The formula used to calculate cable tray capacity is: Cable Tray Capacity = (Tray Width × Tray Depth × Fill Ratio) / Cable Cross-sectional Area Where: Tray Width is the internal width of the cable tray in meters (or millimeters). Enter the dimensions of the cable tray, the desired fill ratio, and the diameter of the cables to calculate the cable tray capacity. The following formula is. Our free calculator helps you determine the correct tray size based on NEC and IEC standards. 5 inches, in a 4-inch deep cable tray. For mixed cables, sum the areas of all individual cables.