To calculate the size of the cut-out in the cable tray in this situation you divide the distance between sets by the width of the cable tray ie. Our free calculator helps you determine the correct tray size based on NEC and IEC standards. Select Fill Standard: Choose 40% for power cables (NEC compliant) or 50% for. Cable tray sizing looks simple on paper, but in real projects it affects cable safety, thermal performance, maintainability, future expansion, and inspection approval. Cable tray fill capacity is governed by electrical codes (typically NEC Article 392) which. Determine the total usable cross-sectional area of the cable tray by multiplying its width by its height (or depth).