Usable depth is the space inside the tray that is available for cables to fit after taking into account the tray profile and installation clearances. The calculator then estimates tray area. Check the computed fill percentage, the recommended tray width, and any warning on overfill. Calculate cable tray sizing and fill capacity based on tray dimensions, cable diameter, number of cables, and maximum fill percentage per electrical code. NEC 392 recognizes several cable tray types, each. Determine the total usable cross-sectional area of the cable tray by multiplying its width by its height (or depth).