Dimensions of underground fiber optic cable ducts

Underground fiber optic cables are typically installed in ducts ranging from 25 mm to 40 mm for sub-ducts, with micro-ducts as small as 5 mm to 14 mm inside diameter.

Standard Duct Sizes

Fiber optic cables are usually placed in sub-ducts (innerducts) with an inside diameter of 25 mm to 40 mm, which are then installed inside larger telecommunications conduits. These innerducts provide mechanical protection, reduce friction during cable installation, and allow for future upgrades without extensive excavation. For smaller fiber cables, micro-ducts are used, typically with an inside diameter of 5 mm to 14 mm.

Micro-ducts can be nested within innerducts or installed directly into larger conduits, and fiber cables are often blown into these ducts using compressed air.

Conduit Depth and Protection

Underground conduits are generally buried 1–1. 2 meters (3–4 feet) deep to protect against accidental digging and environmental factors. In areas with extreme cold, conduits may be buried deeper to avoid frost damage.

Conduits are made from materials such as PVC, HDPE, or concrete, and may include armored or water-blocking layers for additional protection. Multiple Ducts in a ConduitA single large conduit can accommodate three or four sub-ducts, allowing multiple fiber cables to share the same pathway. This modular approach improves space efficiency and simplifies future expansions.

SummarySub-duct (innerduct) diameter: 25–40 mmMicro-duct diameter: 5–14 mmConduit depth: 1–1. 2 meters standard, deeper in cold climatesMaterials: PVC, HDPE, concrete, often with armored or water-blocking layersCapacity: Multiple sub-ducts can fit inside a single conduit for future growth These dimensions ensure mechanical protection, ease of installation, and scalability for modern fiber optic networks, including urban, industrial, and smart city deployments.