Power Transformer Home

Power Transformer Co.   
The Power Transformer Resource     

transformer guide
Small kVA 3 Phase Transformers Guide

Distribution Transformers

Distribution Transformers

Distribution Transformers convert high-voltage electricity to lower voltage levels acceptable for use in homes and business.

To Order Distribution Transformers Call TEMCo.

Or 1-510-490-2187
Link: Distribution Transformer

Electrical energy is passed through distribution transformers to reduce high-distribution voltage levels down to end-use levels. Nearly all energy used in the United States passes through at least one distribution transformer before being consumed by an end-use appliance, motor, or other piece of equipment. Transformers are found in all sectors of the economy: residential, commercial, and industrial.

Distribution transformers are generally categorized in several ways:

type of insulation: liquid-immersed or dry-type
number of phases: single-phase or three-phase
voltage level (for dry-type): low or medium

General Purpose Distribution Transformers

 They are generally used for supply appliance, lighting, motorized machine and power loads from electrical distribution systems. They are either ventilated or totally enclosed, and are available with either aluminum or copper windings in standard ratings from 50VA up to 750 kVA.

Non-Linear (K-Factor) Distribution Transformers

The use of electronic equipment has continued to grow in both offices and industrial plants. Offices now include computers, fax machines, copier, printers, cash registers, UPS's and solid-state ballasts. They all contribute to the distortion of the current waveform and the generation of Harmonics. The K-Factor transformer are designed to tolerate the harmonic distortion associated with these loads.

Energy Efficient Distribution Transformers

These distribution transformers are energy efficient, dry type, general purpose transformers to meet NEMA TP-1 specifications. In the past several years, there has been an accelerated rate of change to introduce energy efficiency standards for transformers. Many products have preceded the initiatives for transformers in an overall effort to optimize manufactured goods for environmental concerns. Governments are encouraging users to consider energy efficient products like dry type transformers, in an effort to help reduce carbon dioxide emissions. In addition to the benefits to the environment, energy efficient transformers also can realize substantial savings in operating costs thereby have a direct impact on the initial investment evaluated over a period of time. 

Distribution Transformer Basics

Copper or aluminum conductors are wound around a magnetic core to transform current from one voltage to another. Liquid insulation material or air (dry-type) surrounds the transformer core and conductors to cool and electrically insulate the transformer.

Since small distribution transformers do not generate much heat, a higher proportion of theses tend to be dry-type. Dry-types are less flammable, and are therefore often selected for use when they must be located in confined spaces on a customer's premises.

Distribution transformers are used in electric power systems. The final part of the distribution system at medium voltage are the distribution transformers. Due to the Iow impedance voltage, this type of power distribution transformer will not substantially limit the short circuit current in the case of a fault on its secondary side. It is therefore common practice that power distribution transformers have to be type tested to their ultimate short-circuit conditions. Power distribution transformers may be oil filled or dry-filled.  Manufacturer offers a complete range of power distribution transformers, which can be certified in one test access very efficiently.

You use a power distribution transformer when you have no interest in what happens below the LV terminal of the power distribution transformer. You use a power distribution transformer element if you are interested in modeling loads or lines on the LV side of the power distribution transformers. If you have a node with a power distribution transformer, any loads on that node are assumed to be applied at the LV side of the power distribution transformer. Modeling the power distribution transformer give you the benefit of computer values for secondary voltages, and transformer losses due to series (or copper) losses and due to no load (or core energization losses.

Transformer Design Characteristics:
Transformers consist of two primary components:

  • A core made of magnetically permeable material.

  • A conductor, or winding, typically made of a low resistance material such as aluminum or copper.

Transformer Loss Basics

Core Losses are constant. Winding Losses increase exponentially with the square of the load.

A Transformer uses the core's magnetic properties and current in the  primary winding (connected to the source of electricity) to induce a current in the secondary winding (connected to the output or load). Alternating current in the primary winding induces a magnetic flux in the core, which in turn induces a voltage in the secondary winding. A voltage step-down results from the exchange of voltage to current, and its magnitude is determined by the ratio of turns in the primary and secondary windings. A transformer with 50 primary turns and five secondary turns would step the voltage down by a factor of 10, for example from 13,500 volts to 1,350 volts.

A given transformer's energy output is lower than the level specified by the nameplate rating due to inefficiencies in both the core and the windings. In general, transformer losses are less than two percent of the total transformer load. 

The magnitude of the losses is dependent upon the loading of the transformer. Core losses remain constant while winding losses increase with the square of the load. Thus, for a transformer with an average load of 25 percent, the core losses may represent approximately 75 percent of total energy losses. Conversely at 100 percent of rated load, the winding losses may represent more than 80 percent of total energy losses. Thus, core losses make-up a greater share of total losses at lower transformer loads, while the winding losses make-up a greater share of total losses at higher transformer loads.

Many different distribution transformer designs are available, depending on the loading patterns and needs of the end-user. Transformer engineers modify transformer design and vary material depending upon circumstances. Transformer design includes variation of :

  • The material used for the core

  • The material used for the winding.

  • The material that insulates the core and the winding.

  • The number of phases of the current that passes through the transformer.

  • Mounting and the rated size.

The following describe these factor in more detail.

Core Material

Transformer cores are usually made of either grain-oriented silicon steel or amorphous metal. Silicon steel comes in a variety of grades, each with its own conductive and efficiency characteristics. Amorphous metal, a more costly but highly efficient material, can significantly reduce core losses. Constructing the core of laminated sheets, insulated from each other, also reduce losses, but adds to the cost, weight and volume of the transformers.

The type of core material preferred by a utility is usually dependent on the cost of its core losses and the expected transformer loading levels. Since the marginal cost of energy for electricity utilities is usually rather low, the financial incentive for moving to high efficiency transformer materials may be limited.

Winding Material

Generally, copper and aluminum are used for transformer windings. As with silicon steel, these materials are available in a variety of grades and thicknesses, each with their own efficiency characteristics. The types of windings chosen by the transformer designer are also dependent on the cost of a specific utility's losses and on assumed transformer loading levels.

Insulating Material

The majority of utility distribution transformers are liquid filled. The non-conducting liquid (mineral oil is most commonly used) serves to electrically insulate and cool the transformer. As the core temperature of the transformer rises, the efficiency decreases, so an efficient cooling method improves performance. Typically, transformers perform best at temperatures below 55 C above the ambient temperature.

Liquid-filled transformers transfer heat more efficiently than dry-type transformers and are generally preferred for larger applications. Most liquids used in transformers now are non-flammable.


Transformers may be designed to step down a single alternating current from one voltage to another, call single-phase transformers, or contain three primary and three secondary windings and therefore provide the output in three-phases. Three-phase transformers induce a more constant magnetic flux and output voltage necessary for motors, heating, ventilating, air-conditioning and other large equipment. Technically, the three-phase transformer is equally efficient to the single-phase transformer.

In the US, over 95% of average annual transformer sales comprise single-phase distribution units, mostly in the 15kVA to 25kVa range.  Since the larger transformers tend to be three-phase, these comprise 38% of the total MVA sold each year.


Distribution transformers are either mounted on an overhead pole or on a concrete pad at ground level. There is some evidence to suggest that pole mounted transformers dissipate heat more easily than pad mounted units and may therefore be more fully loaded.

Correct Sizing

Properly sizing a transformer for a given application has a significant impact on the overall transformer efficiency and energy loss level. Oversized transformers are lightly loaded, and consequently lose more energy from excess core losses than optimally sized transformers. Conversely, undersized transformers operate at higher load levels and experience high load losses.

Energy Losses from New Distribution Transformers

A new 50kVA single-phase utility transformer will produce approximately 70 MWh in energy losses. An average 1,500kVA liquid-filled three-phase transformer will produce approximately 1.4GWh in losses over its life time.

Studies in the US have shown that new, efficient transformer designs could reduce energy losses and associated air emissions by 10 percent to more than 40 percent, depending on materials used and the loading pattern of the transformer.

Advance in transformer design have produced substantial transformer efficiency improvements over the past 20 years. The most significant improvements have been made in core technologies with the use of high-efficiency silicon-steel and amorphous metal. Due to the large numbers of transformers in service and the constant nature of energy losses, only small increases in efficiency are need to produce significant economic and environmental gains.

Sources of Transformer Efficiency

Transformer energy losses can be reduced by improving the efficiency of the core or windings. The relative importance of core and winding losses depends on the loading on the transformer and the cost of each type of loss to the utility.

Core Loss Reductions

Since the majority of transformer losses at low load levels are due to core inefficiencies, much of the research on reducing transformer losses has concentrated on building more efficient cores. Core losses result from cyclic changes in the magnetic state of iron, and "eddy-current" losses caused by the flow of small currents in the iron. Core losses can be reduced by improving the magnetic permeability of the core material or by using a core material that offers less magnetic resistance.

Considerable progress in reducing core losses has been made over the past twenty years, primarily through material improvements. In the early 1970's manufactures introduced more efficient silicon-steels. The four main grades of silicon-steel used in transformers are M2, M3, M4 and M6(decreasing in efficiency). Differences are due mainly to the chemical composition and the rolling techniques used in manufacture of the core. The increased domestic availability of higher grades of silicon-steel (M2 and M3) and new manufacturing  processes has led to the improved efficiency of silicon-steel distribution transformers.

Amorhous metal, a highly efficient material used in transformer cores, possesses good magnetic properties, low inherent magnetic resistance losses, and high resistively. Due to its ability to be constructed into very thin sheets, "eddy-current" losses are significantly reduced. Amorphous metals have been found to reduce core losses by as much as 70 percent. However, the cost of transformers with more efficient cores increases due to the following factors:

  • Increasing core efficiency requires the use of more core material.

  • The larger core size associated with the energy-efficient transformer necessitates the use of additional winding material, generally resulting in lower winding efficiencies and other cost.

  • The thin lamination of amorphous metal tends to make the core material more difficult to handle.

  • Certain types of efficient transformers may encounter specific problems, such as the difficulties associated with larger and heavier transformer design.

Winding Loss Reductions

Winding losses, or load-losses, arise from the conducting material's inherent resistance to the flow of electrical current. Winding losses increase with the square of the transformer load. Efficiency gains can be achieved by using materials with lower resistively or greater diameters. For example, distribution transformer coils made with low resistively conductors, such as copper, can have considerable lower load losses than those made with other materials. However, low resistively conductors often cost more than other conducting material.

Importance of Sizing

Overall transformer efficiency depends critically on the percent of time that the transformer is heavily or lightly loaded. the load factor.

Transformers need to be sized to cope with expected peak loads, rather that average loads, and therefore where there is a large disparity between these two, the load factor will be small.

For example, distribution transformers serving primarily residential loads regularly carry average loads that are only 15 percent to 20 percent of the transformer's rated capacity but also must be designed to support peak morning and evening loads. Because of the wide gap between peak and non-peak loads, and the relatively limited amount of time that the transformer is peak-loaded, average transformer loading tends to be fairly low. In this case, total losses may be mainly attributed to core losses.

Larger distribution transformers, used more often in transforming power for commercial or industrial customers, tend to be loaded at higher average levels over the course of the year. Transformers that serve businesses operating from 9:00 am to 5:00 pm, for example, typically experience a consistent and relatively higher load throughout the day. In this circumstance, it is likely that load losses will make the major contribution to total losses.

Correctly sizing a transformer is therefore critical to the quantity and source of losses, and optimizing transformer design for efficiency remains a complex task.

To complicate matters, the marginal cost of energy varies dramatically throughout the day, altering the cost of energy losses and the cost-benefit of installing more efficient transformers from the utility perspective.

In addition, transformer loading patterns tend to change over time. Homeowners may accumulate more appliances and equipment (or new houses built in the area), or businesses may expand and consequently increase the load on the transformer. Generally, utilities estimate load growth when sizing and purchasing transformers. In the Us it has been calculated that, on average, utilities size single-phase transformers so that transformer peak load at installation is approximately 88 percent of its capacity, and 157 percent of capacity at the end of its service life.

In an effort to improve transformer sizing practices,  the U.S. Environmental Protection Agency as part of the Energy Star Transformer Program, has developed technical tools to enable utilities to enter utility-specific loading information in order to optimize transformer size and total owning cost which providing reliability and energy-efficiency.

Power Transformer Sources: Power Transformer * Step Up Transformer * Step Down Transformer * Electrical Transformer * Isolation Transformer * Toroidal Transformer * Acme Transformer * High Voltage Transformer * Distribution Transformer * Transformer Manufacturer * Three Phase Transformer * Dry Type Transformer * 3 Phase Transformer * Cast Transformer * Voltage Transformers * Variac * Voltage Stabilizer * Voltage Regulators

Power Transformer Information:

Power Transformer HomeContact Power Transformer Co.

Power Transformer Types

Step Up and Step Down Transformers Step Up and Step Down Transformers to Power transformers to step-up ( raise) or step-down (lower) the electrical voltage.
Isolation Transformers Isolation Transformers allows signal or power to be taken from one device and fed into another without electrically connecting the two.
Toroidal Transformers Toroidal Transformers are devices that transfer electrical energy from one electric circuit to another, without changing the frequency, by electromagnetic induction.
Custom Transformers
Custom Transformers are designed to meet certain performance specifications and size requirement that you require. There is a wide range of custom transformer types.
Buck Boost Transformers
Buck Boost Transformers is a ideal solution for changing line voltage by small amounts. Often used to buck (lower), or boost (raise) the voltage from 208v to 240v for lighting applications.
Pole Mounted Transformers
Pole Mounted Transformers are mounted to poles for overhead electrical lines. Used in various applications. Are available in single phase or three phase transformers.
Medium Voltage Transformers
Medium Voltage Transformers are used with a medium range of voltages. They come in a full range from liquid-filled, convention dry type as well as cast coil.
Pad Mounted Transformers Pad Mounted Transformers are a excellent choice for commercial and industrial such as manufacturing facilities, refineries, office buildings, schools, hospitals, restaurants, and retail stores. They come in various sizes and can be used underground as well.
High Voltage Transformers High Voltage Transformers typically these voltage transformers are used in power transmission applications. High voltage transformers are also used in microwave.

Power Transformer Manufacturer

  • ACME Transformers - With Acme Electric being in business over 80 years, they have always believed in offering there customers superior service, quality and technical expertise in the transformer market.
  • AMVECO Transformers - AMVECO designs and manufactures toroids transformers, current transformers, and auto transformers. Most AMVECO products are custom designed utilizing their state-of-art proprietary CAD programs.  The AMVECO engineers can quickly generate designs in a matter of hours, if needed.
  • Federal Pacific TransformersFederal Pacific is a division of Electro- Mechanical Corporation, a privately held, American owned company founded in 1958. Federal pacific offers dry-type transformers from .050 KVA through 10,000 KVA single and three phase, up to 34.5 KV, 150 KV BIL with UL approval through 15 KV.
  • Marcus Transformer - Ever since they opened their doors for business a half a century ago, they have been a leader in innovative transformer design. As a family-owned company they are proud of the reputation they have earned for making quality-built transformers that deliver exceptional performance and savings.
  • Hammond TransformersHammond Manufacturing was founded in 1917 in Guelph, Ontario, Canada. In the last 3 decades it has expanded to the US and the international markets offering many types of power transformers. 
  • TEMCo Transformers - TEMCo  a family-owned business has been manufacturing and distributing electrical products since 1968. They  focus on transformers that significantly reduce power consumption over 30 percent compared to competitive makes.
  • GE Transformers - GE has been a key player in the energy industry for more than a century.  Since the installation of their first steam turbine in 1901. They have become number one provider of high-technology power generation equipment.
  • Jefferson Electric Transformers - Jefferson Electric has been a pioneer and innovator of magnetic products since 1915. Jefferson broad line of dry-type transformers are backed by quality assurance systems so stringent that each and every unit gets thoroughly tested before it goes out there door.
  • More power transformer brands - Check out more companies by clinking this link.

 Power Transformer Types

  • Distribution Transformers - Distribution Transformers are generally used in electrical power distribution and transmission power. This class of transformer has the highest power, or volt-ampere ratings. and the highest continuous  voltage rating.
  • Substation Transformers - Substation transformers are large devices which usually weigh tens of thousands of pounds.   They are filled with tens of thousands of gallons of heat transfer fluid.  Although they are typically 99.8% efficient in the transforming of electricity from one voltage to another, processing hundreds of Mega Volts-Amps of electricity force the liberation of hundreds of BTUs per second.
  • Medical Grade Isolation
     Transformer -
    Medical Grade Transformers generally refer to the transformers used in medical devices as well as hospital, biomedical and patient care equipment. There are a number of strict safety rules, guidelines and laws governing the design, construction and the test of these transformers.
  • Drive Isolation Transformer - They are used to adjust the speed of induction motors by varying the frequency. They are becoming more popular because of their impact on energy saving, and they are easy to control.
  • Toroidal Transformers - Toroidal transformers are more efficient than the cheaper laminated EI types of similar power level. Some of the advantages are smaller size, lower weight, less mechanical hum, (making them superior in audio amplifier), low-off-load loss.
Capacitor for Motor Resources


 Power Transformer Types

  • Step-Up Transformers - A Step-Up Transformer is one whose secondary voltage is greater than its primary voltage.  This kind of transformer "steps up" the voltage applied to it. -
  • Step-Down Transformers - A Step-Down Transformer is  designed to reduce voltage from primary to secondary.  They can range from sizes from .05 KVA to 500 KVA
  • Isolation Transformers - An Isolation Transformer is a device that transfers energy from the alternating current (AC) supply to an electrical or electronic load.  It isolates the windings to prevent transmitting certain types of harmonics.
  • Buck Boost Transformers - Buck Boost Transformers make small adjustments to the incoming voltage. They are often used to change voltage from 208v to 240v for lighting applications.  Major advantages of Buck boost transformers include; low cost, compact size and light weight. 
  • High Voltage Transformer - There are many different types of voltage transformers. A High Voltage Transformer operates with high voltages. Typically, these voltage transformers are used in power transmission applications, where voltages are high enough to present a safety hazard.
  • Medium Voltage Transformers - A Medium Voltage Transformer can be connected directly to a primary distribution circuit and generally has the most load diversity. These voltage transformers have installation practices that are generally in accordance with application recommendations from the Institute of Electrical and Electronic Engineers (IEEE).
  • Low Voltage Transformers - A Low Voltage Transformer is an electrical device that transforms 120 volts (line voltage) into 12 volts or 24 volts (low voltage). Some uses for low voltage transformer are in landscaping lighting.
  • Single Phase Transformers - In electrical engineering, single-phase electric power refers to the distribution of electric power using a system in which all the voltages of the supply vary in unison. Single-phase distribution is used when loads are mostly lighting and heating, with few large electric motors.
  • Three Phase Transformers - Three Phase Transformers must have 3 coils or windings connected in the proper sequence in order to match the incoming power and therefore transform the power company voltage to the level of voltage needed while maintaining the proper phasing or polarity.
  • Custom Transformers - Custom Transformers are designed for a certain performance specifications and size requirements.  The company works with your engineering specification. 
  • Industrial Control Transformers - Industrial Control Transformers are used to convert the available supply voltage to the required voltage to supply industrial control circuits and motor control loads.
  • Pad Mounted Transformers - Pad Mounted Transformers are usually single phase, or three phase, and used where safety is a main concern. Typical applications; restaurant, commercial building, shopping mall, institutional. 
  • Pole Mounted Transformers - Pole Mounted Transformers are used for distribution in areas with overhead primary lines. Outside a typical house one can see one of these devices mounted on the top of an electrical pole.
  • Oil Filled Transformers - Oil Filled Transformers are transformers that use insulating oil as insulating materials.  The oil helps cool the transformer. Because it also provides part of the electrical insulation between internal live parts, transformer oil must remain stable at high temperatures over an extended period.
  • Dry Type Transformers - Dry-Type Transformers are available for voltages up through 34.5 kV (although the most common upper limit is 15) and KVA ratings up through 10,000 (with 5000 as the usual limit). Dry-type use air as a coolant, lowering health and environmentally concerns.
  • Auto Transformers - An Autotransformer is an electrical transformer with only one winding. The winding has at least three electrical connection points called taps. Autotransformers are frequently used in power applications to interconnect systems operating at different voltage classes, for example 138 kV to 66 kV for transmission. Another application is in industry to adapt machinery built for 480 V supplies to operate on the local 600 V supply.
  • More power transformer types - Read further about additional transformer types and their uses.



 Power Transformer Term Definitions

  • Electrical Transformers - Electrical Transformers are devices used to raise or lower the voltage of alternating current. For instance, power is transported over long distance in high voltage power lines and then transformers lower the voltage so that the power can be used by a business or household.
  • Isolating Transformers - An Isolating Transformer is a transformer, often with symmetrical windings, which is used to decouple two circuits.  An Isolation transformer allows an AC signal or power to be taken from one device and fed into another without electrically connecting the two circuits. Isolation transformers block transmission of DC signals from one circuit to the other, but allow AC signals to pass. 
  • Transmission Power Lines - A Transmission Line is the material medium or structure that forms all or part of a path from one place to another for directing the transmission of energy, such as electromagnetic or acoustic waves as well as electric power transmission. Components of transmission lines include wires, coaxial cables,  dielectric slabs, option fibers, electric power lines, and waveguides.
  • Transformer Voltage - The measure of the amount of force on a unit charge because of the surrounding charge.
  • Transformer Phase - Most transformer are either single phase or three phase.
  • Transformer Frequency - The transformer cannot change the frequency of the supply. If the supply is 60 hertz, the output will also be 60 hertz.
  • Transformer K Factor - Some transformers are now being offered with a k-factor rating. This measure the transformer's ability to withstand the heating effects of non-sinusoidal harmonic currents produced by much of today's electronic equipment and certain electrical equipment.
  • Primary Voltage - The coil winding that is directly connected to the input power.
  • Secondary Voltage - The coil winding  supplying the output voltage.
  • Harmonic Cancellation - Harmonic cancellation is performed with harmonic canceling transformers also known as phase-shifting transformers. A harmonic canceling transformer is a relatively new power quality product for mitigating harmonic problems in electrical distribution systems. This type of transformer has patented built-in electromagnetic technology designed to remove high neutral current and the most harmful harmonics from the 3rd through 21st.
  • Weatherproof - Enclosed transformers come with a weatherproof standard set by NEMA.
  • Epoxy Encapsulated - A process in which a transformer or one of its components is completely sealed with epoxy or a similar material. This process is normally preferred when a unit might encounter harsh environmental conditions.
  • More power transformer terms - Such as inductor, ground fault, core saturation, current transformer, faraday shield, etc.

Related Transformer Products

  • Voltage Regulators - A Voltage Regulator is an electrical regulator designed to automatically maintain a constant voltage level.  It may use an electromechanical mechanism, or passive or active electronic components. Depending on the design, it may be used to regulate one or more AC or DC voltages.
  • AC Line Reactor - AC Line Reactors is a three phase transformer used in conjunction with AC variable frequency and DC motor drive. They are a bi-directional protective filtering device.
  • Line Power Conditioners - Power or Line Conditioners regulate, filter, and suppress noise in AC power for sensitive computer and other solid state equipment.
  • DC Power Supplies - Conversion of one form of electrical power to another desired form and voltage. This typically involves converting 120 or 240 volt AC supplied by a utility company to a well-regulated lower voltage DC for electronic devices.
  • Rotary Phase Converters - Rotary Phase Converters are commonly used in home or small commercial or industrial settings. Rotary phase converters convert single-phase power into three-phase power. This is a very cost-effective way to power three-phase electric motors and other three phase equipment.
  • Frequency Converters - A Frequency Changer or Frequency Converter is an electronic device that converts alternating current (AC) of one frequency to alternating current of another frequency.
  • Voltage Converters - A Voltage Converter changes the voltage of an electrical power source and is usually combined with other components to create a power supply.
  • Magnetic Motor Starters - Magnetic Motor Starters are essentially heavy duty relays mounted in boxes, often equipped with heater/thermal overloads matched to the motor they start.
  • Motor Starting Auto Transformers - An Auto Transformer starter uses an auto transformer to reduce the voltage applied to a motor during start. The auto transformer may have a number of output taps and be set-up to provide a single stage starter, or a multistage starter.

For an additional resource the Best of Industry Web Directory : Electrical Power Transformer Directory section is quite useful.