Use NEC 392 for tray rules, but still size conductors from NEC 310. In practice, cable tray dimensions are a system of interrelated measurements —width, depth, length, and material thickness—that directly affect cable fill compliance, heat dissipation, structural loading, and long-term expandability. From an engineering standpoint, cable tray dimensions are not. Ladder cable tray is available in widths of 6, 9, 12, 18, 24, 30, 36, 42 and 48 inches with rung spacings of 6, 9, 12 or 18 inches. Note that wider rung spacings and wider cable tray widths decrease the overall strength of the cable tray. It is grounded on 40 years of experience in the manufacturing.