Cable Grouping Factor

When cables are installed close together, the heat generated by each cable affects the temperature of its neighbours. This mutual heating raises conductor temperatures above what they would reach in isolation, requiring each cable to carry less current to stay within its insulation temperature limit. The grouping factor Cg quantifies this reduction: for two circuits touching on a surface, Cg is typically 0.80; for six circuits it drops to 0.57; and for twenty circuits it can be as low as 0.38. The factor depends on the number of circuits (not cables — a three-phase circuit with neutral counts as one circuit), the arrangement (bunched, single-layer on a tray, or trefoil), and whether cables are touching or spaced. Grouping factors assume all circuits are fully loaded — if some circuits are lightly loaded, the engineer may apply reduced grouping factors using the methods in BS 7671. Spacing cables apart on a perforated tray can significantly improve the grouping factor compared to bunching in conduit. For large installations, cable routing that minimises grouping can avoid the need for oversized conductors and the associated material cost.