Copper or aluminium windings: what really changes in the design
Aluminium is lighter and usually cheaper per ampere, but it needs about 60 % more conductor area. That extra area moves the core, the impedance and the tank, not just the bill of materials.
· 7 min read · Design
The basic trade
Aluminium has roughly 1.6 times the resistivity of copper and about a third of its density. To carry the same current with the same resistive loss, an aluminium conductor needs about 60 % more cross-section, yet it still weighs only around half as much as the copper it replaces.
Whether that is cheaper depends on the metal prices on the day you quote, and those have moved a long way in both directions in recent years. What does not change is the geometry: the aluminium winding is bigger, and everything around it has to make room.
A bigger winding moves everything else
The extra conductor area shows up as radial build. A thicker winding needs a wider core window, which means longer yokes and more core steel, which raises the no-load loss and the core cost. The larger active part needs a larger tank and more oil. A design that is cheapest on copper can lose its advantage once the aluminium version has been grown to fit.
Impedance moves as well, because leakage reactance depends on the radial builds and the mean turn length. Converting a copper design to aluminium by simply enlarging the conductors usually puts the impedance outside its band, and the turns or the winding height then have to change to bring it back.
Strength, joints and the standard
Aluminium has a lower proof stress than copper, so the winding's ability to withstand the radial and axial forces of a short circuit needs checking rather than assuming. Jointing aluminium to copper leads and bushings needs the right technique to avoid corrosion and loosening over the service life.
Some standards and some buyers treat the two metals differently. India's IS 1180 sets loss limits that do not depend on the conductor, while some utility specifications state a preference or a separate set of guarantees for aluminium. Read the tender before assuming either metal is acceptable.
Decide with both designs in front of you
The only reliable way to choose is to design the unit properly in both metals, each meeting every guarantee, and compare the evaluated cost: material at today's rates plus the capitalised losses. That comparison often surprises people in both directions.
It also shows where each design sits against its limits, which matters as much as the price. A copper design with comfortable margins can be worth more than an aluminium design that only just passes.