8 July 2008

CONDUIT SIZING FOR A FIBER OPTIC BACKBONE

Jeffrey Gregg Wolford

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Conduit Sizing for a Fiber Optic Backbone

Purpose: This paper discusses current methods for placing fiber optic cable and small copper cables in underground outside plant, explores the pros and cons of each method, and examines some alternative methods.

Background: The standard size and material for telecommunication conduits has been four inch diameter polyvinylchloride (PVC) plastic conduit which was implemented prior to the advent of the first telephone trials of fiber optic cable in the spring of 1977. The introduction of fiber optic cable necessitated the need for smaller, segmented pathways and by 1978 Endot began making innerduct to support fiber optic and small copper cable installation. Recently, fabric textile innerduct, developed by Jerry Allen of TVC Inc., has supplemented rigid and corrugated flexible innerduct. Fabric textile innerduct occupies less space and provides greater conduit segmentation. Assuming three, three-inch, three-cell units installed in each conduit, there would be nine separate pathways provided by cells versus a maximum of four paths using 1.25” diameter rigid or corrugated flexible innerduct. In both cases there is void space between the innerduct and wall of the conduit to install an additional small cable.

Methods of Conduit Segmentation: Currently there are three methods of handling fiber optic cable and small copper cables in underground outside plant: no segmentation; rigid and flexible innerduct (conduit); and textile fabric innerduct.

No Segmentation: The un-segmented method works well when multiple cables are pulled at the same time. The practice is common when distribution cables from the voice and data nodes are home run to each end user buildings. The following example of this practice is from West Point United States Military Academy, Maintenance Hole 5 (figure). Note there are thirteen fiber optic cables in the top left conduit. This indeed is a large number of cables without separation. According to Corning Cables Systems, the maximum number cables sharing a single pathway should not exceed three cables, although they did acknowledge more than three cables per pathway occur frequently in the field. The key during installation is to insure that the maximum pulling tension is not exceeded.

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Figure 1 West Point Maintenance Hole 5

Cables can also be installed into conduits with existing cables without segmentation. The following example is again from West Point, Maintenance Hole 237 (figure 2).

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Figure 2West Point Maintenance Hole 237

Note the conduit on the right. There are at least five cables that have been installed at different times. The argument against using this method is there is potential to damage the existing cable during installation and increased difficulty removing a single cable from a bundle that was installed together as the cables tend to twist around each other. Although cable manufacturers do not recommend more than three cables being placed together without separation, the method is quite successful based on the number field observations. Note also the rigid innerduct. In this area of the post, rigid innerduct was placed in a continuous run between several maintenance holes to reduce the number of pull points, thus allowing for longer continuous pulls. Continuous runs make cable identification difficult, and many times are not properly racked in the maintenance hole. Once again, in all of these situations, the key is to insure that the maximum pulling tension is not exceeded during installation.

Rigid and Flexible Innerduct: The next method uses either rigid innerduct or corrugated flexible innerduct (conduit). This requires installation of the innerduct prior to the installation of cable. Rigid inner duct such as the smooth wall type is observed in the above picture, right conduit. As mentioned previously, this method can improve cable installation times by providing longer continuous pathways that are beyond the short maintenance hole spacing that is required for large copper cables. Rigid innerduct works extremely well for long installation runs where blown fiber methods are used. For underground building entrances that utilize a 90° sweep into building, this method offers advantages over fabric textile innerduct. If the first cable

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is installed in the outer most cell of textile innerduct placed through a 90° sweep, the cable will collapse the other cells against the inside bend of the sweep, making the installation of additional cable more difficult if the initial cable was pulled in too tightly.

The following diagram shows five 1.00 inch (inside diameter) innerducts placed in standard four-inch conduit and four 1.25 inches (inside diameter) innerducts placed in a standard four-inch conduit.

Figure 3 Four-Inch Conduit with 1” and 1-1/4” Innerduct

The table below shows the percentage area occupied by the innerduct. Clearly there is enough void space between the innerducts to install more cables, provided the route is relatively straight or has minimum bends. As stated earlier, filling the innerduct with cable is not recommended and care must be taken not to exceed maximum recommended pulling tension. This should only be done in situations where in areas to avoid excessive digging costs where future growth and need for additional cables in the future is not foreseen and with careful consideration by the engineer.

The maximum number of innerducts that can be installed in a conduit is calculated as follows:

D - Inside diameter of the outerduct.d - Outside diameter of the innerduct.Note the calculation uses radiansN – Number of Innerducts - Angle from the horizontal axis of the outerduct to a line that passes through the cent

of the outerduct and is tangent to the innerduct.

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1

1

11

1

1.25

1.25

1.25

1.25

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Figure 4 Innerduct in Outer Duct

Next calculate the number of innerducts:

And simplifying yields”

Clearly, if 1-1/4-inch innerduct is used, the innerduct itself can accommodate more than one cable, if the fiber optic cables are 60 strands or less. Multiple cables should also be installed at the same time, as stated before. Additional cables should not be installed into innerduct with existing cable except in carefully evaluated situations as stated in my comment above.

Fabric Textile Innerduct: The newest method of segmenting standard four-inch conduits is textile innerduct. The material allows for greater segmentation thus allowing for potentially greater utilization of the main conduit.   Over the past few years the practice has been to install up to three, three-inch, three-cell innerduct, for a total of nine cells per four-inch conduit. The

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current practice is not to mix fiber optic cables with copper voice cables. If the conduit with textile innerduct is reserved only for fiber optic cables, the question then becomes, is this an efficient use of the infrastructure? The following picture omits the textile innerduct for clarity. For conduit runs that are reasonably simple and straight it is clear the amount of fiber optic strands that could be installed into a four-inch conduit, limiting the maximum size to 288 strands per cable, becomes excessive.

Figure 5 Four Inch Conduit with 288 Strand Single Mode Fiber Optic Cables

Let us examine the percentage of large fiber optic cables installed on a typical project. The following data was extracted from the Independent Government Cost Model database. Cables with strand counts of twenty-four or less are treated as building entrance cables and are omitted.

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Table 1 IN/OUT Normalized Cable Data

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IN/OUT CABLE USAGE NORMALZED

41.36%

26.13%25.00%

2.10% 1.87% 1.33% 0.94% 0.92% 0.25% 0.06% 0.03%0.00%

5.00%

10.00%

15.00%

20.00%

25.00%

30.00%

35.00%

40.00%

45.00%

IN/O

UT

48S

M

IN/O

UT

96S

M

IN/O

UT

144

SM

IN/O

UT

36S

M

IN/O

UT

288

SM

IN/O

UT

240

SM

IN/O

UT

192

SM

IN/O

UT

120

SM

IN/O

UT

168

SM

IN/O

UT

72S

M

IN/O

UT

156

SM

Chart 1 IN/OUT Normalized Cable Data

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Table 2 UG Cable Normalized Cable Data

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UNDERGROUND BACKBONE CABLE USAGE NORMALZED

25.25%

23.31%

20.92%

18.79%

4.65%3.85%

1.60% 1.35%

0.27%

0.00%

5.00%

10.00%

15.00%

20.00%

25.00%

30.00%

UG 96SM UG 144SM UG 48SM UG 72SM UG 36SM UG 192SM UG 288SM UG 120SM UG 240SM

Chart 2 UG Cable Normalized Cable Data

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Combining both Underground and In/Out fiber yields the following:

Table 2 UG Combined Cable Normalized Cable Data

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Combined Normalized Cable Useage

34.81%

22.00%21.04%

15.89%

1.77% 1.58% 1.12% 0.79% 0.77% 0.21% 0.03%0.00%

5.00%

10.00%

15.00%

20.00%

25.00%

30.00%

35.00%

40.00%

48 Strand 72 Strand 144 Strand 96 Strand 36 Strand 288 Strand 240 Strand 192 Strand 120 Strand 168 Strand 156 Strand

Chart 3 Combined Cable Normalized Cable Data

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From the above analysis we can conclude that the predominate cable sizes are 48 to 144 strands. The only place where there be would a high concentration of large strand fiber optic cables would be near the communication nodes (MCN and ADN) and only for a few maintenance hole segments until the cable routes diverge and begin to taper.

The above analysis suggests that the possibility of filling a four-inch conduit with fiber optic cable is unlikely. Now let us investigate how many fiber optic cables can physically be placed in various sizes of innerduct. The same formulas for calculating the maximum number of innerducts in conduit is applied to determining the maximum number of cables per innerduct. The following table lists the values for standard innerduct.

Table 3 Standard Innerduct Dimensions

The following table lists the standard outside diameter for fiber optic cable, the maximum number of cables that be placed in a given innerduct under ideal conditions, and the percent fill for each scenario.

Table 4 Maximum Number Fiber Optic Cables per Innerduct/ConduitFrom the above table we see that the one-inch innerduct can only accommodate a single cable. A graphical representation of selected cables is depicted below:

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Figure 6 Selected Cables in 1-1/4”, 1-1/2”, and 2” Innerduct/ Conduit

The above information suggests that a more optimal conduit size for a fiber optic cable underground infrastructure may not be a four-inch conduit segmented with either rigid, corrugated flexible, or fabric textile innerduct.

Other Influencing Factors: Before an alternative design method is presented, we should consider developments in voice, data, and converged networks. As copper prices continue to climb and the cost of multiplexing over fiber, using Dense Wave Division Multiplexing (DWDM), becomes more attractive, the need for large copper trunk and distribution cables decreases. Buildings with large voice user requirements could more easily and affordably be serviced by placing a remote line self in each building and connecting them to the Dial Central Office (DCO) via fiber optic cable. Buildings requiring less than four-hundred lines could be connected using copper cable. To be accurate and correct, a cost benefit analysis should be done comparing using traditional copper cable from the DCO verses using a remote line self connected with fiber optic cable based on the total number of lines. Also, with the advances in data network technology the data rates have increased beyond the physical and practical limits of copper cable. Even with exotic shielded twist-pair technology used to increase the distances that can be support by copper, copper cable simply can no longer meet the faster data rate demands nor even remotely come close to spanning the distances that are easily covered by single mode fiber optic cable.

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288

1.550"/1.250" 1.968"/1.610" 2.375"/2.067"

1.550"/1.250" 1.968"/1.610" 2.375"/2.067"

144144

144

144144144144

144 144

1.968"/1.610" 2.375"/2.067"1.550"/1.250"

288

1.000"/1.349"

1.000"/1.349"

144

1.000"/1.349"

8 July 2008

Converged networks most always incorporate various flavors of DWDM. Since DWDM allows for higher bandwidths over two strands of fiber, the future need for very large fiber optic cables will also diminish. Only small strand fiber optic cables would be required in the core network.

An Alternate Approach: Realizing that most of the volume of a four-inch conduit dedicated to fiber optic cables, partitioned with textile innerduct, may never be fully exploited, an alternate approach should be considered. A suggested alternative method would be to replace the four-inch conduit with a two-inch four-way conduit system. The aforementioned system fits in the standard maintenance hole knock out and would provide a rigid wall of separation between fiber optic cables and copper cables. One of conduits in a two-inch four-way system can accommodate copper cables up to P4-24PF. Although a 1-1/4-inch six-way system is available, it would require a modification to the standard knock to be used and would not be able to accommodate copper cables larger than P1-24PF.

There is an advantage to using two-inch four-way system in lieu of using four-inch conduit partitioned with textile innerduct. Since fiber optic cables are smaller and lighter than copper communication cables, and since fiber can be pulled or blown distances greater than 600-foot maintenance hole spacing limit, one or all of the two-inch conduits could be connected through the maintenance hole. Connecting the conduits through the maintenance hole would reduce the number of maintenance holes would have to be entered during cable installation and would be extremely advantageous when blown fiber installation methods are employed. With textile innerduct, every maintenance hole in the pathway would have to be entered during cable installation.

Finally, in the presentation we noted several small fiber optic cables can be placed in a two-inch conduit. For larger cables where up to three cables wound fill the conduit, the cables could be installed without the need for separation, following the suggested guidance of Corning Cable Systems.   With conduits potentially holding more than three fiber optic cables or continuing with the practice of 100% separation between cables, a smaller version of fabric textile innerduct is available.

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