Fill Limits: For power cables, the fill must not exceed 40% of the tray's cross-sectional area; for control cables, it's 50%. Materials: Choose the tray material - aluminum, steel, or FRP -. The most common standards used in our calculator include: According to NEC Article 392. Cables will nearly completely fill the cable tray when reaching the 50% cable fill, due to empty space between the surface of the cables. Cable tray systems play a critical role in organizing and managing electrical cables in both industrial and commercial settings.