Aluminium conductor steel-reinforced (ACSR) is a 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 center 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 cable still depends on the tensile strength of the aluminium; it is only reinforced by the steel. Because of this, its continuous operating temperature is limited to 75 °C (167 °F), the temperature at which aluminium begins to anneal and soften over time. Cable which relies entirely on the steel for strength, and so can be used at temperatures up to 250 °C (480 °F), is called aluminium-conductor steel-supported (ACSS).
ACSR cables are available in several specific sizes, with multiple center 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. Cables generally range from #6 AWG ("6/1"—six outer aluminum conductors and one steel reinforcing conductor) to 2167 kcmil ("72/7"—seventy-two outer aluminum conductors and seven steel reinforcing conductors)
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 (11 or 12 mm for 60 or 50 Hz[5]) where only a small amount of AC current flows.