Optical cable is usually placed in a 25 to 40 mm inside diameter (ID) sub-duct which is placed into an existing larger diameter communications conduit. Most communications conduits can be fitted with three or four sub-ducts. Sub-ducts are often referred to as. CAUTION: Care must be taken to avoid cable damage during handling and placing. Any such damage may alter the cable's characteristics to the extent that the cable section may have to be replaced. Note that Recommendation ITU-T L. 0, in February. Our Utilities Ducting collection offers a comprehensive range of solutions designed to facilitate the efficient routing, protection, and maintenance of essential utilities services, such as electricity, water, gas, and telecommunications. Explore our specialised subcategories below to discover how. Duct fiber optic cables—often called “duct fiber”—are specialized optical cables engineered to be installed within pre-existing ducts (hollow tubes) rather than buried directly in soil or strung from poles.