Plan cable routes before installation to ensure airflow, accessibility, and room for expansion. MicroDucts were developed as a solution to house fiber cables that were smaller in size, but still carried significant capacity. Today, MicroCables range from 6 to 432-fiber. Cable tray types, fill rules for single-conductor and multiconductor cables, ampacity derating, separation requirements, and when to use tray vs conduit. Cable Tray Types and When to Use Each 2. Fill Rules for Multiconductor Cables 3. A rung spacing of 6 to 9 inches (150 to 230 mm) is preferable when. Effective cable tray and conduit system planning is essential for both new installations and retrofit projects.