Size the tray by calculating total cable cross-sectional area and dividing by the allowable fill percentage (typically 40%). Add 20–30% spare capacity for future cables. Standard tray widths are 6, 9, 12, 18, 24, and 30 inches. Our free calculator helps you determine the correct tray size based on NEC and IEC standards. Follow these simple steps: Define Tray Dimensions: Enter the width and depth of your planned cable tray (in mm or inches). Cable tray fill capacity is governed by electrical codes (typically NEC Article 392) which. Use the largest cable diameter in the tray for calculation., Single Core, Multicore) and measure the overall outside diameter (OD).