How do you size cable trays and calculate fill capacity?

Cable tray sizing follows NEC Article 392. Fill limits depend on cable type: multiconductor cables limited to cable tray cross-sectional area, single conductors to 40% fill, and control cables per manufacturer. Calculate required area by summing individual cable cross-sections. Consider cable weight for tray structural rating and maintain minimum bend radius at direction changes. Allow for future expansion (typically 25% spare capacity) and maintain proper cable spacing for heat dissipation.