Fiber optic cable blowing isn’t a new technique for cable installation and, in fact, it has been in use since the late 1990’s. However, the cable blowing technique may be new to some installers. Machines such as the Fiber Optic Cable Blowing Machines offered by GMP [1] help get the job done with ease for both the seasoned installer or the greenhorn. In this article, we discuss one of the key parameters to a successful installation, the fill ratio.

The Fill Ratio

The fill ratio can be calculated for a single fiber optic cable installed in an innerduct or microduct as: d2 ÷ D< 65%


d2  < 65%

The lowercase “d” stands for the outer diameter of the cable, while the uppercase “D” stands for innerduct’s inside diameter.

This means that when the cable outer diameter squared is divided by the innerduct inside diameter squared, the outcome should be less than 65% (or .65) for the most effective cable blowing. Also, the fill ratio should not fall below 35%, as the cable may have more of a tendency to “kink” in the innerduct due to excessive turbulence. Likewise, when the fill ratio is greater than 80% it becomes increasingly likely that there is insufficient space around the cable for the viscous drag principle of the air to act on the cable jacket.

The fill ratio is one of the parameters determining how far a fiber cable can be blown. A larger fill ratio is preferred when the cables are relatively stiff and the duct run is straight. A smaller fill ratio is typically preferred when there are multiple bends in the duct run, allowing more room in the innerduct for the cable to navigate the route.

The installer doesn’t specify the innerduct or the cable, but the installer can easily calculate the fill ratio and determine if the ratio is in the “sweet spot” or not. A fill ratio out of range may explain why the cable installation didn’t proceed as planned.

Tools for Fiber Optic Cable blowing

GMP offers a range of cable blowing machines for conventional fiber optic cable and micro-fiber optic cable. The Tornado and JetStream install conventional fiber optic cable as well as micro-fiber optic cable. The AirStream, Breeze and Whisper install micro-fiber optic cable. The AccelAir2 rounds out the product offering and installs the smallest of fiber optic cables.

Since 1936, GMP has been a top provider of durable and long lasting tools that can get the job done right. Our Cable Blowing Machines are designed for maximum efficiency to successfully complete even the toughest jobs. To learn more about our cable blowing products and accessories, visit our website at