Power Transformer Selection for Cold Storage and Refrigeration Facilities

Cold storage warehouses, refrigerated distribution centers, food processing plants, and industrial refrigeration facilities depend on stable electrical power around the clock. Compressors, condenser fans, evaporator fans, pumps, defrost heaters, control systems, lighting, and material-handling equipment may all operate from the same electrical distribution system.

This makes power transformer selection for cold storage and refrigeration facilities different from selecting a transformer for a simple commercial building.

The transformer must not only support the total connected load. It also needs to handle compressor starting, changing refrigeration demand, long operating hours, motor loads, possible VFD use, future refrigeration expansion, and the environmental conditions of the electrical installation.

A transformer that is too small can lead to voltage drop, overheating, nuisance protection trips, and limited expansion capacity. An excessively large transformer may increase initial investment and unnecessary no-load losses.

The correct selection starts with understanding how the refrigeration system actually operates.

What Should You Check When Selecting a Power Transformer for Cold Storage?

For most cold storage and refrigeration projects, transformer selection should begin with six points:

• Maximum simultaneous electrical load
• Compressor motor size and starting method
• Primary and secondary voltage
• Expected power factor
• Continuous and seasonal operating profile
• Installation environment

The refrigeration capacity of the facility alone is not enough to determine transformer size.

Two cold storage warehouses with similar floor areas may require very different transformer capacities if one uses several large central compressors while the other uses multiple smaller compressor packages with VFD control.

A practical power transformer specification should therefore be based on the electrical load schedule rather than only the building size or refrigeration tonnage.

Why Are Refrigeration Loads Different from Ordinary Building Loads?

A cold storage facility may appear to have a relatively stable electrical demand, but its internal load profile can change significantly during operation.

Refrigeration compressors are usually the largest loads

In many facilities, compressors account for a major portion of the electrical demand.

Depending on system design, compressors may:

• Run continuously for long periods
• Start and stop according to refrigeration demand
• Operate in stages
• Use soft starters
• Use VFD control
• Run several units simultaneously during peak cooling demand

These characteristics directly influence transformer loading and voltage stability.

Condenser and evaporator fans add continuous motor load

Cold rooms require air circulation, while condensers need sufficient airflow or cooling-water circulation.

Although individual fan motors may be much smaller than compressor motors, a large warehouse may contain many fan motors operating simultaneously.

Their combined demand should not be ignored when determining transformer capacity.

Defrost creates another load peak

Electrical defrost heaters can create a significant temporary load.

If several evaporators enter defrost mode while refrigeration equipment remains operating, the transformer may experience a higher simultaneous load than expected.

The electrical designer should therefore consider the actual defrost control strategy rather than simply adding all heater ratings to the refrigeration load.

Loading also changes with outdoor temperature

Refrigeration demand is not constant throughout the year.

During high ambient temperatures, compressors and condensers may operate at higher duty cycles. Refrigerated logistics facilities may also experience increased load during periods of frequent door opening, loading, and product movement.

The power transformer should be selected around realistic peak operating conditions rather than average electricity consumption.

How Do You Calculate the Required Transformer Capacity?

Transformer sizing should begin with the expected simultaneous load.

Simply adding every connected device can result in unnecessary oversizing, while sizing only from average demand can underestimate the actual requirement.

Consider a simplified cold storage project:

If the expected operating power factor is 0.88:

Apparent power = 440kW ÷ 0.88 = 500kVA

This does not automatically mean that a 500kVA transformer should be selected.

The project must still consider:

• Compressor starting current
• Future refrigeration equipment
• Ambient temperature
• Transformer temperature rise
• Load diversity
• Harmonic-producing loads
• Required voltage stability

If a reasonable expansion and operating margin increases the design requirement to approximately 575–600kVA, a standard 630kVA power transformer may become a practical candidate.

However, the final rating should still be verified against motor starting and system voltage-drop calculations.

Avoid adding an arbitrary safety margin

A power transformer should not automatically be oversized by 30%, 50%, or more simply because the facility contains motors.

Too much spare capacity increases initial cost and may leave the transformer lightly loaded for much of its operating life.

The objective is to find enough capacity for reliable peak operation and planned expansion without creating unnecessary oversizing.

How Does Compressor Starting Affect Power Transformer Selection?

This is one of the most important considerations in refrigeration projects.

Large induction motors can draw several times their normal operating current when started directly across the line.

The exact value depends on:

• Motor design
• Compressor type
• Motor power
• Supply voltage
• Starting method
• System impedance

If a large compressor starts while other refrigeration equipment is already running, the temporary current demand can produce a voltage dip.

A transformer may therefore have sufficient kVA capacity during normal operation but still produce unacceptable voltage drop during motor starting.

Direct-on-line starting

Direct-on-line starting is simple but generally creates the highest starting current.

When large refrigeration compressors use this method, transformer impedance and upstream system strength become particularly important.

Soft starter

A soft starter can reduce mechanical stress and limit starting current compared with direct starting.

This may reduce the impact of compressor starting on the transformer and other connected loads.

VFD starting

A VFD allows the compressor motor to accelerate gradually and can significantly reduce the starting-current problem.

VFDs can also adjust compressor speed according to refrigeration demand, improving control under variable operating conditions.

However, VFDs introduce nonlinear loads into the electrical system. Harmonic currents can increase transformer heating and should be considered when a large portion of the refrigeration plant uses variable-frequency drives.

Therefore, installing VFDs does not mean transformer selection can ignore load characteristics.

The transformer should be evaluated for both normal RMS current and the electrical characteristics of the connected drive system.

What Voltage Ratio Should Be Used in a Cold Storage Facility?

The required voltage ratio depends on the local utility supply and the facility distribution system.

For example, a project may receive medium-voltage power at:

• 6kV
• 10kV
• 11kV
• 20kV
• 22kV
• 33kV

The transformer then steps the voltage down to the facility’s low-voltage distribution level, such as 400V or another project-specific voltage.

Large refrigeration facilities may also use medium-voltage motors for high-capacity compressors.

In this case, the electrical distribution architecture can be different from a smaller cold storage warehouse where all compressors operate from the low-voltage bus.

The transformer manufacturer should therefore confirm:

• Incoming voltage
• Required secondary voltage
• Frequency
• Phase
• Vector group
• Impedance
• Tap range
• Earthing arrangement

Voltage selection should be coordinated with the entire electrical system rather than determined by the transformer alone.

Transformer impedance deserves particular attention

Impedance affects both short-circuit current and voltage drop.

A higher transformer impedance can help limit fault current, but it may also increase voltage drop during large motor starting.

For refrigeration facilities with large compressors, impedance should therefore be coordinated with:

• Compressor starting current
• Switchgear short-circuit rating
• Cable voltage drop
• Protection settings
• Acceptable voltage dip

This is one reason why choosing a power transformer only from voltage and kVA is insufficient.

Oil-Immersed or Dry-Type Transformer for Cold Storage?

Both transformer types can be used in refrigeration projects.

The better choice depends primarily on where the transformer will be installed.

Oil-immersed transformer

An oil-immersed transformer is commonly considered for:

• Outdoor substations
• Utility-style transformer areas
• Large industrial refrigeration plants
• High-capacity distribution
• Projects with suitable transformer foundations and safety separation

Oil provides effective insulation and heat transfer, making oil-immersed transformers practical for continuous industrial loads.

For large cold storage parks or food processing facilities where the transformer is installed outside the main building, an oil-immersed design can be a practical option.

Dry-type transformer

A dry-type transformer is often considered when the transformer is installed:

• Inside the building
• In an indoor electrical room
• Near occupied or production areas
• Where oil containment should be avoided
• Where indoor installation requirements favor a dry design

Because dry-type transformers depend strongly on surrounding air for cooling, ventilation around the transformer is important.

This becomes especially relevant in refrigeration facilities because the transformer room should not be treated as part of the cold storage area.

The transformer still produces heat and requires suitable airflow even when the surrounding facility operates at low temperatures.

Neither transformer type should be selected only because one is considered “better.” The installation environment determines which structure is more suitable.

Why Do Transformer Losses Matter in a 24-Hour Cold Storage Facility?

Cold storage facilities often operate continuously.

Even when refrigeration demand falls, the electrical distribution system usually remains energized.

This makes transformer losses important over the entire operating life of the facility.

A transformer has two major categories of losses:

No-load loss occurs whenever the transformer is energized, even when the connected refrigeration load is low.

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

For a cold storage warehouse operating 24hours per day and 365days per year, relatively small differences in transformer losses can accumulate over many years.

This means buyers should not compare transformers only by purchase price.

They should also review:

• No-load loss
• Load loss
• Expected average loading
• Peak loading
• Annual operating hours
• Electricity cost
• Expected service life

The U.S. Department of Energy also distinguishes between no-load and load losses when evaluating distribution transformer efficiency, highlighting why transformer loading matters when considering long-term energy performance.

For refrigeration facilities with stable long operating hours, transformer efficiency can become part of the facility’s total lifecycle cost.

Do not automatically choose the largest transformer

A very large transformer operating at a relatively low average load still has continuous no-load losses.

This is another reason why realistic load calculation matters.

A properly sized power transformer should balance:

• Peak demand
• Motor-starting performance
• Future expansion
• Thermal capacity
• Long-term operating efficiency

What Installation Conditions Should Be Checked?

The cold room temperature itself does not necessarily represent the transformer’s operating environment.

The transformer is normally installed in an electrical room, transformer yard, substation, or another dedicated area.

Several conditions should still be checked carefully.

Ventilation

Dry-type transformers release heat into the surrounding air.

If the electrical room has insufficient ventilation, internal temperature can rise even when nearby refrigerated rooms are cold.

Condensation

Cold storage facilities can create large temperature differences between refrigerated spaces and surrounding areas.

Cable routes, electrical rooms, wall penetrations, and equipment near transitional zones should therefore be designed to reduce moisture and condensation problems.

Transformer insulation should not be exposed to persistent condensation.

Dust, humidity, and corrosive environments

Food processing and refrigeration facilities may contain:

• High humidity
• Cleaning chemicals
• Salt
• Dust
• Ammonia refrigeration equipment
• Other industrial contaminants

The transformer location and enclosure should be selected according to the actual site conditions.

Maintenance access

Transformers need sufficient access for inspection, testing, cleaning, cable work, and possible future replacement.

Do not place the transformer where future refrigeration expansion, piping, ducting, or warehouse equipment will block maintenance access.

Matching a HONLE Power Transformer to Refrigeration Projects

HONLE provides both oil-immersed and dry-type transformer solutions for industrial and commercial power distribution.

For a cold storage or refrigeration project, transformer selection can be based on the actual combination of:

• Utility voltage
• Required secondary voltage
• Transformer capacity
• Compressor starting method
• Simultaneous refrigeration load
• Installation location
• Ambient conditions
• Future cold-room expansion
• Required efficiency
• Project-specific electrical configuration

For outdoor or dedicated substation installations, an oil-immersed transformer may provide a practical solution for continuous industrial power distribution.

For indoor transformer rooms or projects where an oil-free design is preferred, a dry-type transformer may be more suitable.

The important point is not to begin with a transformer model.

Begin with the refrigeration load.

A cold storage facility with several large fixed-speed compressors requires a different electrical evaluation from a refrigerated distribution center using multiple VFD-controlled compressor packages.

By reviewing the actual load profile before transformer selection, the project can achieve better voltage stability, appropriate spare capacity, and more efficient long-term operation.

FAQ

How large should a transformer be for a cold storage warehouse?

There is no fixed transformer size based only on warehouse area. Capacity should be calculated from the simultaneous electrical demand of compressors, fans, pumps, defrost equipment, lighting, controls, and other loads. Power factor, motor starting, future expansion, and operating conditions must also be considered.

Can a 630kVA transformer support refrigeration compressors?

Yes, depending on the compressor load and other simultaneous loads. A 630kVA transformer cannot be evaluated only from compressor nameplate power. The total operating demand, power factor, starting method, transformer impedance, and acceptable voltage drop should be checked.

Does a VFD allow a smaller power transformer to be used?

Not automatically. A VFD can reduce motor starting current and improve load control, which may reduce severe startup demand. However, the transformer must still carry the full operating load, and harmonic effects from multiple VFDs should also be considered.

Is a dry-type transformer suitable for a cold storage facility?

Yes. Dry-type transformers can be suitable for indoor electrical rooms in cold storage facilities. However, the transformer room still requires appropriate ventilation, temperature control, protection from condensation, and adequate maintenance clearance.

Is an oil-immersed transformer better for large refrigeration plants?

Oil-immersed transformers are commonly used in large industrial facilities, particularly for outdoor or dedicated substation installations where effective cooling and higher-capacity distribution are required. The final choice should still depend on the site layout, fire protection strategy, environmental requirements, and electrical design.

Should the transformer be sized for every compressor running at the same time?

The design should reflect the maximum credible simultaneous operating condition. If the refrigeration control system prevents all compressors from operating together, this diversity may be considered. However, the calculation should also examine peak cooling periods, defrost cycles, equipment redundancy, and abnormal operating scenarios.

Can one power transformer supply both refrigeration and general building loads?

Yes. A single transformer can supply refrigeration equipment together with lighting, offices, warehouse equipment, HVAC, and other facility loads when the distribution system is designed accordingly. The combined peak demand and the effect of large compressor starts on other sensitive loads should be checked before deciding whether one transformer or separate transformers are more appropriate.