For a single fiber cable, 1-inch conduit provides adequate space. Choosing the right conduit size is one of the most important steps when installing fiber optic cables. The conduit protects the fragile fiber optic cables from environmental factors and physical damage, ensuring their longevity and optimal. Assuming a straight run up to 3 meters (adding bends reduces capacity!), here is a safe guide for standard stranded (6491X) single core cables: 2 cables (Tight!) *Estimates based on typical UK stranded singles. Always check specific cable manufacturer OD (overall diameter). Stranded:. Innerduct: Smaller diameter (typically 1 to 1.