Microcables For Duct Installation-Fill Ratio, Duct Packing Density

Normally, micro-duct cables are jetted into an appropriately sized micro-duct. To insure that the jetting operation can be done efficiently, micro-duct should be selected to have some space between its inner diameter (ID) and the cable it contains. The recommended fill ratio has been empirically determined from actual jetting experience. It varies depending upon the length of the cable, the number and severity of the bends in the microducts, and the air pressure used. Using the following formula for FILL RATIO, the fill ratio should be no greater than 60 to 70%, although cables have been successfully jetted at higher fill ratios using higher air pressures.

FILL RATIO = (d²/D²) × 100

Where:d = outside diameter of cable and,

D = inside diameter of microduct

This fill ratio should assure sufficient air movement and jetting force can be developed to enable the cable to be placed in its microduct. At very low fill ratios, the placing operation is less effective than at a greater fill ratio. The same formula can be used for microducts being jetted into innerducts or sub-ducts.

Based upon the fill ratio formula, above, a Fill Ratio of approximately 60 to 70%, and empirical data from previous field experience the information in Table 3 has been assembled to show the number of different size microducts that can be field installed in standard sized innerduct.

Table 3 - Micro-Ducts That Can Be Field Installed in Innerduct

Using a 60 to 70% maximum fill ratio, each microduct is sized to accept cables up to a maximum diameter, unique to the ID of the microduct in which it is placed. Table 4 relates this unique maximum diameter for each microduct to the fibre range of Sterlite Micro DUCT-LITE cables encompassed by this microduct size.

When cables are installed into conduit or innerduct, the maximum cable diameter attempted / recommended will normally not exceed 50 to 60% of the conduit or innerduct ID.