Aluminium conductor steel-reinforced is an specific type of high-capacity, high-strength stranded conductor typically used in overhead power lines. The outer strands are high-purity 1350 or 1370 aluminium alloy, chosen for its excellent conductivity, low weight and low cost. The centre strands are of steel for the strength required to support the weight without stretching the aluminium due to its ductility. This gives the conductor an overall high tensile strength.
ACSR cables are available in several specific sizes, with multiple centre steel wires and correspondingly larger quantities of aluminium conductors. For example, an ACSR cable with 72 aluminium conductors that requires a core of 7 steel conductors will be called 72/7 ACSR cable. The higher resistance of the steel core is of little consequence to the transmission of electricity in large diameter conductors since it is located below the skin depth (8.5 or 9 mm for 60 or 50 Hz) where only a small amount of AC current flows.