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Delving into the ‘fill factor’

There is a big difference between the conduit fill specified by TIA 569-B standard and the conduit fill using a fill factor of 40%.


November 1, 2006  


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For this month’s article, I wanted to talk about cable installation in conduits. This has become an important issue in the industry as the diameter of Category 6A and screened Category 6 / 6A cables is much larger than traditional Category 6 and 5e cables.

There are several reasons for the larger size, including higher performance and better alien crosstalk isolation between cables

The maximum number of cables that each conduit size may contain is determined from the cross-sectional area of the cables divided by the cross-sectional area of the conduit. This ratio is defined as the “fill factor.”

A fill factor of 40% is generally used when installing more than two cables in a conduit, e.g. NEC 2005, Chapter 9, Table 1. A single cable is permitted to occupy 53% conduit fill and two cables are limited to 31% conduit fill.

The table below calculates the maximum number of cables that can be put into an EMT conduit based on a 40% fill factor.

For conduit runs of 50 to 100 feet, the installed number should be reduced by 15%, or use the next size larger conduit. Each 90-degree conduit bend may be estimated as equal to the friction of 30 feet of straight level conduit.

If more than two 90-degree bends are to be used in the conduit run, or if the run is to be over 100 feet in length, insert a pull box.

The 40% conduit fill is compared side-by-side with the conduit fill specified in the TIA 569-B standard. TIA 569-B is more conservative in specifying the number of cables permitted for a given conduit size.

When asked about the origin of the TIA conduit fill table, I was told that the values were based on some pull tests that were done at the time the standard was developed.

You will note that there is a significant difference between the conduit fill specified by TIA 569-B standard and the conduit fill using a fill factor of 40%.

This is an important consideration especially when selecting conduit sizes for horizontal distribution.

A task group has been formed in TIA TR 42.3 subcommittee to re-examine these values and to provide some additional guidance. When asked to provide some personal guidance, I have sanctioned the 40% fill numbers in Table 1 above, provided that the conduit length does not exceed 100 feet with a maximum of two 90-degree bends between pull points.

The presence of pulling lubricants or other liquids can be detrimental to the mechanical and electrical performances of LAN/Data cables.

When using lubricants or pulling compounds, make sure to use the lubricant type recommended for communications and/or data cables by that lubricant manufacturer as well as consider the following points: compatibility with LAN/data cables, safety, lubricity, adherence, stability and drying time.

Also, minimize the quantity of pulling lubricant/pulling compound used and make sure cable is dry before testing. Note that most lubricants or pulling compounds are water based; therefore allow sufficient time for the pulling lubricant to dry before testing.

If the pulling lubricant is not dry, the length-corrected attenuation will fail.

Also, annealed copper will begin to permanently elongate (stretch) under a stress of approximately 15,000 lbs./sq. in. During installation the total pulling tension must be equally distributed among all cables. The maximum recommended pulling tensions for specific cable and conductor sizes is as follows: 4-pair / 24 AWG – 25 lbf, 4-pair / 23 AWG – 32 lbf, 4-pair / 22 AWG – 40 lbf.

TIA-569-B, Clause 8.8.2.1 and 8.8.2.2 specifies that no section of conduit shall be longer than 30 metres (100 feet) between pull points, and that no section of conduit shall contain more than two 90-degree bends, or equivalent (see annex B), between pull points (e.g., outlet boxes, telecommunications rooms, pull boxes).

These are more restrictive that the NEC 2005, Article 358.26, which permits up to four quarter bends. These requirements are intended to limit the pull force and a methodology for calculating these is provided in Annex B of the TIA standard.

As you can see, the issue of conduit fill is more complex than it appears on the surface. All the factors are inter-related including the type and number of cables to be installed, the maximum pulling tension, the use of pulling lubricants and their effect on cable performance, and the reserve capacity for the future.

The results of the TIA TR 42.3 Conduit Fill task group are eagerly awaited in the industry.

Paul Kish is Director, Systems & Standards, at Belden CDT.

Disclaimer: The information presented is the author’s view and is not official TIA correspondence