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Guide to transformer cores: types, construction, & purpose

Transformer cores ensure the efficient magnetic coupling between the windings. Learn about core types, how they are constructed, and what they do.

November 13, 2023

A distributed gap core being formed

This article will explore the most common core styles used in distribution transformers. There are two main types we’ll focus on–laminated, and wrapped (distributed gap) cores. To begin with, we’ll take a look at the basic function of the core and its associated parts.

What is a Transformer Core?

A transformer core is a structure of thin laminated sheets of ferrous metal (most commonly silicon steel) stacked together, that the primary and secondary windings of the transformer are wrapped around. 

A single lamination sheet of ferrous metal stacked to make a laminated core

Parts of the Core

A transformer core is composed of limbs and yokes that are joined together to form a single structure around which the coils are placed. The manner in which the respective yoke and limbs join together will depend on the type and design of the core.

A laminated stacked core showing the yokes and limbs
Above is a diagram of a laminated stacked core.


In the above example, the limbs of the core are the vertical sections which the coils are formed around. The limbs can also be located on the exterior of the outermost coils in the case of some core designs. The limbs on a transformer core can also be referred to as legs.


The yoke is the horizontal section of the core which joins the limbs together. The yoke and limbs form a pathway for magnetic flux to flow freely.

What does a transformer core do?

The transformer core ensures efficient magnetic coupling between the windings, facilitating the transfer of electrical energy from the primary side to the secondary side.

A transformer core graphic showing the primary and secondary windings wrapped around the core

When you have two coils of wire side by side and you pass an electric current through one of them, an electromagnetic field is induced in the second coil, which can be represented by several symmetrical lines with direction emanating from north to south pole–called lines of flux. With the coils alone, the path of the flux will be unfocused and the density of the flux will be low. 

Adding an iron core inside the coils focuses and magnifies the flux to make for a more efficient transfer of energy from primary to secondary. This is because the permeability of iron is much higher than that of air. If we think of electromagnetic flux like a bunch of cars going from one place to another, wrapping a coil around an iron core is like replacing a winding dirt road with an interstate highway. It’s much more efficient.

Core Construction

The densely packed laminated sheets that cores are constructed from, reduce overheating and circulating currents in the core and lowers energy loss. Removing any air gaps between the laminations will result in lower core losses and a higher efficiency as well. A core design with large gaps between laminated sheets will yield higher no load or iron losses and a lower efficiency.

Type of material cores are made from

The earliest transformer cores utilized solid iron, however, methods developed over the years to refine raw iron ore into more permeable materials such as silicon steel, which is used today for transformer core designs due to its higher permeability. Also, the use of many densely packed laminated sheets reduces issues of circulating currents and overheating caused by solid iron core designs. Further increases in core design are made through cold rolling, annealing, and using grain oriented steel.

Cold Rolling

Silicon steel is a softer metal. Cold rolling silicon steel will increase its strength–making it more durable when assembling the core and coils together.


The annealing process involves heating the core steel up to a high temperature to remove impurities. This process will increase the softness and ductility of the metal.

Grain Oriented Steel

Silicon steel already has a very high permeability, but this can be increased even further by orienting the grain of the steel in the same direction. Grain oriented steel can increase flux density by 30%.

Core and Coil Assembly Configurations

Another important point to consider is how the core is formed around the windings. The core and coils can be configured with either a shell type or core type design.

A core-type and shell-type transformer core diagram showing the differences in windings and magnetic flux


With a shell-type configuration, the core surrounds the windings. This creates a closed pathway surrounding the windings for magnetic flux to flow. This design also typically yields less energy loss than a core type design. A shell type design is the classification for most distribution class padmounts and substations with a wrapped 5-legged core. 


A core type design is where the windings surround the core steel. In this design, there is no return path (or closed loop) for the magnetic flux around the coils. This design typically yields more energy losses, and it requires more copper or aluminum winding material than a shell-type configuration. 

Three, Four, and Five Limb Cores

Three Limb Core

Three limb (or leg) cores are frequently used for distribution class dry-type transformers–both low and medium voltage types. The three limb stacked core design is also used for larger oil-filled power class transformers. It is less common to see a three limb core used for oil-filled distribution transformers.

Due to the absence of an outer limb(s), the three legged core alone is not suitable for wye-wye transformer configurations. As the picture below shows, there is no return path for the zero sequence flux which is present in wye-wye transformer designs. The zero sequence current, with no adequate return path, will attempt to create an alternate path, either using air gaps or the transformer tank itself, which can eventually lead to overheating and possibly transformer failure.

A three limb transformer core showing no return path for the zero flux

Buried Delta Tertiary Winding

One way this problem is solved is by adding a buried delta tertiary winding which provides a place for the flux to circulate freely. Other solutions for wye-wye transformers would include utilizing a 4-limb or 5-limb core as described below.

A wiring diagram for a buried delta tertiary winding transformer

Four Limb Core

Rather than employ a buried delta tertiary winding, a four limb core design provides one outer limb for return flux. This type of core design is very similar to a five limb design as well in its functionality, which helps to reduce overheating and additional transformer noise.

A four limb transformer core diagram

Five Limb Core

Five-legged wrapped core designs are the standard for all distribution transformer applications today (regardless of whether or not the unit is wye-wye). Since the cross sectional area of the three inner limbs surrounded by the coils is double the size of the three limb design, the cross sectional area of the yoke and outer limbs can be half that of the inner limbs. This helps conserve material and reduce production costs as well.

A five limb transformer core showing the windings and core
5-limb distributed gap core.

Types of transformers cores

There are two main types of transformer cores, distributed gap and laminated core (sometimes referred to as stacked)

Distributed Gap Core

Commonly referred to as a wrapped core, this core design is used in 3-ph and 1-ph distribution transformers. Distributed gap cores utilize a simple core clamp design which leads to faster assembly times and lower manufacturing costs. Another advantage of the distributed gap design is that there is only one cut per layer of laminated sheet. This type of core configuration is also referred to as a shell since the windings are surrounded by the core. As we mentioned earlier, this shell type design typically yields lower transformer losses. It’s efficient and lower cost design makes it the ideal choice for distribution transformers.

A distributed gap core

To form these distributed gap cores, multiple layers of laminated metal are placed inside a compression machine that forms their general shape.

(The picture below shows a distributed gap design. Each laminated sheet is formed around another and fit together with a step-lap joint)

A diagram of a distributed gap core with the step-lap joint

Laminated Core

A laminated core showing the step-lap mitred edge

Also referred to as a stacked core or E core, this type of core design employs a step-lap mitred configuration for the core sections. The laminations for each limb are grouped together in groups of 5 - 7 sheets–these groups are referred to as books. These sections of core steel are mitre cut and fit together as shown below.

A stacked or laminated core showing the mitre cut joined

(the photo above shows several books step-lapped and stacked together to form a point where an outer limb and the yoke are joined together by a mitre cut). 

A diagram showing the mitre cuts on a laminated or stacked core design

(The figure above shows the mitre cuts in the core which join the yoke and limbs together)

Amorphous & Nanocrystalline Cores

Although less common, amorphous cores are worth mentioning here–especially with growing demands for higher efficiency transformers (Check out our article on Transformer DOE Efficiency Standards). Known as tape wound cores, these core designs employ densely packed ribbon-like strips of highly permeable metallic-glass material. This makes for a more brittle core construction, but a higher efficiency. To keep the sheets together, amorphous cores are often encased in epoxy, or taped together. The no load losses are lower and core saturation levels are higher than conventional core designs. Amorphous cores are more costly than conventional cores, but their higher efficiency makes them a popular choice for special, custom applications.

There will be some loss of energy with any core configuration, but the better the core design, the less the no load losses. A well built transformer core help offset operating costs of the transformer and extend its lifespan.


Core designs have come a long way since the early days of transformer manufacturing, but the basic principles remain the same. Fill out the form below if you have any questions about transformer core design or are looking for a quote on a specific transformer.

Also check out our page on how transformers work for more information about cores, coils and electromagnetic induction.

Maddox padmount transformer loaded on truck

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