A power transformer may show the expected secondary voltage when it is lightly loaded or operating at no load, but the measured voltage can fall once the connected equipment begins drawing current. A small change is a normal part of transformer operation. A larger or unexpected voltage drop, however, may indicate that the transformer, the load, or the wider electrical system needs closer review.
The important point is that transformer voltage drop is not caused by one factor alone. Winding resistance, leakage reactance, transformer impedance, load current, power factor, tap position, and the voltage available on the primary side all influence the voltage that finally reaches the load.
As a power transformer manufacturer, HONLE considers these parameters together when matching transformer specifications to industrial, commercial, utility, and infrastructure projects. HONLE manufactures both oil-immersed and dry-type transformers and also supplies higher-voltage oil-immersed power transformer solutions for 35kV and 110kV applications.
Understanding why voltage drops under load therefore helps not only with troubleshooting an existing transformer but also with specifying a new transformer more accurately.
Why Does Transformer Voltage Change When Load Is Applied?
A power transformer does not have zero internal impedance.
Its windings have electrical resistance, and the magnetic construction of the transformer produces leakage reactance. When very little current flows, the voltage lost across these internal elements is small. As load current increases, the internal voltage drop becomes more noticeable.
This is why the secondary terminal voltage measured at no load is usually slightly higher than the voltage measured at rated load.
The basic relationship can be understood without treating the transformer as an ideal device:
No load: current is low → internal voltage drop is small → secondary terminal voltage remains relatively high.
Under load: current increases → voltage drop across the transformer impedance increases → secondary terminal voltage decreases.
This difference between no-load and loaded secondary voltage is closely related to transformer voltage regulation.
Voltage regulation is commonly expressed as:
Voltage Regulation (%) = (No-Load Voltage − Full-Load Voltage) / Full-Load Voltage × 100
For example, if the secondary voltage is 415V at no load and falls to 400V near rated load, the voltage regulation is approximately:
(415 − 400) / 400 × 100 = 3.75%
That does not automatically mean that anything is wrong with the transformer. The significance of the drop depends on the transformer design, loading, power factor, tap setting, upstream supply, downstream cables, and the voltage tolerance required by the connected equipment.
Transformer Impedance Is One of the Main Reasons for Voltage Drop
Transformer percentage impedance is often discussed in relation to short-circuit current, but it also has an important influence on normal loaded voltage.
The transformer winding can be represented approximately by two internal components:
- winding resistance, or R
- leakage reactance, or X
Together, these create the transformer impedance.
When load current flows through that impedance, part of the transformer’s internal voltage is consumed before the voltage reaches the secondary terminals.
A simplified expression for the voltage change under a lagging load is:
ΔV ≈ I(R cosφ + X sinφ)
where:
- I is load current,
- R is effective transformer resistance,
- X is leakage reactance,
- φ represents the load power-factor angle.
The formula looks more complicated than the practical idea behind it.
The more current the transformer carries, the more important its internal resistance and reactance become. At the same time, the power factor determines how strongly the resistive and reactive parts influence the final voltage.
This is why two transformers with the same rated capacity and voltage ratio do not necessarily show exactly the same loaded-voltage behavior if their impedance characteristics are different.
It is also why impedance should not be selected only from the perspective of fault-current limitation. Increasing or decreasing transformer impedance can affect several parts of the electrical system, including short-circuit current, voltage regulation, motor starting performance, and parallel operation.
Load Current Determines How Much of the Internal Voltage Drop Becomes Visible
A power transformer carrying 20% of its rated current will normally experience much less internal voltage drop than the same transformer operating close to full load.
As current increases, the voltage drop across the internal impedance also increases.
This becomes especially noticeable in facilities where the load changes substantially during operation.
Consider an industrial transformer feeding several production lines. During a quiet operating period, only lighting, controls, ventilation, and a few auxiliary machines may be running. Secondary voltage may remain close to the expected nominal value.
Later, several large motors, compressors, pumps, welding machines, or process loads may operate simultaneously. The transformer current rises, and the measured secondary voltage may fall.
The fact that voltage changes with loading does not by itself indicate a transformer fault. The more useful question is:
Does the voltage change correspond reasonably to the actual load, transformer specification, and system conditions?
If the transformer is operating close to or beyond its intended loading for long periods, voltage performance must also be considered together with temperature rise and thermal loading. Oversizing should not be used as the automatic solution, but neither should a transformer be selected with so little margin that ordinary production peaks create unacceptable system voltage.
Power Factor Can Make Voltage Drop More Severe
Load current alone does not tell the complete story.
Power factor changes the relationship between the resistive and reactive components of transformer voltage drop.
Many industrial systems contain inductive loads such as:
- induction motors,
- pumps,
- compressors,
- fans,
- conveyors,
- HVAC equipment,
- and other motor-driven machinery.
These loads often operate at a lagging power factor.
Because transformer leakage reactance forms an important part of the internal impedance, a lower lagging power factor can increase the voltage drop seen at the secondary terminals.
This explains a situation that sometimes causes confusion during troubleshooting: two operating conditions may have similar kW demand but produce different transformer currents and different secondary voltages.
For example, a plant drawing 500kW at a power factor of 0.95 requires less apparent power than the same 500kW load operating at a power factor of 0.75.
The approximate apparent power would be:
500kW / 0.95 ≈ 526kVA
compared with:
500kW / 0.75 ≈ 667kVA
The power transformer therefore has to carry substantially more current to deliver the same useful active power when the power factor is poor.
That higher current, together with the phase relationship between current and voltage, can increase voltage drop.
This is one reason a transformer should not be selected from kW demand alone. Rated kVA, expected power factor, load characteristics, and operating profile all need to be considered.
Motor Starting Can Cause a Much Deeper but Temporary Voltage Drop
One of the clearest examples of loaded transformer voltage drop occurs when a large motor starts.
An induction motor can draw several times its normal running current during direct-on-line starting. The exact starting current depends on motor design, starting method, connected mechanical load, and system conditions.
That high current flows not only through the motor circuit but also through the impedance of the transformer and the upstream and downstream electrical system.
As a result, the secondary voltage can dip sharply for a short period.
This is different from a transformer that continuously operates with a secondary voltage that is too low.
For example, suppose a factory normally operates at 400V. When a large compressor starts, the bus voltage temporarily falls and then quickly recovers after the motor reaches speed. The issue is primarily related to the transient starting condition.
If the voltage remains low after the motor has reached normal running speed, other factors should also be investigated.
For projects with large motors, the transformer therefore needs to be evaluated not only for continuous kVA demand but also for motor starting conditions. Motor size, starting current, starting method, transformer impedance, available system fault level, and acceptable voltage dip all affect the result.
Using a soft starter or VFD may reduce starting current in suitable applications, but this should be evaluated as part of the complete motor and electrical system rather than treated as a universal solution.
The Transformer May Not Be the Only Source of the Voltage Drop
A common troubleshooting mistake is to measure low voltage at the equipment and immediately conclude that the transformer is responsible.
The transformer is only one part of the complete path between the utility or generating source and the final load.
Voltage can also be lost through:
The upstream network. If the primary voltage supplying the transformer falls during peak demand, the secondary voltage will normally fall as well. A transformer cannot maintain a fixed secondary voltage indefinitely when its incoming voltage changes unless the system includes appropriate voltage-regulation equipment or tap-changing capability.
Long low-voltage cables. Large current flowing through a long cable can create substantial voltage drop. This is especially important on the low-voltage side because current becomes much higher after the transformer steps voltage down.
Undersized conductors. A cable may meet one design requirement but still produce more voltage drop than desired over a long distance or under higher actual load.
Connections and terminals. Loose, damaged, oxidized, or overheated connections can create abnormal resistance. These conditions should be investigated because they can cause both voltage loss and localized heating.
Switchgear and distribution equipment. Busbars, breakers, contactors, disconnectors, joints, and other components also form part of the current path.
This distinction becomes important when measurements are taken.
If the voltage is correct directly at the transformer secondary terminals but significantly lower at a machine located 150m away, the transformer may not be the main problem.
If voltage is already too low at the transformer terminals, then transformer loading, impedance, tap position, incoming primary voltage, and transformer condition deserve more attention.
Measurement location therefore matters as much as the voltage value itself.
Tap Position Can Correct Voltage Ratio, but It Cannot Solve Every Voltage Problem
Transformers commonly provide tapping arrangements that allow the effective turns ratio to be adjusted within a defined range.
These taps are useful when the actual system voltage differs from the nominal design value.
For example, if the incoming network normally operates slightly below the expected primary voltage, an appropriate tap position may help bring the secondary voltage closer to its intended operating level.
However, tap adjustment should not be used as a shortcut for every low-voltage condition.
If a secondary bus is low because:
- the transformer is heavily overloaded,
- power factor is poor,
- a large motor creates excessive starting current,
- the downstream feeder is undersized,
- or the upstream network is unstable,
changing the transformer tap may raise the general voltage level without correcting the underlying cause.
It can also create another problem: voltage may become too high when the load decreases.
For this reason, tap selection should be based on actual primary voltage, required secondary voltage, load behavior, transformer tap range, and system operating conditions.
Off-circuit tap changers must also only be adjusted according to the transformer design and appropriate operating procedures. Larger power transformer applications may use on-load tap-changing arrangements where voltage must be regulated while the transformer remains energized, but whether OLTC is required depends on the network and project specification.
How Much Voltage Drop Is Acceptable?
There is no single percentage that can be applied to every transformer and every project.
Acceptability depends on both the transformer and the equipment being supplied.
A transformer may be operating within its expected voltage-regulation characteristics while the downstream equipment still experiences an unacceptable terminal voltage because additional voltage is being lost in long feeders.
Conversely, a small measured voltage change may be completely normal for one installation but important in another installation containing voltage-sensitive process equipment.
The assessment should therefore separate three different questions:
What is happening inside the transformer? Review loading, percentage impedance, winding characteristics, tap position, and expected voltage regulation.
What is happening in the wider power system? Check primary voltage, feeder lengths, cable sizes, connections, switchgear, and load distribution.
What voltage does the load actually require? Motor starting, control electronics, production machinery, drives, lighting, HVAC equipment, and other loads may have different acceptable voltage ranges.
Instead of deciding that a transformer has “too much voltage drop” from one measurement, it is more useful to record voltage and current under several operating conditions.
For example:
| Operating Condition | Transformer Load | Secondary Voltage | What It Can Indicate |
|---|---|---|---|
| No load / very light load | Very low | Highest measured value | Reference secondary voltage |
| Normal production | Moderate | Slightly lower | Normal operating regulation |
| Peak production | High | Lower | Effect of transformer and system impedance |
| Large motor starting | Short high-current event | Temporary dip | Starting-current effect |
| Load removed | Low again | Voltage recovers | Load-related voltage change |
The exact values will differ from project to project. The pattern of change is often more informative than one isolated voltage reading.
When Should a Larger or Different Power Transformer Be Considered?
Low voltage does not automatically mean that the transformer needs to be replaced with a larger unit.
Before increasing transformer capacity, the cause of the voltage drop should be identified.
If the transformer is correctly loaded but the main voltage loss occurs in a long secondary feeder, increasing transformer kVA may have little effect. Increasing conductor size, changing the distribution arrangement, or moving the transformation point closer to the load may deserve more attention.
If the main issue occurs only during motor starting, the motor starting method and permissible voltage dip should be evaluated before changing the transformer.
If incoming utility voltage varies substantially, transformer capacity alone will not solve the variation.
A different transformer becomes more relevant when the existing unit is genuinely undersized for the actual or expanded load, its impedance does not suit the system requirement, its tap range cannot accommodate the real network voltage, or the project operating conditions have changed substantially from the original design.
For a new project, it is much easier to address these questions before the transformer is manufactured.
Useful design information includes the expected maximum kVA, normal operating load, major motors, starting methods, power factor, load variation, primary voltage range, required secondary voltage, allowable voltage dip, cable arrangement, and future expansion.
These parameters allow voltage performance to be evaluated as part of the electrical system instead of being considered only after commissioning.
Working With HONLE on Power Transformer Voltage Performance
Transformer voltage performance starts with the specification.
Two transformers may show the same rated capacity and voltage ratio on a basic datasheet while differing in impedance, tapping arrangement, winding design, cooling requirements, loss characteristics, accessories, and suitability for a particular load.
HONLE manufactures oil-immersed and dry-type transformer solutions for industrial, commercial, infrastructure, renewable-energy, and power-distribution applications. The company’s transformer range also includes 35kV and 110kV oil-immersed power transformers for projects requiring higher voltage classes.
For project-specific power transformer selection, HONLE can review parameters including rated capacity, primary and secondary voltage, frequency, vector group, impedance, tapping requirements, major load characteristics, cooling, installation conditions, testing, and documentation before the configuration is finalized.
This is particularly important where voltage performance is affected by large motors, fluctuating industrial loads, long operating periods near peak demand, or network conditions that differ from a simple constant-load calculation.
The objective is not to eliminate every change in secondary voltage. Some voltage regulation is inherent in a real transformer.
The more practical objective is to make sure that the transformer impedance, voltage ratio, tap arrangement, capacity, and system design work together so that the connected equipment receives an acceptable voltage under the operating conditions the project will actually experience.
FAQ
Is it normal for transformer secondary voltage to drop under load?
Yes. A real transformer has internal resistance and leakage reactance, so its secondary terminal voltage normally decreases to some extent as load current increases. The amount depends on transformer impedance, current, power factor, and system conditions.
Does higher transformer impedance cause more voltage drop?
Other factors being equal, higher impedance generally produces a larger voltage change as load current flows. However, impedance also affects short-circuit current and other system characteristics, so lower impedance is not automatically better.
Can an overloaded transformer cause low voltage?
Heavy loading increases current through the transformer impedance and can therefore increase secondary voltage drop. If the transformer is operating beyond its intended duty, thermal performance must also be evaluated.
Why does voltage drop when a large motor starts?
A motor may draw several times its rated running current during starting. That high current flowing through transformer and system impedance creates a temporary voltage dip. The voltage normally recovers as the motor accelerates and starting current decreases.
Does poor power factor increase transformer voltage drop?
A low lagging power factor can increase transformer current for a given kW load and can also increase the influence of transformer reactance on voltage regulation. Both effects can contribute to a larger voltage drop.
Can changing the transformer tap fix low voltage?
It can correct the effective voltage ratio when the system voltage differs from the expected design value, but it does not correct every cause of low voltage. Transformer overload, poor power factor, motor starting, long cables, or low upstream voltage should be investigated separately.
How can I tell whether voltage drop comes from the transformer or the cable?
Measure voltage at several points under the same operating condition. Comparing primary voltage, transformer secondary-terminal voltage, distribution-panel voltage, and equipment-terminal voltage can help show where most of the voltage is being lost.
What information is useful when specifying a power transformer for stable loaded voltage?
Important information includes rated capacity, primary and secondary voltage, expected load profile, power factor, major motor loads, starting method, transformer impedance requirements, tap range, allowable voltage variation, feeder arrangement, installation environment, and future expansion.