The primary and secondary windings of oil filled transformers are made with aluminium or electrolytic copper with enamelled insulation, Nomex or fiber glass according to the dimensions of conductor and insulating class.

The choice of windings type depends on the required voltage and power range.

Every high and low voltage winding layer is insulated with pure cellulose paper sized in such a way as to withstand all mechanical and electric stress.
Cast resin power transformers are made with disk windings of the typology adequate to voltage and currents involved.
For higher currents transposition cable or other alternative solutions are applied.

For higher voltages all technical and constructive details are adopted to uniform the electric field and distribute the capacity on the windings so that to protect the transformer from over-voltages, impulsive phenomena etc.