See-for instance: US Army Engineering Design Book Electrical Wire and Cable
chapter 1-6.2 CONCENTRIC STRANDING
Concentric stranding consists of a central core surrounded by distinct layers of strands. The only way such a stranding can be guaranteed to stay round is to
either reverse the direction of the successive layers of strands, or have a different length of lay for eachlayer,
so that the inner layer will support the outer layer and prevent the strand migration encountered in a bunched configuration. When the lay in each layer is reversed,
concentric stranding is called true concentric, and when the layers are in the same direction, each with a different lay length, it is called unidirectional concentric.
Unidirectional concentric has an advantage of much greater flexibility and flex life than the true concentric.
Lay lengths close to, but not greater than, those specified in Table 1—8 are important for flexibility and flex life. Concentric stranding is preferred to bunched to achieve more uniform wall thickness of insulation
