Innerducts in a conduit - considerations and calculations

Calculating Number of Innerducts in a Conduit (a.k.a. outerduct)

Use 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.

picture courtesy of Premier Conduit


Typical dimensions of corrugated innerducts

Trade Size

Nominal OD

Nominal OD
Nominal ID
Nominal ID

Innerduct Calculator

outer conduit ID (trade size) innerduct OD (inches) Fill ratio (0.3 - 0.7) number of innerducts

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