Conduit Sizing For A Fiber Optic Backbone 4f3w1r

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 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 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 is installed 4 of 16

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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 fourinch conduit and four 1.25 inches (inside diameter) innerducts placed in a standard four-inch conduit. 1

1

1.25 1

1

1.25

1

1.25 1.25

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.

Inside Outside Diameter Diameter 1" Innerduct 1-1/4" Innerduct 4" Conduit

1.000 1.250 4.026

1.349 1.550

Cross Sectional Number of innerducts Total Area Area 1.429 1.887 12.730

5 4

7.146 7.548

Percent fill (Total Area of ID/Conduit Area) 56% 59%

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 radians N – Number of Innerducts θ - Angle from the horizontal axis of the outerduct to a line that es through the cent of the outerduct and is tangent to the innerduct.

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Figure 4 Innerduct in Outer Duct  d  θ = arcsin  D−d Next calculate the number of innerducts: N=

2π 2θ

N=

π θ

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 6 of 16

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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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Cable IN/OUT 48SM IN/OUT 96SM IN/OUT 144SM IN/OUT 36SM IN/OUT 288SM IN/OUT 240SM IN/OUT 192SM IN/OUT 120SM IN/OUT 168SM IN/OUT 72SM IN/OUT 156SM

Number Total of Normalized Length in Weighted Combined Cable Usage Purchases Purchases Feet Length Weighting Normalized 186 0.2322097 1,655,172 0.307335 0.071366 41.36% 147 0.1835206 1,323,234 0.245700 0.045091 26.13% 159 0.1985019 1,170,468 0.217334 0.043141 25.00% 91 0.1136080 171,550 0.031854 0.003619 2.10% 38 0.0474407 366,741 0.068097 0.003231 1.87% 44 0.0549313 225,555 0.041881 0.002301 1.33% 39 0.0486891 178,903 0.033219 0.001617 0.94% 41 0.0511860 166,384 0.030894 0.001581 0.92% 21 0.0262172 89,283 0.016578 0.000435 0.25% 17 0.0212235 25,883 0.004806 0.000102 0.06% 18 0.0224719 12,394 0.002301 0.000052 0.03% 801

1 5,385,567

Table 1 IN/OUT Normalized Cable Data

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1.000000

0.172536

100%

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IN/OUT CABLE USAGE NORMALZED 45.00% 41.36% 40.00%

35.00%

30.00% 26.13% 25.00%

25.00%

20.00%

15.00%

1.33%

0.94%

0.92%

0.25%

0.06%

0.03%

IN/OUT 120SM

IN/OUT 168SM

IN/OUT 72SM

IN/OUT 156SM

1.87%

IN/OUT 192SM

IN/OUT 36SM

IN/OUT 144SM

IN/OUT 96SM

IN/OUT 48SM

0.00%

2.10%

IN/OUT 288SM

5.00%

IN/OUT 240SM

10.00%

Chart 1 IN/OUT Normalized Cable Data

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

Number of Normalized Purchases Purchases 169 0.155474 170 0.156394 185 0.170193 194 0.178473 130 0.119595 79 0.072677 54 0.049678 69 0.063477 37 0.034039 1087

1.000000

Total Length in Weighted Combined Cable Usage Feet Length Weighting Normalized 2,182,924 0.234378 0.036440 25.25% 2,003,616 0.215126 0.033644 23.31% 1,652,203 0.177395 0.030191 20.92% 1,415,196 0.151948 0.027119 18.79% 522,576 0.056108 0.006710 4.65% 712,305 0.076479 0.005558 3.85% 434,037 0.046602 0.002315 1.60% 285,880 0.030695 0.001948 1.35% 104,952 0.011269 0.000384 0.27% 9,313,689

1.000000

0.144310

Table 2 UG Cable Normalized Cable Data

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100.00%

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 UNDERGROUND BACKBONE CABLE USAGE NORMALZED 30.00%

25.25% 25.00%

23.31% 20.92%

20.00%

18.79%

15.00%

10.00%

4.65%

5.00%

3.85% 1.60%

1.35% 0.27%

0.00% UG 96SM

UG 144SM

UG 48SM

UG 72SM

UG 36SM

Chart 2 UG Cable Normalized Cable Data

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UG 192SM

UG 288SM

UG 120SM

UG 240SM

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

Cable

Total Number of UG Purchases Cable (UG Cable) Length

UG 12SM UG 24SM UG 36SM UG 48SM UG 72SM UG 96SM UG 120SM UG 144SM

605 194 130 185 194 169 69 170

3,080,345 2,503,639 522,576 1,652,203 1,415,196 2,182,924 285,880 2,003,616

UG 192SM UG 240SM UG 288SM

79 37 54

712,305 104,952 434,037

Cable IN/OUT 6SM IN/OUT 12SM IN/OUT 24SM IN/OUT 36SM IN/OUT 48SM IN/OUT 72SM IN/OUT 96SM IN/OUT 120SM IN/OUT 144SM IN/OUT 156SM IN/OUT 168SM IN/OUT 192SM IN/OUT 240SM IN/OUT 288SM

IN/OUT Cable 17 721 238 91 186 147 108 41 159 18 21 39 44 38

Number of Total Puchases IN/OUT Normalized (IN/OUT Total Cable Normalized Normalized Combined Cable Cable) Purchases Length Purchases Length Weights Useage 25,883 17 25,883 0.004529 0.000963 0.000004 0.000029 3,224,928 1326 6,305,273 0.353223 0.234613 0.082871 0.560513 2,065,976 432 4,569,615 0.115077 0.170031 0.019567 0.132343 171,550 221 694,126 0.058871 0.025828 0.001520 0.010284 1,655,172 371 3,307,375 0.098828 0.123064 0.012162 0.082261 1,323,234 341 2,738,430 0.090836 0.101894 0.009256 0.062603 1,301,021 277 3,483,945 0.073788 0.129634 0.009565 0.064698 166,384 110 452,264 0.029302 0.016828 0.000493 0.003335 1,170,468 329 3,174,084 0.087640 0.118105 0.010351 0.070009 12,394 18 12,394 0.004795 0.000461 0.000002 0.000015 89,283 21 89,283 0.005594 0.003322 0.000019 0.000126 178,903 118 891,208 0.031433 0.033161 0.001042 0.007050 225,555 81 330,507 0.021577 0.012298 0.000265 0.001795 366,741 92 800,778 0.024507 0.029796 0.000730 0.004939 3754 26,875,165

Table 2 UG Combined Cable Normalized Cable Data

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1.000000

1.000000

0.147848

100%

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Combined Normalized Cable Useage 40.00%

35.00%

34.81%

30.00%

25.00% 22.00%

21.04%

20.00% 15.89% 15.00%

10.00%

5.00% 1.77%

1.58%

1.12%

0.79%

0.77%

240 Strand

192 Strand

120 Strand

0.21%

0.03%

168 Strand

156 Strand

0.00% 48 Strand

72 Strand

144 Strand

96 Strand

36 Strand

288 Strand

Chart 3 Combined Cable Normalized Cable Data 13 of 16

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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. Inside Diameter (inches) 1" Innerduct 1-1/4" Innerduct 1-1/2" Innerduct 2" Innerduct

1 1.25 1.61 2.067

Outside Diameter (inches) 1.349 1.55 1.986 2.375

Internal Cross Sectional Area (square inches) 0.7854 1.2272 2.0358 3.3556

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.

Fiber Count Range 2-60 61-72 71-96 97-120 121-144 145-216 217-240 241-288

1" 1-1/4" 1-1/2" 2" Nominal Cross 0.7854 SQ. IN 1.2272 SQ. IN 2.0358 SQ. IN 3.3556 SQ. IN Outside Sectional Number Percent Number Percent Number Percent Number Percent Diameter Area cables Fill cables Fill cables Fill cables Fill 0.52 0.21237 1 27.04% 3 51.92% 6 62.59% 9 56.96% 0.54 0.22902 1 29.16% 3 55.99% 5 56.25% 8 54.60% 0.63 0.31172 1 39.69% 1 25.40% 4 61.25% 6 55.74% 0.7 0.38485 1 49.00% 1 31.36% 3 56.71% 5 57.34% 0.76 0.45365 1 57.76% 1 36.97% 2 44.57% 5 67.60% 0.79 0.49017 1 62.41% 1 39.94% 2 48.15% 4 58.43% 0.82 0.52810 1 67.24% 1 43.03% 1 25.94% 4 62.95% 0.91 0.65039 1 82.81% 1 53.00% 1 31.95% 3 58.15%

Table 4 Maximum Number Fiber Optic Cables per Innerduct/Conduit From 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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288

1.000"/1.349"

144

288

1.550"/1.250"

1.968"/1.610" 144

144

144

2.375"/2.067" 144 144 144

144 144

1.000"/1.349"

1.550"/1.250"

1.968"/1.610"

2.375"/2.067"

1.000"/1.349"

1.550"/1.250"

1.968"/1.610"

2.375"/2.067"

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 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 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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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 fourinch 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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