While 40% is a good rule of thumb for pathways to meet present and future cable installation requirements, most telecom professionals aim for a maximum fill ratio of 70 to 80% for fiber innerduct. A conduit is a protective tube or channel that houses the fiber optic cables, shielding them from moisture, dust, physical stress, and other environmental factors. It also facilitates cable management and ease of maintenance. With these assemblies we mention in this article, the widest point of. The Fiber Optic Association, Inc. I being searching, but I feel I need to ask the right questions before I'll do any design. My current plan is to run 2" or 3" PVC conduit across the two building (clamped to the underside of a metal stairwell and on each building mount a 10x10 (or whatever size is recommended) PVC box. Premise innerduct is a flexible, non-metallic, corrugated raceway that has long been an essential conduit system for protecting fiber optic cables installed throughout telecommunications spaces and pathways.