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Fiber Optic Cable and Fiber Innerduct Filling Ratio

Fiber optic innerducts are smooth wall or corrugated tubes made with HDPE (outside plant OSP), PVDF or PVC (indoor applications).

Innerduct is used in applications where several fiber cables must be protected. The corrugated construction allows innerduct to easily bend at a fairly large radius without collapsing the interior. It also provides mechanical protection against crushing.

Fibers are deployed through fiber innerducts which in turn are drawn through conduit laid in the ground. Innerducts can subdivide each section of the larger duct.


100mm diameter conduits are the most popular. As a rule of thumb, 2~4 pcs of 1” (25.4mm) innerduct can be pulled through a 100mm conduit.

The most popular fiber optic innerduct sizes are 1” (25.4mm) and 1.25” (31.8mm). Fiber cables with a maximum diameter of 1” (25.4mm) can be pulled in 1.25” innerduct. The general idea is that a fiber cable can fill no more than 60~70 percent of the area of an innerduct.

Multiple cables can be pulled at once, as long as the tensile load is applied equally to all cables.

If future cable pulls in the same duct or conduit are a possibility, fiber optic innerduct should always be used. Without it, future fiber cable pulls can entangle existing operating fiber cables and cause service interruption.

Innerduct should be installed as straight as possible since twists can increase cable pulling tension which in turn can cause reliability issue.

Fiber cable filling ratios requires that high fiber counts in prediction of future needs. On cable jacket can be more densely packed with fibers than multiple cable jackets. The cost of extra fibers is usually small when these extra fibers are not terminated until needed.

An innerduct may also be used in a cable tray to isolate fibers from copper cables.

As with all cabling components, innerduct must meet all the proper ratings to be placed in plenum space.

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