The conduit's minimum inside cable diameter must be large enough to accommodate the cable, at least 0. 75 inches for single-mode fiber cable and 0. 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. The conduit protects the fragile fiber optic cables from environmental factors and physical damage, ensuring their longevity and optimal. How large are the fiber optic conduits used in the telecommunications industry? The telecommunications industry uses fiber-optic conduits substantially for underground and surface-level installations.