This step‑by‑step approach helps you determine width, depth, support spacing, and allowable load with confidence. Plan 20–30% spare capacity for growth. Select Fill Standard: Choose 40% for power cables (NEC compliant) or 50% for. Save your cable tray sizing calculator results as branded PDF, Excel, or Word reports with full standard references and clause numbers. Cable tray fill is the proportion of usable cross-sectional area inside a cable tray occupied by installed cables. Typical values: Formula 2: Cable Area Calculation Where: This helps determine how many cables fit in the tray based on available area. You need to install 50 power cables, each with a diameter of 0. 5 inches, in a 4-inch deep cable tray.