What Is Conduit and Innerduct?
Conduit and innerduct definitions for OSP fiber builds: flexible HDPE conduit, ribbed vs silicore innerduct linings, and how FCC designs duct systems.
Conduit and innerduct are the underground and riser pathway system used to protect and organize fiber optic cable. Conduit is the outer pipe, commonly flexible HDPE installed by trenching, plowing, or directional drilling, while innerduct is smaller tubing installed inside that conduit to subdivide it into separate raceways for individual cables or cable groups.
Conduit: The Outer Pathway
Conduit is the outer protective pipe that houses fiber optic cable underground or in risers. The most common OSP conduit material is high-density polyethylene (HDPE), which is flexible, not rigid. Flexible HDPE conduit ships coiled on reels, bends around underground obstacles, and feeds continuously through horizontal directional drilling (HDD) bores without joints every few feet. Rigid PVC conduit is used in shorter, straight applications such as building entrances, risers, and handhole stub-outs, where sections are glued or threaded together. Conduit is sized by outer diameter (commonly 1 inch to 6 inches) based on the number of cables or innerducts it needs to carry and the pulling tension the route requires.
Innerduct: Subdividing the Conduit
Innerduct is smaller tubing installed inside a larger conduit to split it into separate raceways, so multiple cables, or multiple network owners, can share one bore without contacting each other. Innerduct comes with different interior linings, and these linings are not interchangeable. Ribbed innerduct has raised longitudinal ribs molded into the inner wall, which reduce the surface area of contact between the cable jacket and the duct wall to lower pulling friction. Silicore innerduct is a separate technology: a smooth, low-friction coating applied directly to the interior surface, without ribs, typically specified when cable will be air-assisted (jetted or blown) rather than pulled. Project designs should call out one lining type or the other based on the installation method planned.
Conduit and Innerduct in Duct Bank Design
On larger OSP builds, contractors group multiple conduits together in a duct bank, sometimes encased in concrete at road crossings or high-traffic areas and direct-buried elsewhere. Each conduit in the bank may carry two, three, or four innerducts, sized to the cable diameter and pull length of that route. Working out the mix of conduit size, innerduct count, and lining type before construction avoids re-trenching later when new capacity is needed. Fiber Construction Company designs and installs conduit and innerduct systems as part of underground OSP builds nationwide, coordinating conduit selection with the engineering and permitting requirements of each route.
Conduit and Innerduct, answered
What Is Conduit and Innerduct?
Conduit and innerduct are the underground and riser pathway system used to protect and organize fiber optic cable. Conduit is the outer pipe, commonly flexible HDPE installed by trenching, plowing, or directional drilling, while innerduct is smaller tubing installed inside that conduit to subdivide it into separate raceways for individual cables or cable groups.
Is HDPE fiber conduit rigid or flexible?
HDPE (high-density polyethylene) conduit is flexible. It ships on reels, bends around obstacles, and feeds continuously through directional drilling bores. Rigid PVC conduit is used instead for short, straight runs such as building entrances or riser stub-outs, where sections are joined with fittings rather than fed off a reel.
What is the difference between ribbed and silicore innerduct?
They are different linings, not the same thing. Ribbed innerduct has raised interior ribs molded into the wall that reduce contact area during a cable pull. Silicore innerduct has a smooth, low-friction coating on the inner wall instead of ribs, typically chosen when cable will be air-jetted rather than pulled.
How many innerducts go inside one conduit?
It depends on the diameters involved. A common configuration is three or four 1.25 inch innerducts inside a 4 inch conduit, but the actual count is set by cable size, pull length, and the fill ratio called for in the route's engineering design.