Industrial pure aluminium foil is used for transformer windings and for other wiring use. Aluminium has higher resistivity to Copper so needs a larger wire size than Copper to achieve the same number of turns or in a given winding space Aluminium windings will have a larger resistance than if Copper is used.

In North America, aluminum is the predominant choice of winding material for low-voltage, dry-type transformers larger than 15 kilovolt-amperes(kVA). The primary reason for choosing aluminum windings is its lower initial cost. 

The cost of copper base metal has historically proven to be much more volatile than the cost of aluminum, so that the purchase price of copper conductor generally is the most expensive choice.

Also, because aluminum has greater malleability and is easier to weld, it is the lower-cost manufacturing choice. However, reliable aluminum connections require more discipline and expertise on the part of transformer installers than that needed for copper connections.

Aluminum wound transformers are lighter in weight than copper wound equivalents.The HV windings are wound from aluminium foil interleaved with an insulating foil.  The LV windings are wound form single-aluminium sheets and interleaved with cast-resin impregnated fibreglass fabric. Foil type windings are subject to less electrical stress than conventional windings resulting in higher AC and impulse voltage withstand characteristics. Thermal expansion coefficients of aluminium and cast resin are similar and resulting thermal stress due to load change is kept to a minimum.

Related article:  1350 Aluminum Foil and Its Applications