Dry-Type Transformer No-Load Loss vs Load Loss: What Is the Difference?

For a dry type transformer, no-load loss and load loss describe two different sources of electrical loss. They are measured under different test conditions, respond differently to operating load, and can lead to very different annual energy costs.

No-load loss is mainly associated with energizing the magnetic core and remains present whenever the transformer is energized, even if the secondary supplies almost no load.

Load loss is mainly associated with current flowing through the windings and increases substantially as transformer loading increases.

For an EPC engineer or procurement team, simply comparing one “efficiency” number is therefore insufficient. A transformer serving a lightly loaded commercial building should not necessarily be evaluated in the same way as a transformer operating near rated capacity in an industrial plant.

The correct comparison starts with the expected load profile, followed by separate review of guaranteed no-load loss and load loss.

1. The Engineering Difference at a Glance

Parameter	No-Load Loss	Load Loss
Common engineering term	Core loss / iron loss	Copper and stray load loss
Main location	Magnetic core	Windings and conductive structures
Main driver	Applied voltage and frequency	Load current
Exists at zero secondary load?	Yes	Very small
Changes strongly with load current?	No	Yes
Typical test condition	Rated frequency and specified excitation voltage, other winding open	Current circulated with the opposite winding short-circuited
Temperature correction	Normally not treated like winding load loss	Reported/corrected according to applicable reference-temperature requirements
Major procurement concern	Continuous energization losses	Losses under medium/high loading
Relevant test	No-load loss and current test	Short-circuit impedance and load loss test

IEC 60076-1 defines load loss in relation to rated-frequency current under short-circuit test conditions and defines total losses as the combination of no-load and load losses, excluding auxiliary consumption from that definition.

The practical point is simple:

No-load loss is mainly determined by how the transformer is energized. Load loss is mainly determined by how heavily it is loaded.

2. What Causes No-Load Loss in a Dry-Type Transformer?

A transformer can consume active power even when there is no useful secondary load.

Once rated AC voltage is applied to the energized winding, alternating magnetic flux is established in the core. This creates losses principally associated with the magnetic material.

The major components are:

Hysteresis loss

The core is magnetized and demagnetized during every AC cycle. Energy is dissipated during this repeated magnetic reversal.

Core material, flux density, frequency and magnetic characteristics affect this component.

Eddy-current loss

Changing magnetic flux induces circulating currents within the conductive core laminations.

Thin insulated laminations, appropriate electrical steel and core design are used to limit these currents.

Other core-related losses

Core joints, local flux distribution, manufacturing tolerances and structural effects can also influence measured no-load loss.

This is why two transformers with the same:

• rated capacity,
• voltage ratio,
• vector group,
• impedance,

can still have different no-load losses.

The magnetic design may be different even though their nameplate ratings appear similar.

3. Does No-Load Loss Change with Transformer Loading?

Under normal operating conditions, it does not change in proportion to load.

A 1000 kVA transformer supplying only 100 kVA still requires its core to remain magnetized.

If the transformer remains energized for 24 hours, the no-load loss continues throughout those 24 hours.

That makes no-load loss particularly important for transformers with long energized hours and low average utilization.

Examples include:

• office or commercial buildings with large night-time load reductions;
• standby distribution systems that remain energized;
• seasonal facilities;
• transformer installations sized for future capacity expansion.

Voltage and frequency also matter. Excessive voltage or an unfavorable voltage-to-frequency condition can increase core flux and affect excitation behavior, so no-load loss should not be interpreted only as a function of kVA loading.

4. What Causes Load Loss?

Load loss appears principally when current flows through the transformer windings.

The most familiar component is winding resistance loss:

P = I²R

where:

• P = resistive loss,
• I = winding current,
• R = winding resistance.

Because current is squared, increased loading has a strong effect on this loss.

Load loss also contains effects beyond simple DC resistance.

Winding eddy-current loss

Alternating magnetic fields induce additional currents inside the winding conductors.

Conductor dimensions, winding arrangement and leakage flux distribution influence the result.

Stray load loss

Leakage flux may produce eddy-current heating in conductive transformer components.

For a dry-type transformer, winding geometry and structural design therefore influence measured load loss as well as basic conductor resistance.

For procurement purposes, treating load loss as nothing more than “copper loss” can be an oversimplification.

5. Why Load Loss Is Temperature-Sensitive

Winding resistance changes with temperature.

For metallic winding conductors, resistance rises as conductor temperature increases. This directly affects the resistive component of load loss.

That is why a measured load-loss value should be interpreted together with the reference-temperature basis specified by the applicable standard and project specification.

A buyer should not compare:

• Supplier A's uncorrected test-room result

with

• Supplier B's guaranteed loss at the specified reference condition

as though they were directly equivalent.

The comparison basis must be consistent.

This issue is especially important during FAT review.

6. How Transformer Loss Changes with Load

For preliminary engineering evaluation under normal sinusoidal conditions, total transformer loss can often be approximated as:

Ptotal ≈ P0 + β²Pk

where:

• P0 = no-load loss;
• Pk = load loss at rated current under the stated reference condition;
• β = actual load / rated load.

This is an engineering approximation rather than a complete harmonic or thermal model.

Using this relationship:

Transformer Load	Approximate Relative Load Loss
0%	0%
25%	6.25%
50%	25%
75%	56.25%
80%	64%
100%	100%
120%	144%

This explains why load loss becomes much more significant when the transformer operates close to rated current.

At low loading, no-load loss may dominate.

At high loading, load loss can dominate.

7. A Practical Calculation Example

Consider a hypothetical dry-type transformer with:

• rated capacity: 1000 kVA;
• no-load loss P0: 2.0 kW;
• rated load loss Pk: 12.0 kW.

These figures are illustrative only and are not universal values for a 1000 kVA transformer.

At 50% loading:

Load loss:

12 × 0.5² = 3.0 kW

Approximate transformer loss:

2.0 + 3.0 = 5.0 kW

At 80% loading:

12 × 0.8² = 7.68 kW

Approximate transformer loss:

2.0 + 7.68 = 9.68 kW

At full load:

2.0 + 12.0 = 14.0 kW

The loss profile is therefore very different at 50% and 100% loading even though the transformer itself has not changed.

8. At What Load Does Load Loss Equal No-Load Loss?

A useful engineering indicator is the loading point at which both losses are approximately equal.

From:

P0 = β²Pk

the approximate crossover loading is:

β = √(P0 / Pk)

Using the illustrative values above:

β = √(2 / 12)

β ≈ 0.408

So at roughly 41% loading, the calculated load loss becomes similar to the no-load loss.

Below this point, core-related loss represents a larger share of transformer loss.

Above this point, load-dependent loss becomes progressively more influential.

This helps explain why transformer loss optimization should be based on the expected operating profile rather than only rated kVA.

9. Which Loss Should a Buyer Prioritize?

Neither should automatically be prioritized.

The correct answer depends on operating conditions.

Scenario 1: Commercial building with low night load

Suppose a dry-type transformer is energized continuously but:

• operates around 60–70% load during business hours;
• drops below 20% at night;
• remains energized all year.

A lower no-load loss can have substantial lifecycle value because the core remains energized during thousands of low-load hours.

Scenario 2: Industrial production transformer

Consider a transformer serving a production line that:

• operates for long shifts;
• normally runs at 75–90% capacity;
• has a relatively stable load.

Load loss becomes much more influential.

A small improvement in rated load loss may produce larger energy savings than a similar numerical reduction in no-load loss.

The purchasing decision should therefore be based on:

loss value × operating load × operating hours

rather than the lowest individual watt figure in a supplier datasheet.

10. How Harmonics Affect the Comparison

The simple β² relationship assumes a relatively conventional load.

Projects containing:

• variable-frequency drives;
• UPS systems;
• rectifiers;
• data-processing equipment;
• other nonlinear loads

may have substantial harmonic current.

Higher RMS current and frequency-dependent eddy-current effects can increase winding and stray losses beyond what a simple fundamental-frequency load calculation suggests.

For a heavily nonlinear project, the engineer should review:

• harmonic spectrum;
• expected RMS current;
• transformer thermal capability;
• conductor and winding design;
• derating requirements where applicable.

A low rated load-loss number alone does not prove that a transformer is suitable for a severe harmonic environment.

11. How No-Load Loss and Load Loss Are Tested

For dry-type transformers within the relevant scope, IEC 60076-11 includes both measurement of short-circuit impedance and load loss and measurement of no-load loss and current among its routine-test items.

No-load loss test

One winding is energized at the specified frequency and voltage condition while the other winding is open.

The test determines:

• no-load loss;
• no-load current.

Voltage accuracy and waveform quality matter because core excitation is directly affected by the applied voltage.

IEC 60076-1 also notes that no-load loss measurement requires careful control of excitation conditions.

Load loss test

For a two-winding transformer, one winding is short-circuited and sufficient voltage is applied to the other side to circulate the required current.

The applied test voltage is much lower than normal rated system voltage because only the transformer impedance needs to be overcome.

The test is used to establish:

• load loss;
• short-circuit impedance.

This is one reason impedance and load loss should both be reviewed during FAT, rather than examining only one value.

12. What Should Be Checked on the FAT Report?

For projects where guaranteed losses are part of the technical specification, the FAT or routine-test documentation should allow the buyer to identify clearly:

• transformer serial number;
• rated capacity;
• HV/LV rated voltage;
• tapping used for the test;
• rated frequency;
• measured no-load loss;
• measured no-load current;
• measured load loss;
• reference-temperature basis for load loss;
• measured short-circuit impedance;
• applicable acceptance criteria;
• guaranteed values from the approved datasheet.

The measured figures should be compared with the approved technical specification and applicable standard, not with generic catalogue values.

Loss tolerances should also be evaluated according to the contract and applicable standard requirements rather than assumed from an arbitrary percentage.

13. Low No-Load Loss vs Low Load Loss: A Design Trade-Off

Procurement specifications sometimes simply state:

“Transformer shall have minimum losses.”

That is incomplete.

Suppose two technically compliant transformers have different loss profiles:

Option	No-Load Loss	Rated Load Loss
Transformer A	Lower	Higher
Transformer B	Higher	Lower

Transformer A may perform better economically in a low-utilization installation.

Transformer B may perform better under sustained high loading.

The correct choice depends on:

• annual operating hours;
• average load;
• peak load;
• loading distribution;
• electricity cost;
• expected service period;
• harmonic characteristics;
• cooling-system auxiliary consumption where applicable.

A lifecycle-loss comparison is therefore more useful than comparing only transformer purchase price.

14. Do Cooling Fans Count as Transformer Loss?

Not in the same way.

If the dry-type transformer uses AN/AF cooling, forced-air fans consume auxiliary electrical power when operating.

IEC's definition of total transformer losses distinguishes transformer losses from auxiliary-plant consumption.

For lifecycle-energy calculations, however, fan consumption should not be ignored.

If forced ventilation is frequently required, the project evaluation should consider:

• fan motor power;
• annual AF operating hours;
• control logic;
• maintenance requirements.

This power should normally be reported separately rather than incorrectly added to the guaranteed no-load or load-loss figures.

15. Why Loss Data Can Create Procurement Risk

Several common specification errors can make supplier quotations impossible to compare accurately.

Risk 1: Only total loss is requested

A single total-loss figure hides the balance between no-load and load loss.

Ask for both separately.

Risk 2: Load-loss temperature basis is unclear

Measured values at different conductor temperatures cannot simply be compared side by side.

Define the applicable reference basis.

Risk 3: Buyer compares catalogue values with guaranteed values

A typical catalogue value is not automatically a contractual guarantee.

The RFQ should identify whether losses are:

• typical;
• maximum guaranteed;
• subject to contractual tolerance.

Risk 4: Load profile is ignored

The lowest rated total loss may not produce the lowest annual energy consumption for the actual application.

Risk 5: Harmonic loading is omitted

A transformer optimized for conventional sinusoidal loading may not be the appropriate design for a significant nonlinear load.

16. What Loss Information Should Be Included in a Dry-Type Transformer RFQ?

For a project where loss performance matters, the RFQ should preferably include:

• rated capacity, kVA;
• primary voltage;
• secondary voltage;
• rated frequency;
• vector group;
• impedance requirement;
• winding conductor requirement, if specified;
• cooling method, such as AN or AN/AF;
• enclosure and IP requirement;
• maximum or guaranteed no-load loss, if required;
• maximum or guaranteed load loss, if required;
• applicable loss-evaluation method;
• expected load profile;
• harmonic information where relevant;
• testing standard;
• FAT requirements;
• quantity.

If the project specification does not define loss limits, suppliers should still provide both values in the technical offer so the EPC team can make a meaningful comparison.

17. How to Compare Two Supplier Quotations

Do not rank two dry-type transformers only by quoted price.

Use a technical comparison table containing at least:

Comparison Item	Supplier A	Supplier B
Rated capacity
Voltage ratio
Vector group
Impedance
No-load loss
Load loss
Loss reference condition
No-load current
Cooling
Enclosure / IP
Winding material
Applicable standard
Routine-test documentation

Only after the electrical specifications have been normalized should lifecycle losses and commercial price be compared.

This avoids selecting a low-priced transformer whose electrical design does not match the project requirement.

FAQ

1. Is no-load loss the same as core loss?

In practical transformer engineering, no-load loss is often called core loss or iron loss because the dominant components originate in the magnetic core. The measured no-load loss is the actual test quantity and should be taken from the approved datasheet or routine-test report.

2. Is load loss the same as copper loss?

Not exactly. Winding I²R loss is a major component, but measured load loss can also include winding eddy-current and other stray load losses. “Copper loss” is therefore a useful informal term but not always a complete technical description.

3. Does no-load loss disappear when the transformer has no load?

No. If the transformer remains energized at its normal excitation condition, the magnetic core continues to generate no-load loss even if the secondary load is near zero.

4. What happens to load loss at 50% transformer load?

As a first-order engineering approximation under conventional conditions, load loss varies approximately with current squared. At 50% rated current, the load-dependent loss is therefore around 25% of its rated-current value.

5. Which is more important, no-load loss or load loss?

It depends on the load profile. No-load loss becomes more influential in continuously energized, lightly loaded transformers. Load loss becomes more influential where the transformer operates at medium or high load for long periods.

6. Are no-load loss and load loss checked during routine testing?

For dry-type transformers within the relevant IEC 60076-11 scope, measurement of no-load loss/current and measurement of short-circuit impedance/load loss are included in the routine-test structure. Project-specific FAT requirements should still be defined in the purchase specification.

7. Can two 1000 kVA dry-type transformers have different losses?

Yes. Core material, magnetic flux density, conductor material, conductor cross-section, winding geometry, leakage-flux design and manufacturing decisions can produce different loss values even when rated capacity and voltage ratio are identical.

8. What should I request from a supplier before comparing transformer losses?

Request the guaranteed no-load loss, guaranteed load loss, reference condition, short-circuit impedance, applicable standard and routine-test reporting basis. The expected project load profile should then be used for lifecycle evaluation.

CTA

If you are preparing a dry-type transformer RFQ, provide the rated capacity, primary and secondary voltage, frequency, vector group, impedance, cooling method, enclosure/IP requirement, expected load profile, quantity and any specified no-load or load-loss limits.

The technical specification can then be reviewed before quotation so that no-load loss, load loss, impedance and FAT requirements are evaluated on the same engineering basis, rather than comparing catalogue values alone.