Cable Dimension Considerations

Determination of a conduit’s dimensions begins with the largest cable, or group of
cables or innerducts, intended for occupancy. From a functional viewpoint, selection
of diameter can be broken down into the following general considerations:
1. The inside diameter of the conduit is determined by the cable diameter and
placement method (pulling or air-assisted pushing).
2. Pulling cables into underground conduits requires sufficient free clearance and is
typically further distinguished by classifying the cables into two groups: power
and coax (short lengths) and fiber (long lengths). Additionally, electrical cable
fill is controlled by the National Electric Code (Chapter 9), whereas, dielectric, or
fiber optic cables, are not.
3. Long pulling lengths require low volume fill, i.e. 36% max.
4 Short pulling lengths may be filled up to 53%, or up to the latest NEC limitations
for groups of cables.
5. Push-blow installation methods for long length fiber cables utilize higher volume
fills, i.e. up to 70% max.
6. Innerducts are smaller diameter conduits, intended for placement into larger
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Duct and Conduit
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conduits or casings. Their purpose is to subdivide the larger conduit space
into discrete continuous pathways for incorporation of fiber optic cables.
Diameters of conduits and innerducts are often specially designed to maximize
the conduit fill.
Using these guidelines, one can determine the minimum ID of the conduit or
innerduct. When over-sizing a conduit for power, coaxial or multi-pair telecom
cables, the more room the better. This rule does not necessarily apply for push-blow
methods of installation. Here, it is found to be more difficult to push a cable with
additional clearance since a cable tends to form a helix, which transfers some of the
axial load laterally into the wall causing friction. The air velocity moving over the
cable can also be maximized with a minimum volume of air when the free volume
is low. Higher air velocities result in improved drag forces on the cable, thus aiding
with its placement.