Transformer effect - the phenomenon occurring in steel-core aluminium overhead conductors, which causes redistribution of current density in non-ferrous layers of strands.¹⁾

Typical composite conductors used in overhead lines, made from all-aluminium strands or with steel-reinforced core

^{S. Zurek, E-Magnetica.pl, CC-BY-4.0}

Such redistribution causes increased effective AC resistance and decreased internal inductance of the conductor. The effect is a function of several factors, linked more directly to DC resistance, skin depth and proximity effect, and therefore also: total current flowing through the conductor, frequency, operating temperature, tensile stress of the steel core, number of layers of strands, as well as the lay lengths and their directions.

The transformer effect is stronger for non-even number of non-ferrous layers, especially 1 or 3. For even number of layers the effect self-cancels to a large degree.

• Overhead conductor