal connections and supports;
- Protection clearing time;
- Every permitted operating mode.
HONLE supplies oil-immersed and dry-type power transformers together with medium- and low-voltage switchgear for industrial substations and power distribution projects. By confirming the power transformer and switchgear parameters before production, potential mismatches in rated current, impedance, fault level, breaker capacity, and busbar withstand strength can be identified earlier.
Welcome to contact HONLE with your single-line diagram, transformer capacity, voltage ratio, impedance requirement, system fault level, cable or bus-duct data, and intended operating arrangement. Our team will review the available information and help coordinate the transformer and switchgear parameters for your project.