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our Innerduct Calculator
or read about required considerations below
There
is no industry standard or code that governs installation of innerducts
in conduits similar to rules of Chapter 9 of NEC(NFPA 70) that govern
installation of electrical cables inside conduits. Similar considerations
can be applied, however. As with electrical cables, total cross-section
of all innerducts should be less than that of the conduit, and some
spare space should be left for installation.
The amount
of free space left is controlled by the volume fill ratio. Different
manufacturers recommend different fill ratios, however most stick
with the same familiar 40% fill ratio that is derived directly from
NEC. We suggest that the fill ratio be considered within 30%-70% range,
based on such factors as number of bends, length of the conduit run
as well as material of the conduit. Short run of solid HDPE provides
least friction to pull, and therefore can tolerate 30% fill ratio
whereas long concrete conduit run will cause more friction and the
fill ratio should be lowered to 30% or so.
Many
types of innerduct have corrugated design that adds additional considerations
to the calculations. The ribs are intended to cause less friction
while pulling cable inside the innerduct. However, the drawback of
having the ribs is also significant: it will make it almost impossible
to pull another innerduct into the same outerduct later. Therefore
the initial pull should consist of as many innerducts as possible
with as large diameter as possible to fully utilize the space available
inside the conduit.
Additionally,
the ribs increase the conduit's outer diameter (OD) and decrease its
inner diameter (ID), which needs to be taken into consideration when
calculating the cross-section area. Below is a table with typical
values for the OD and ID based on the specified trade size of the
corrugated innerduct. Make sure you request actual dimensions from
the manufacturer. |