EXPLAINER SERIES: Fill Ratio for Subdividing Existing Conduits and Overrides
April 13, 2026
In our Explainer Series, we break a critical concept in conduit networks into handy pieces of knowledge that pay dividends time after time. Here, it’s fill ratio for subdividing existing conduits and OverRides.
What is Fill Ratio?
As we established in our previous blog, fill ratio is a measure of the space taken up by one thing (or things) inside another. Here, we are concerned with the space taken up by multiple cables or MicroDucts inside a larger conduit. In this context, Dura-Line recommends a maximum fill ratio of 55%, once again, based upon years of experience and thousands of successful installations.
What is Subdividing
Subdividing is the process of dividing the space inside a larger traditional conduit (with typical sizes including 1”, 1.25”, 2”, 4”) into smaller compartments. Depending on the size of the larger duct, this can be done with loose MicroDucts or bundles of MicroDucts, also known as FuturePath®. This approach allows infrastructure owners to use their available space more efficiently and add new fiber cables in the future without the cost and hassle of digging.
What is an OverRide?
An OverRide is an innovative installation method, which allows an existing larger conduit to be subdivided, even when it already contains a cable or cables. In an OverRide, a special piece of equipment called a “Y-block” isolates the existing cable(s) so the larger duct can be pressurized and one or more MicroDucts can be installed via air-jetting. OverRides are a low-risk, high-reward solution, which can be up to 80 percent faster and more cost effective than traditional network construction.
Why is Fill Ratio Important in This Context?
In simple terms, working out your fill ratio indicates whether the MicroDucts you intend to install into a larger conduit – occupied or unoccupied by cables – will fit. But it also allows you to find the optimal number and size of MicroDucts for your project. For example, you might choose a higher number of smaller pathways for lower-count cables if you need to route fiber to several distributed locations. Or for high-fiber count express transport, like in a middle-mile network, you may opt for fewer pathways with a larger diameter to accommodate larger cables.
How is it Calculated?
In simple terms, working out your fill ratio indicates whether the MicroDucts you intend to install into a larger conduit – occupied or unoccupied by cables – will fit. But it also allows you to find the optimal number and size of MicroDucts for your project. For example, you might choose a higher number of smaller pathways for lower- -count cables if you need to route fiber to several distributed locations. Or for high-fiber-count express transport, like in a middle-mile network, you may opt for fewer pathways with a larger diameter to accommodate larger cables.
Is There Anything Else to It?
While fill ratio is a key aspect of a subdivision or OverRide installation, it is not the only success factor. The age, material, and condition of the existing conduit as well as any bends or elevation changes in the pathway can impact installation performance – as can the combination of installation equipment being used. Finally, always remember to install all your MicroDucts at the same time and not incrementally, as space will be reduced and complexity increased with each additonal pathway.