Pad-Mounted Transformer Sizing Guide: kVA, Primary Voltage and Secondary Voltage (8/19/2026)

Q: How many amps can a 1500 kVA transformer supply at 480 volts?

A 1500 kVA three-phase transformer rated 480 V on the secondary can supply approximately 1,804 amperes at rated output.

Q: How is primary voltage selected for a pad-mounted transformer?

Primary voltage must match the nominal electrical distribution system supplying the transformer, including applicable system configuration, grounding and voltage class.

Q: How is secondary voltage selected for a pad-mounted transformer?

Secondary voltage is selected according to the downstream distribution system and connected loads. Common industrial examples include 480Y/277 V and 208Y/120 V.

Q: Does a larger transformer increase available fault current?

It can. Transformer kVA and percent impedance affect available secondary fault current, so increasing transformer capacity may require reevaluation of downstream short-circuit ratings, protective-device coordination and arc-flash calculations.

Pad-mounted transformer sizing requires matching transformer kVA, primary voltage and secondary voltage to the actual electrical system and load profile. Engineers should evaluate demand, continuous loads, motor starting, future expansion, system configuration, available fault current, impedance, grounding and utility requirements before selecting a transformer.

By LarsonElectronics.com, August 19, 2026

Correctly sizing a pad-mounted transformer requires three fundamental decisions: how much apparent power the transformer must supply, what primary voltage is available and what secondary voltage the facility requires. These correspond to the transformer's kVA rating, primary voltage and secondary voltage. However, a reliable industrial design also considers demand, continuous loading, motor starting, nonlinear loads, future expansion, system grounding, transformer impedance, available fault current and site conditions.

For industrial projects, transformer sizing should begin with the electrical load study and one-line diagram rather than selecting a transformer from a catalog. A transformer that is too small may experience excessive loading and temperature rise, while excessive oversizing can increase capital cost and no-load losses without improving system performance.

This guide focuses primarily on three-phase liquid-immersed pad-mounted distribution transformers used for industrial and commercial power distribution. IEEE C57.12.34-2022 covers three-phase, 60 Hz, liquid-immersed, self-cooled, compartmental-type pad-mounted distribution transformers rated 10 MVA and smaller, with high-voltage systems through 34.5 kV nominal and low-voltage systems through 15 kV nominal.

Pad-Mounted Transformer Sizing Starts With kVA, Primary Voltage and Secondary Voltage

The three basic ratings required to begin selecting a pad-mounted transformer are:

kVA rating - the apparent-power capacity required to serve the load
Primary voltage - the voltage of the electrical distribution system supplying the transformer
Secondary voltage - the voltage required by the facility or downstream distribution equipment

For example, an industrial facility might require a 1500 kVA, three-phase transformer supplied from a 13.8 kV distribution system with a 480Y/277 V secondary.

That information establishes the basic transformer rating, but it is not a complete transformer specification. Phase, frequency, winding connections, grounding, impedance, BIL, taps, radial or loop feed, dead-front or live-front construction, protection and other characteristics must also be defined before equipment is ordered.

Transformer kVA Represents Apparent-Power Capacity

Transformers are rated in kilovolt-amperes rather than kilowatts because their thermal loading depends on voltage and current, not simply the real power consumed by the load.

For a balanced three-phase system:

kVA = (1.732 × Line Voltage × Line Current) / 1000

For a single-phase system:

kVA = (Voltage × Current) / 1000

These formulas are useful for calculating apparent load from measured or expected current. They should not be used in isolation to select the final transformer rating because the load profile may include motors, intermittent equipment, continuous loads and future expansion.

Transformer kVA Should Be Based on Demand Rather Than Connected Load Alone

Connected load is the sum of equipment that could theoretically operate on the system. Actual transformer demand may be different because many industrial loads do not operate simultaneously or continuously at full nameplate rating.

A transformer sizing study should evaluate:

Connected load
Maximum expected demand
Continuous loads
Intermittent loads
Motor loads
Motor starting requirements
Heating loads
Lighting loads
Variable frequency drives
UPS and rectifier loads
Welders and other cyclic loads
Harmonic-producing equipment
Planned facility expansion

For an existing facility, measured demand data can be particularly useful. For a new facility, the engineer typically develops a load schedule using equipment ratings, expected utilization and applicable demand factors.

A Three-Phase Current Calculation Helps Translate kVA Into System Amperage

Once transformer kVA and secondary voltage are known, the transformer's rated secondary current can be calculated.

For a three-phase transformer:

Current = (kVA × 1000) / (1.732 × Line Voltage)

For example, the full-load secondary current of a 1500 kVA transformer at 480 V is approximately:

(1500 × 1000) / (1.732 × 480) = 1,804 A

This immediately provides useful information for downstream engineering. A 1500 kVA, 480 V transformer can supply approximately 1,804 A at rated three-phase output, subject to its design and operating conditions.

The engineer can then evaluate switchgear bus ratings, conductors, protective devices and other downstream equipment against the expected load and transformer capacity.

Primary Voltage Must Match the Actual Source System

The primary voltage is determined by the electrical system feeding the transformer. It should not be selected independently from the utility or facility distribution system.

Industrial medium-voltage systems may use nominal voltages such as:

4.16 kV
12.47 kV
13.2 kV
13.8 kV
24.9 kV
34.5 kV

The exact source voltage should be confirmed from the facility one-line diagram, utility service information or measured and documented system characteristics as appropriate.

Voltage alone is not sufficient. The engineer should also identify the source system configuration and grounding method because these characteristics influence transformer winding connections, insulation requirements and protection.

Primary Voltage Class and BIL Are Different Specifications

Primary voltage identifies the nominal operating system voltage. Basic impulse insulation level, or BIL, describes insulation withstand capability for standardized impulse voltage stresses.

A transformer therefore should not be specified using primary voltage alone when BIL is an important project requirement. Insulation coordination, system voltage class, surge protection and utility or facility standards should be considered when establishing the required BIL.

IEEE C57 standards provide the technical framework for these transformer dielectric requirements.

Secondary Voltage Must Match the Facility Distribution System

The transformer's secondary voltage is determined by the voltage required by downstream loads and distribution equipment.

Common industrial secondary configurations include:

480Y/277 V
208Y/120 V
240 V delta
480 V delta
Medium-voltage secondary systems for larger distribution applications

A 480Y/277 V system is common in industrial and commercial facilities because it can supply 480 V three-phase loads while also providing 277 V line-to-neutral for compatible lighting and other loads.

A 208Y/120 V system is more commonly associated with facilities requiring significant quantities of 120 V utilization equipment.

The appropriate secondary voltage should be determined from the facility distribution architecture rather than selected solely because a particular transformer is readily available.

Primary and Secondary Winding Connections Affect the Electrical System

Voltage ratings do not completely describe a transformer. Winding connections must also be specified.

A common industrial arrangement is a delta primary with a grounded-wye secondary, but the correct configuration depends on the source system, facility grounding design, protection requirements and loads.

For example, a transformer specified as 13.8 kV to 480Y/277 V communicates considerably more useful information than simply stating "13.8 kV to 480 V."

The winding configuration affects phase relationships, grounding behavior and zero-sequence current paths and should therefore be coordinated with the overall power-system design.

Continuous Loads Must Be Considered in the Electrical Design

Transformer sizing should be coordinated with applicable National Electrical Code requirements for the installation. NEC Article 450 addresses transformers and transformer installations, while other NEC articles govern conductors, overcurrent protection, grounding and bonding and specific load types.

The NEC treatment of continuous loads is particularly important when sizing feeders and overcurrent protection associated with transformer systems. A continuous load is generally one where the maximum current is expected to continue for three hours or more, and applicable NEC rules may require conductors and overcurrent devices to be sized accordingly.

This should not be simplified into a universal rule that every transformer must be sized at 125% of all connected load. Transformer selection should be based on the actual load calculation, equipment ratings, applicable NEC provisions, manufacturer requirements and engineering design criteria.

Motor Starting Can Affect Transformer Sizing

Large motors can draw substantial current during acceleration. A transformer may have sufficient steady-state kVA capacity for a motor load while still experiencing an unacceptable voltage drop during starting.

For facilities containing large motors, engineers should consider:

Motor horsepower
Starting method
Locked-rotor current
Starting frequency
Simultaneous motor starts
Permissible voltage drop
Other loads operating during starting
Transformer impedance

For example, a manufacturing facility may operate several motors continuously while periodically starting a large compressor. The transformer must support both the running load and the transient starting condition without creating unacceptable voltage disturbance for other equipment.

Variable Frequency Drives and Nonlinear Loads Require Additional Evaluation

Modern industrial facilities frequently contain variable frequency drives, UPS systems, rectifiers, data processing equipment and other nonlinear loads. These devices can produce harmonic currents that increase transformer losses and heating.

Where nonlinear loading is significant, transformer sizing should account for the harmonic characteristics of the load rather than relying solely on fundamental-frequency kVA.

The appropriate solution may involve transformer design considerations, harmonic mitigation or other system-level measures depending on the application.

Future Expansion Should Be Planned Without Excessively Oversizing the Transformer

Industrial facilities frequently add production equipment, process lines, pumps, HVAC equipment or data loads after the original electrical system is installed. Providing reasonable capacity for known or likely expansion can prevent premature transformer replacement.

However, specifying a transformer dramatically larger than the expected load is not automatically beneficial. Excessive oversizing can increase:

Initial equipment cost
No-load energy losses
Physical footprint
Installation requirements
Potential available fault current depending on transformer characteristics

A practical approach is to identify known future loads and establish a documented growth allowance rather than selecting an arbitrary oversized transformer.

Transformer Impedance Influences Available Secondary Fault Current

Transformer impedance is not simply a manufacturer data point. It directly affects the available short-circuit current on the secondary system.

As a simplified approximation, ignoring upstream source impedance, maximum symmetrical secondary fault current at the transformer terminals can be estimated from:

Fault Current ˜ Full-Load Current × (100 / Percent Impedance)

For example, consider a 1500 kVA, 480 V transformer with approximately 1,804 A full-load current and 5.75% impedance:

1,804 × (100 / 5.75) ˜ 31,374 A

This simplified calculation suggests approximately 31.4 kA of symmetrical fault current at the transformer secondary terminals before accounting for upstream source impedance and other system impedances.

The actual short-circuit study should incorporate the complete electrical system. The example illustrates why transformer kVA and impedance must be considered together when selecting downstream switchgear and protective equipment.

Larger Transformer kVA Can Increase Available Fault Current

Increasing transformer capacity can have consequences beyond providing more load capacity. If voltage and percent impedance are similar, a larger transformer can make substantially more fault current available on the secondary system.

This becomes important when an industrial facility replaces an existing transformer with a larger unit to support expansion. Existing switchboards, panelboards, motor control centers and circuit breakers should be evaluated to verify that their short-circuit current ratings and interrupting ratings remain adequate.

A transformer upgrade should therefore be coordinated with a short-circuit study rather than treated solely as a capacity increase.

Pad-Mounted Transformer Ratings Should Be Selected From Standard Sizes When Practical

Pad-mounted distribution transformers are commonly available in standardized kVA ratings, although available ratings and configurations vary by manufacturer and application.

Example Transformer Rating	Approximate Full-Load Current at 480 V, Three Phase
500 kVA	601 A
750 kVA	902 A
1000 kVA	1,203 A
1500 kVA	1,804 A
2000 kVA	2,406 A
2500 kVA	3,007 A

These currents are calculated from rated kVA and 480 V three-phase output and are provided as engineering reference values. They do not represent recommended transformer sizes for a particular facility.

Utility Requirements Can Determine the Transformer Configuration

When a pad-mounted transformer connects directly to utility distribution, the serving utility may specify significant portions of the transformer design.

Utility requirements can address:

Primary voltage
Voltage class
Winding configuration
BIL
Radial or loop feed
Dead-front or live-front construction
Bushing interfaces
Fusing
Metering
Grounding
Concrete pad requirements
Clearances and access

These requirements should be obtained before transformer procurement. Ordering equipment first and attempting to obtain utility approval later can produce expensive compatibility problems.

Radial or Loop Feed Does Not Change the Basic kVA Calculation

Radial-feed and loop-feed configurations describe how the primary underground distribution system connects to the transformer. They do not fundamentally change the calculation of transformer kVA required by the secondary load.

However, the selected feed arrangement affects primary bushings, cable terminations, switching and the physical configuration of the high-voltage compartment.

IEEE C57.12.34-2022 specifically addresses connector, bushing and terminal arrangements for radial- and loop-feed systems within the scope of the standard.

Site Conditions Can Require Transformer Derating or Special Design Consideration

Transformer ratings assume specified service conditions. Installations outside those conditions may require engineering review.

Site conditions that should be identified during sizing include:

High ambient temperature
High elevation
Severe loading cycles
Coastal or corrosive environments
Industrial contamination
Restricted airflow
Seismic requirements
Flood exposure

IEEE C57.12.00-2021 establishes general electrical and mechanical requirements for liquid-immersed distribution, power and regulating transformers within its scope and provides an important foundation for specifying this equipment.

Replacement Transformer Sizing Requires Review of the Existing Electrical System

When replacing a pad-mounted transformer, increasing or decreasing kVA should not be done without evaluating the rest of the electrical system.

For a replacement project, engineers should document:

Existing transformer kVA
Primary voltage
Secondary voltage
Phase and frequency
Winding connections
Percent impedance
BIL
Tap configuration
Radial or loop feed
Dead-front or live-front construction
Bushing arrangement
Secondary terminals
Existing load
Available fault current
Physical dimensions
Concrete pad and cable-entry configuration

Changing transformer size can affect feeder loading, voltage regulation, fault current, protective-device coordination and arc-flash study results. A replacement transformer should therefore be evaluated as part of the distribution system rather than as an isolated component.

A Practical Pad-Mounted Transformer Sizing Example Shows the Selection Process

Consider an industrial facility with a calculated diversified demand of 1,050 kVA. Engineering has also identified approximately 200 kVA of planned near-term expansion. The facility receives 13.8 kV three-phase power and uses a 480Y/277 V distribution system.

The initial planning load becomes approximately 1,250 kVA. Rather than automatically choosing a transformer from that number alone, the engineer would evaluate available standard ratings, load characteristics, motor starting, continuous loading, harmonics, expected growth and operating margin.

If engineering analysis supports selection of a 1500 kVA transformer, the preliminary description might be:

1500 kVA
Three phase
60 Hz
13.8 kV primary
480Y/277 V secondary
Winding connection per system design
Impedance per short-circuit and coordination study
BIL per system requirements
Feed arrangement per site distribution design
Protection and taps per project specification

This is still a preliminary selection. The complete specification must address the remaining electrical, mechanical, environmental and utility requirements before procurement.

IEEE Standards Establish the Technical Framework for Pad-Mounted Transformer Selection

IEEE C57 standards provide important requirements and engineering guidance for liquid-immersed transformers. Relevant standards for many industrial pad-mounted transformer projects include:

IEEE C57.12.34-2022 for qualifying three-phase pad-mounted, compartmental-type, self-cooled distribution transformers rated 10 MVA and smaller, with high-voltage systems through 34.5 kV nominal and low-voltage systems through 15 kV nominal.
IEEE C57.12.00-2021 for general electrical and mechanical requirements applicable to liquid-immersed distribution, power and regulating transformers within its scope.
IEEE C57.12.90-2021 for testing of liquid-immersed distribution, power and regulating transformers.
IEEE C57.12.28-2023 for enclosure integrity requirements applicable to qualifying pad-mounted equipment.

The exact standards applicable to a project depend on transformer type, rating, voltage and application. Specifications should reference standards whose scopes actually match the equipment being purchased.

NEC Requirements Should Be Coordinated With Transformer Sizing

The National Electrical Code affects the installation surrounding a pad-mounted transformer. NEC Article 450 addresses transformers and transformer vaults, including requirements related to transformer protection and installation.

Other NEC requirements may apply to:

Primary and secondary conductors
Overcurrent protection
Grounding and bonding
Medium-voltage installations
Equipment clearances
Liquid-insulated transformer installations
Specific occupancies and loads

The applicable NEC edition is determined by the authority having jurisdiction. Utility standards and local requirements may also apply and can differ from requirements for customer-owned equipment.

Transformer Sizing and Transformer Specification Are Related but Different Tasks

Sizing determines the electrical capacity and voltage transformation required by the application. Specification defines the complete transformer needed to perform that job.

A sizing decision might determine that a facility requires a 1500 kVA transformer with a 13.8 kV primary and 480Y/277 V secondary.

The final specification must then establish additional requirements such as winding configuration, impedance, BIL, taps, radial or loop feed, dead-front or live-front construction, bushings, fusing, insulating liquid, accessories, enclosure construction and applicable standards.

Industrial buyers can review available industrial transformers and use the electrical requirements developed during the sizing study to narrow the appropriate equipment configuration.

After-Sales Support Should Be Planned Before the Transformer Arrives

Large transformers and switchgear may require field services before energization. Installation planning should address equipment placement, assembly requirements, electrical connections, inspections, testing and commissioning.

Larson Electronics provides after-sales support for (LV) low-voltage, (MV) medium-voltage and (HV) high-voltage transformers and switchgear, including assembly, installation, inspection and commissioning support across North America.

Coordinating these activities during procurement can help identify installation requirements before equipment reaches the jobsite and support an orderly transition from delivery through energization.

Frequently Asked Questions About Pad-Mounted Transformer Sizing

How do you size a pad-mounted transformer

Begin with the facility load calculation and determine maximum expected demand, continuous loads, motor starting requirements, nonlinear loads and planned expansion. Select transformer kVA based on the resulting load profile, then confirm primary voltage, secondary voltage, system configuration, impedance and other electrical requirements.

What does kVA mean on a pad-mounted transformer

kVA is the transformer's apparent-power rating. For a balanced three-phase system, kVA equals approximately 1.732 multiplied by line voltage and line current, divided by 1000.

How many amps can a 1500 kVA transformer supply at 480 volts

A 1500 kVA, three-phase transformer rated 480 V on the secondary can supply approximately 1,804 A at rated output. Actual application loading must remain consistent with transformer ratings and operating conditions.

How is primary voltage selected for a pad-mounted transformer

Primary voltage must match the nominal distribution system supplying the transformer. Engineers should verify the actual utility or facility system voltage, configuration, grounding and applicable voltage class rather than selecting primary voltage independently.

How is secondary voltage selected for a pad-mounted transformer

Secondary voltage is selected according to the downstream electrical distribution system and connected loads. Common industrial examples include 480Y/277 V and 208Y/120 V, although other voltages may be required.

Should a transformer be oversized for future growth

Reasonable capacity for documented future expansion can be included in transformer sizing, but excessive oversizing may increase equipment cost and no-load losses. Future loads should be estimated and incorporated into the engineering load study rather than using an arbitrary oversizing percentage.

Does a larger transformer increase available fault current

It can. Transformer kVA and percent impedance both affect available secondary fault current. Increasing transformer capacity may require reevaluation of downstream equipment short-circuit ratings, protective-device coordination and arc-flash calculations.

Which IEEE standard applies to three-phase pad-mounted transformers

IEEE C57.12.34-2022 applies to three-phase, 60 Hz, liquid-immersed, self-cooled, compartmental-type pad-mounted distribution transformers rated 10 MVA and smaller within the high- and low-voltage limits defined by the standard.

Does Larson Electronics provide transformer installation and commissioning support

Yes. Larson Electronics provides after-sales support for low-voltage, medium-voltage and high-voltage transformers and switchgear, including assembly, installation, inspection and commissioning support across North America.

For assistance selecting or specifying a pad-mounted transformer for an industrial, utility or infrastructure project, contact Larson Electronics to discuss the load, primary voltage, secondary voltage and application requirements.

Larson Electronics Building Trust Since 1973.