The concentric stranding is most commonly used for power cable conductors. The construction of concentric-type cable consists of a central core surrounded by one or more layers of helically applied wires.
The first layer has six wires and each subsequent layer has six more wires than the preceding layer. In this type of cable construction, the core consists of single wire and all of the strands have the same diameter.
The first layer over the core contains 6 wires, the second contains 12 wires, the third 18, and so on. The following types of strandings are used in this application.
Class B: This class of stranding is used exclusively for industrial power cables for application in 600 V, 5 kV, and 15 kV power systems. The cable stranding usually consists of 7 (#2 AWG), 19 (#4/0 AWG), 37 (500 kcmil), or 61 (750 kcmil) strands.
Classes C and D: These classes are used where a more flexible cable is required. Class C uses 19, 37, 61, or 91 strands and class D uses 37, 61, 91, or 127 strands for the #2 AWG, #4/0 AWG, 500 kcmil, and 750 kcmil cable construction, respectively.