What information is needed to order a custom transformer?

Manufacturers typically need transformer capacity, primary and secondary voltages, phase, frequency, winding connections, impedance, insulation requirements, cooling method, installation conditions, and applicable industry standards.

Can a custom transformer be manufactured with nonstandard voltages?

Yes. Custom transformers can be engineered for specialized voltage requirements when the design meets applicable electrical, thermal, insulation, and mechanical requirements.

Why is impedance important when specifying a custom transformer?

Transformer impedance affects available short-circuit current, voltage regulation, and parallel load sharing. It should be selected based on system engineering requirements.

What information is needed to replace an existing industrial transformer?

Replacement projects should include existing transformer nameplate data, electrical drawings, physical dimensions, installation photographs, connection requirements, and any changes to the facility's electrical loads.

What factory tests are required for custom transformers?

Required tests depend on the transformer type and applicable standards. Common tests include winding resistance, voltage ratio, impedance, losses, and dielectric withstand testing. Additional tests may be specified for particular applications.

Can custom transformers be designed for data centers and industrial applications?

Yes. Custom transformers can be designed for industrial facilities, utilities, data centers, and specialized electrical systems. The manufacturer must evaluate the application's loading, electrical performance, environmental conditions, and applicable standards.

What Information Manufacturers Need to Design and Build a Custom Transformer (10/22/2026)

Custom transformer manufacturing requires detailed electrical specifications, including kVA or MVA capacity, primary and secondary voltages, phase configuration, frequency, impedance, insulation class, cooling method, and installation conditions. Providing accurate load requirements, applicable IEEE or CSA standards, environmental conditions, and dimensional constraints helps manufacturers design safe, reliable transformers for industrial and utility applications.

By LarsonElectronics.com, October 22, 2026

Designing and manufacturing a custom transformer requires accurate electrical, mechanical, environmental, and application-specific information. Unlike standard catalog transformers, custom-built units may be engineered to accommodate unusual voltages, specialized loads, restricted installation spaces, demanding environmental conditions, or utility-specific requirements.

Manufacturers typically need the transformer capacity, primary and secondary voltages, phase configuration, frequency, winding connections, impedance requirements, insulation system, cooling method, installation environment, and applicable industry standards. Providing these details early helps prevent design changes, manufacturing delays, and equipment compatibility problems.

Essential Information Required to Manufacture a Custom Transformer

A complete transformer specification should define the electrical performance, construction requirements, operating environment, and intended application. The following information provides a practical starting point for industrial buyers, electrical engineers, and procurement teams.

Specification	Information Required	Engineering Importance
Transformer Capacity	kVA or MVA rating and load profile	Establishes continuous loading and thermal design requirements.
Primary Voltage	Nominal supply voltage and operating range	Determines winding voltage and insulation requirements.
Secondary Voltage	Required output voltage and regulation limits	Establishes equipment compatibility and output performance.
Phase and Frequency	Single-phase or three-phase; 50 Hz or 60 Hz	Affects core design, winding configuration, and losses.
Winding Connections	Delta, wye, grounded wye, or other configurations	Determines grounding, phase displacement, and system compatibility.
Impedance	Required percentage impedance and tolerance	Influences fault current, voltage regulation, and parallel operation.
Insulation Requirements	Voltage class, BIL, and applicable dielectric requirements	Establishes insulation withstand performance.
Cooling Method	Dry-type, liquid-immersed, natural, or forced cooling	Determines heat dissipation and thermal capacity.
Installation Conditions	Indoor or outdoor location, ambient temperature, altitude, and environmental exposure	Affects insulation, cooling, enclosure design, and service life.
Mechanical Requirements	Dimensions, weight limits, mounting, and cable entry	Ensures the transformer fits and can be installed safely.

Determining the Required Transformer kVA or MVA Capacity

Transformer capacity is one of the first specifications manufacturers need. The required rating depends on connected loads, expected demand, duty cycle, power factor, and anticipated future expansion.

For a balanced three-phase system, apparent power can be estimated using:

kVA = (1.732 × Voltage × Current) / 1,000

Voltage is the line-to-line RMS voltage, and current is the line current in amperes.

Industrial Transformer Sizing Example

Consider a manufacturing facility requiring a three-phase transformer to supply a 480-volt distribution system with an expected continuous load of 600 amperes.

kVA = (1.732 × 480 × 600) / 1,000

Calculated load = approximately 499 kVA

A 500 kVA transformer could be considered based on the calculated apparent power. However, the final rating must account for continuous loading, ambient temperature, load growth, motor starting, harmonic currents, and applicable installation requirements.

For equipment with significant nonlinear loads, such as variable frequency drives, rectifiers, and certain data center power systems, the manufacturer may require harmonic spectra, current distortion measurements, or additional thermal design information.

Specifying Primary and Secondary Transformer Voltages

Manufacturers need the nominal primary and secondary voltages, maximum and minimum operating voltages, and any required voltage adjustment capability.

For example, a custom industrial transformer might be specified with a 13,200-volt primary and a 480Y/277-volt secondary.

The specification should identify whether each voltage is line-to-line or line-to-neutral, the required neutral availability, and the intended system grounding arrangement.

Transformer Tap Requirements

Tap changers or fixed tap connections allow transformer voltage ratios to be adjusted for supply voltage variations.

Common specifications may include de-energized taps providing two 2.5% steps above and below nominal primary voltage. This arrangement is an example rather than a universal requirement.

Engineers should specify the tap range, step size, location, and whether adjustment must occur only when de-energized or under load.

Choosing Single-Phase or Three-Phase Transformer Construction

Transformer phase configuration must match the electrical distribution system and connected equipment.

Single-phase transformers are commonly used for specialized machinery, control circuits, and single-phase distribution loads. Three-phase transformers are used extensively in industrial facilities, utility distribution, manufacturing operations, and commercial power systems.

For three-phase transformers, manufacturers also need the winding connections and required phase displacement.

Common configurations include delta-wye, wye-wye, delta-delta, and wye-delta. Each has different grounding, neutral availability, and system integration characteristics.

When transformers will operate in parallel, the manufacturer must also evaluate voltage ratios, tap positions, polarity, phase sequence, phase displacement, impedance, and impedance angle to support proper load sharing.

Establishing Transformer Impedance and Short-Circuit Requirements

Transformer impedance affects voltage regulation, available short-circuit current, and coordination with downstream protective devices.

Impedance is typically expressed as a percentage based on the transformer's rated voltage and capacity.

For a simplified three-phase calculation, the transformer-limited symmetrical short-circuit current can be approximated by:

Fault Current = Rated Secondary Current / Per-Unit Impedance

Consider a 1,000 kVA transformer with a 480-volt secondary and 5.75% impedance.

Rated secondary current = 1,000,000 / (1.732 × 480) = approximately 1,203 A.

Approximate fault current = 1,203 / 0.0575 = 20,922 A.

This represents an idealized estimate at the transformer secondary terminals, assuming rated voltage, an effectively infinite upstream source, and neglecting other system impedances and contributions. Actual available fault current must be established through a short-circuit study.

Custom transformer specifications should identify required impedance, applicable tolerances, available upstream fault levels, and short-circuit withstand requirements.

Defining Transformer Insulation Class and BIL

Insulation requirements depend on system voltage, grounding, expected overvoltages, and applicable equipment standards.

For medium-voltage and high-voltage transformers, manufacturers may require a specified basic impulse insulation level (BIL), which establishes standardized lightning impulse withstand performance.

A 15 kV-class transformer may have a 95 kV BIL in certain applications, but the required withstand level must be determined from the actual equipment specification and insulation coordination study.

Engineers should also identify surge arrester requirements, system grounding characteristics, and any special dielectric testing or insulation coordination requirements.

Applicable IEEE references include IEEE C57.12.00 for liquid-immersed transformers, IEEE C57.12.01 for dry-type transformers, and relevant IEEE C62 standards for surge protection.

Selecting Dry-Type or Liquid-Immersed Transformer Construction

The cooling medium and insulation system influence transformer construction, thermal performance, installation requirements, and maintenance.

Dry-Type Transformers

Dry-type transformers use solid insulation systems and air or other gas for cooling. They are frequently selected for indoor electrical rooms, industrial facilities, commercial buildings, and applications where liquid containment is undesirable.

Manufacturers may require the insulation thermal class, permitted winding temperature rise, enclosure requirements, and cooling method.

Liquid-Immersed Transformers

Liquid-immersed transformers use insulating fluid to provide dielectric insulation and transfer heat away from the windings and core.

Specifications should identify the required insulating liquid, cooling classification, temperature rise, fluid containment provisions, and applicable fire safety requirements.

Common cooling designations include ONAN and ONAF under IEC terminology, with corresponding IEEE cooling designations such as KNAN or KNAF for certain less-flammable liquid systems. Cooling designations must be selected according to the actual insulating liquid and applicable standard.

For outdoor industrial installations, liquid-immersed transformers may also require radiators, corrugated cooling fins, conservator arrangements, pressure relief devices, and fluid monitoring accessories.

Identifying Environmental and Installation Requirements

Manufacturers need to understand where and how the transformer will operate.

Important installation information includes:

Indoor or outdoor installation.
Maximum and minimum ambient temperatures.
Installation altitude above sea level.
Humidity, condensation, and moisture exposure.
Salt spray, chemical contamination, or corrosive atmospheres.
Dust, vibration, seismic, or mechanical loading requirements.
Available ventilation and cooling airflow.
Required enclosure protection and physical access restrictions.

High-altitude installations may require adjustments to insulation clearances or thermal ratings. Elevated ambient temperatures may also require increased cooling capacity or reduced loading.

For hazardous locations, the complete installation must be evaluated against the applicable area classification and electrical code. A conventional transformer enclosure should not be assumed suitable for a classified location without appropriate engineering review and certification.

Providing Transformer Dimensions and Mechanical Specifications

Custom transformers are frequently designed to fit existing substations, electrical rooms, equipment skids, or replacement footprints.

Manufacturers may request maximum dimensions, allowable weight, lifting restrictions, mounting arrangements, and required access clearances.

Additional mechanical requirements can include:

Primary and secondary cable entry locations.
Terminal and bushing configurations.
Bus duct or cable connection interfaces.
Pad-mounted or substation-style construction.
Enclosure material and finish.
Required lifting lugs, jacking points, and transportation restraints.
Seismic anchoring and structural support requirements.

For replacement transformers, existing equipment drawings, photographs, nameplate data, and field measurements can help manufacturers identify compatibility requirements.

Specifying Efficiency, Losses, and Temperature Rise

Transformer efficiency and losses influence operating costs, thermal performance, and equipment life.

Manufacturers may need maximum no-load losses, load losses, efficiency targets, allowable temperature rise, and expected operating load profile.

In the United States, applicable transformer efficiency requirements may fall under U.S. Department of Energy regulations in 10 CFR Part 431. Whether a particular custom transformer is covered depends on its design, ratings, and regulatory classification.

Canadian projects may be subject to Natural Resources Canada energy efficiency regulations and applicable provincial requirements.

Engineers should specify whether guaranteed losses, special efficiency targets, or loss evaluation factors are required for procurement.

Defining Transformer Accessories and Monitoring Equipment

Industrial and utility transformers may require additional protection, monitoring, and control accessories.

Depending on transformer construction, these can include:

Winding and liquid temperature indicators.
Resistance temperature detectors (RTDs).
Pressure relief devices.
Liquid level gauges and drain valves.
Sudden-pressure or gas-actuated protective devices.
Surge arresters and grounding provisions.
Cooling fans and automatic fan controls.
Current transformers for metering or protection.
Remote monitoring contacts and communication interfaces.

The manufacturer should receive the required accessory list, control voltages, alarm and trip functions, and communication requirements before the design is finalized.

IEEE, NEC, and CSA Standards for Custom Transformer Manufacturing

Applicable standards depend on transformer type, voltage class, installation location, and contractual requirements.

IEEE Transformer Design and Testing Standards

Commonly applicable IEEE standards include:

IEEE C57.12.00: General requirements for liquid-immersed distribution, power, and regulating transformers.
IEEE C57.12.90: Test code for liquid-immersed distribution, power, and regulating transformers.
IEEE C57.12.01: General requirements for dry-type distribution and power transformers.
IEEE C57.12.91: Test code for dry-type distribution and power transformers.
IEEE C57.110: Recommended practice for establishing transformer capability when supplying nonsinusoidal load currents.
IEEE C57.12.70: Terminal markings and connections for distribution and power transformers.

These standards are not universally applicable to every custom transformer. Specialty transformers, instrument transformers, rectifier transformers, and other equipment categories may be governed by additional or different standards.

National Electrical Code Requirements

NFPA 70, National Electrical Code (NEC), Article 450 addresses transformer installation requirements, including applicable protection, ventilation, and installation provisions.

Article 110 establishes general requirements for electrical equipment installation, including working space, equipment suitability, and installation in accordance with applicable instructions.

For medium-voltage installations, applicable requirements in NEC Articles 490 and 450 should also be evaluated. Article 242 may apply to surge arresters and other overvoltage protection equipment.

The NEC is primarily an installation code rather than a comprehensive transformer manufacturing standard. Equipment design and factory testing must be evaluated against the appropriate product standards and project specifications.

Canadian Transformer Requirements

Canadian projects may reference CSA C22.1, Canadian Electrical Code, Part I, along with applicable CSA transformer standards, including relevant portions of the CSA C88 and CSA C9 series.

Provincial and territorial electrical regulations, utility requirements, and project specifications may establish additional design, approval, and installation obligations.

Factory Testing and Documentation Required for Custom Transformers

Manufacturers should identify the required factory tests, inspection procedures, and documentation before production begins.

Typical transformer tests may include:

Winding resistance measurements.
Voltage ratio and phase relationship verification.
Impedance and load-loss measurements.
No-load loss and excitation-current measurements.
Applied-voltage and induced-voltage dielectric tests.
Insulation resistance measurements where specified.
Temperature-rise testing when required.
Insulating liquid tests for applicable liquid-immersed equipment.

Routine, design, and other specified tests must be distinguished according to the applicable IEEE or CSA standard. Not every test is required for every transformer.

Procurement documentation may also require certified drawings, test reports, wiring diagrams, accessory manuals, installation instructions, and inspection records.

Practical Example of a Custom Industrial Transformer Specification

Consider a manufacturing facility replacing an aging transformer that supplies a 480-volt motor control center from a 13.2 kV utility distribution system.

The initial specification might include:

Capacity: 1,000 kVA.
Primary voltage: 13,200 volts, three-phase.
Secondary voltage: 480Y/277 volts.
Frequency: 60 Hz.
Winding configuration: Delta primary, grounded-wye secondary.
Impedance: 5.75%, subject to engineering review.
Construction: Outdoor liquid-immersed transformer.
Cooling: Natural liquid and air cooling.
Primary BIL: 95 kV, subject to system insulation coordination.
Tap arrangement: De-energized primary taps, with range to be confirmed.
Installation: Existing outdoor concrete pad.
Accessories: Temperature indication, pressure relief, liquid level indication, and grounding provisions.
Standards: Applicable IEEE C57 requirements and local electrical installation codes.

Before manufacturing, the engineer would also confirm available fault current, protection coordination, grounding, load characteristics, installation clearances, dimensional constraints, and utility requirements.

Providing this information early allows the manufacturer to evaluate feasibility, identify potential design conflicts, and develop a more accurate technical proposal.

Common Mistakes When Ordering Custom Transformers

Incomplete specifications are a frequent source of delays and design revisions.

Common procurement errors include:

Providing only the transformer kVA rating and voltage.
Failing to identify the required winding connection or neutral configuration.
Omitting maximum operating voltage and tap requirements.
Ignoring impedance requirements and available fault current.
Failing to disclose nonlinear loads or unusual duty cycles.
Not specifying installation altitude, temperature, or environmental exposure.
Assuming a replacement transformer will fit an existing footprint without dimensional verification.
Leaving testing, documentation, and certification requirements undefined.

These issues can often be avoided by preparing a complete technical specification before requesting quotations.

Frequently Asked Questions About Custom Transformer Manufacturing

Minimum Information Needed to Request a Custom Transformer Quote

At minimum, manufacturers generally need the transformer capacity, primary and secondary voltages, phase, frequency, intended application, construction type, and installation location. Additional information such as impedance, winding connections, insulation levels, cooling requirements, and dimensions may be necessary before engineering and production can begin.

Whether Custom Transformers Can Be Built With Nonstandard Voltages

Yes. Custom transformers can be designed for specialized primary and secondary voltages, provided the electrical, insulation, thermal, and mechanical requirements are technically feasible and compliant with applicable standards.

Importance of Transformer Impedance in Custom Designs

Transformer impedance affects short-circuit current, voltage regulation, and load sharing during parallel operation. It should be established through system studies rather than selected solely from a typical catalog value.

Whether Manufacturers Need Existing Transformer Nameplate Information

For replacement projects, nameplate information is highly valuable. It identifies existing electrical ratings and may provide impedance, insulation, cooling, and connection details. Drawings and site measurements are also important because nameplate data alone may not establish mechanical compatibility.

Whether Custom Transformers Require Factory Testing

Yes. Applicable transformer standards specify required tests based on equipment type and rating. Additional tests may be requested by the purchaser to verify temperature rise, dielectric performance, losses, or other project-specific requirements.

Whether Custom Transformers Can Be Designed for Data Centers and Industrial Facilities

Yes. Custom transformers can support data centers, manufacturing facilities, renewable energy systems, utilities, and specialized industrial equipment. Applications involving harmonic-rich loads, redundancy, high fault levels, or restricted installation spaces may require additional engineering considerations.

Preparing a Complete Custom Transformer Specification

A successful custom transformer project begins with clearly defined electrical, mechanical, environmental, and performance requirements. Manufacturers need sufficient information to establish transformer capacity, winding design, insulation coordination, cooling requirements, physical construction, and applicable testing procedures.

Industrial buyers should coordinate with qualified electrical engineers and equipment manufacturers to confirm that the proposed transformer is compatible with the facility's power system, protection equipment, installation conditions, and operating requirements.

For additional information about available configurations and applications, visit Larson Electronics Industrial Transformers.

Larson Electronics offers after-sales support for low-voltage (LV), medium-voltage (MV), and high-voltage (HV) transformers and switchgear across North America, including assembly, installation, inspection, and commissioning services, subject to project requirements and availability.

For custom transformer specifications, engineering requirements, and application assistance, contact Larson Electronics.

Larson Electronics Building Trust Since 1973.