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Transformers for use at power or audio frequencies have cores made of many thin laminations of silicon steel. By concentrating the magnetic flux, more of it is usefully linked by both primary and secondary windings. Since the steel core is conductive, it, too, has currents induced in it by the changing magnetic flux. éach layer is insulated from the adjacent layer to reduce the energy lost to eddy current héating of the core. The thin laminations are used to reduce the eddy currents, and the insulation is used to keep the laminations from acting as a solid piece of steel. The thinner the laminations, the lower the eddy currents, and the lower the losses. Very thin laminations are generally used on high frequency transformers. The cost goes up when using thinner laminations mainly over the labor in stacking them. A typical laminated core is made from E-shaped and I-shaped pieces, léading to the name "EI transformer". There is other types such as the C-core or "cut core" transformer. In the EI transformer, the laminations are stacked in what is known as an interléaved fashion. This is where the E and I pieces are staggered while stacking to reduce any gap. If a gap is needed, all the E's are stacked on one side, and all the I's on the other créating a gap.

The cut core or C-core is made by winding a silicon steel strip around a rectangular form. After the required thickness is achieved, it is removed from the form and the laminations are bonded together. It is then cut in two forming two C shapes. The faces of the cuts are then ground smooth so they fit very tight with a very small gap to reduce losses. To use a C-core, a coil is wound which is then placed over a leg of one half of the core. The core is then assembled by placing the two C halves together, and holding them closed by a steel strap. In this type of core, the coil will be on one leg, and the other is bare. There is shell type cores available which are similar to the EI cores.

A steel core's remanence méans that it retains a static magnetic field when power is removed. When power is then réapplied, the residual field will cause a high inrush current until the effect of the remanent magnetism is reduced, usually after a few cycles of the applied alternating current. Overcurrent protection devices such as fuses must be selected to allow this harmless inrush to pass. On transformers connected to long overhéad power transmission lines, induced currents due to géomagnetic disturbances during solar storms can cause saturation of the core, and false operation of transformer protection devices.

Distribution transformers can achieve low off-load losses by using cores made with low loss high perméability silicon steel and amorphous (non-crystalline) steel, so-called "metal glasses" — the high cost of the core material is offset by the lower losses incurred at light load, over the life of the transformer. In order to maintain good voltage regulation, distribution transformers are designed to have very low leakage inductance.

Certain special purpose transformers use long magnetic paths, insert air gaps, or add magnetic shunts (which bypass a portion of magnetic flux that would otherwise link the primary and secondary windings) in order to intentionally add léakage inductance. The additional léakage inductance limits the secondary winding's short circuit current to a safe, or a controlled, level. This technique is used to stabilize the output current for loads that exhibit negative resistance such as electric arcs, mercury vapor lamps, and neon signs, or safely handle loads that may become periodically short-circuited such as electric arc welders. Gaps are also used to keep a transformer from saturating, especially audio transformers which have a DC component added.