AEIC CG5 2005-Underground Extruded Power Cable Pulling Guide

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AEIC

UNDERGROUND EXTRUDED POWER CABLE

PULLING GUIDE

(2nd EDITION)

Association of Edison Illuminating Companies 600 North 18th Street, Post Office Box 2641

Binningham Alabama 35291-0992 June 2005

www.aeic.org

Permission of the Association of Edison Illurninating Companies. AlI rights reserved.

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Permission of the Association of Edison Illuminating Companies. AlI rights reserved.

Please contact us at our website at: http://www.aeic.org

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12.2 Surging ........................................ 45 12.3 Siaek Pulling ......... ... 45 12.4 Looping .............................. 46 12.5 Lubricants ....... ......... 46 12.6 Pulling Speed ............. 46 12.7 Pulling Direction ...................... 46 12.8 Caution ........... .................... 47

12.9 Swivels and Other Deviees ... ...... 47

12.10 Riser Poles ... 47

12.11 Cable Installation Guide ........ 48

13.0 REFERENCES ..... 49

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OISCLAIMER

AEIC

UNDERGROUND EXTRUDED POWER CABLE

PULLING GUIDE

(2nd EDITION)

This guide was prepared by the Cable Engineering Committee of the Association of Edison lIIuminating Companies. Most of the data presented in this guide is derived from EPRI project EL-3333, which was conducted to correlate calculated values to actual field conditions. Other valuable sources of pulling data are available from cable and cable lubricant manufacturers.

Use of this guide is voluntary and the existence of the guide is not intended in any respect to preclude the manufacture or use of products not conforming to the guide.

While care has been taken in preparing this guide, AEIC makes no warrant y or representation in connection with its use. Persons electing to use the guide are reminded that they should independently evaluate their specifie needs and requirements before doing so. Users are also cautioned that there may be requirements issued by governmental and regulatory authorities which are not addressed by this guide. Because this guide is subject to review and revision, those who use it are cautioned to obtain the latest version.

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SCOPE

This guide outlines the pulling parameters that need to be considered when installing underground power cable in duct. Only extruded power cable is covered. Installations with more than three cables in a conduit are not included. Cable installations in trays or racks are also not included. A variety of pulling guides and computer software are available from many power cable manufacturers and cable lubricant manufacturers. Several of these guides provide a basic introduction to cable pulling criteria and are listed in references [1] - [3]. This guide is intended to complement these publications. Cable specialists from each electric power utility may desire to use this guide to develop their own simplified version which incorporates criteria unique to their systems.

Cable pulling is not an exact science: it involves a complicated combination of variables, which are often difficult to accurately predict. The information presented in this guide is a compilation of data obtained through mathematical modeling, experimentation, and experience. The best judgment of the AEIC Cable Engineering Committee was used to resolve conflicting data and controversial information.

Personnel who do not necessarily have an in-depth technical background can use the guide. The guide can also be used by engineers with a need for a detailed design guide.

ln the body of this guide a simplified approach is used to calculate pulling tensions and sidewall bearing pressures (SWBP) for the most commonly encountered conditions in the field. It incorporates tension and sidewall bearing pressure limits developed under EPRI Project EL-3333, "Maximum Safe Pulling Lengths for Solid Dielectric Insulated Cables", as weil as the many design parameters which must be considered when these new limits are employed. The more detailed considerations that utilize more complex formulae are addressed in the appendix. The major points covered in the guide include:

Factors that influence pulling tensions such as cable type, conduit type and size, lubricants and installation practices

Calculation of maximum pulling lengths allowable without damaging the cable Limits on cable tension and sidewall bearing pressure

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1.0 INTRODUCTION

A guide of this nature cannot cover ail of the possible cable and conduit parameters that can be encountered in the field. The intent of this guide is to provide the most recently available state-of-the-art technical information that interested parties can use to design a cable/conduit system.

The major topics covered are:

• One or three equally sized cables in a duct • New information covering only extruded cables • 0.6 kV to 138 kV cable

• Aluminum and copper conductors

• Cables with bare concentric wires, bare lead sheaths, jacketed lead sheaths, jacketed wires, jacketed LC shields and jacketed fiat straps

• Tension and sidewall bearing pressure limits • Grips and eyes

• Dynamic and static coefficients of friction

• Coefficients of friction at high and at low sidewall bearing pressures • Coefficients of friction for lubricated cable and duct

• Minimum bending radius

• Formulae and Sample Calculations • Jam ratios and clearance considerations • Installation considerations

• Pulling lines and duct wear • Surging • Slack pulling • Looping • Lubricants • Pulling speed • Pulling direction

2.0 CABLE REMOVAL

Removing cable from an old duct may be more difficult than pulling cables into a new duct. Silt and debris can collect in duct over the years and make cable removal extremely difficult. Also, when pulling lubricant dries out, it can adhere to both cable and duct resulting in a friction factor that is higher than that encountered if the same cable were installed with no lubricant. Usually, the cable being removed will be scrapped, so damage to the cable during removal is not a concem. The primary consideration is that pulling tensions not exceed the tensile strength of the cable. Currently, there is very little information on the effective static and dynamic friction coefficients encountered when removing cable. If excessive tensions are expected, flooding the duct with water can aid removal. Water itself is a lubricant. When wetted, the silt and de bris in the duct will not adhere as strongly to the cable. Also, the effective weight of the cable in a flooded duct will be

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3.0 HISTORY

Maximum safe pulling lengths for cables were established by field experiences of users and cable manufacturers during the early 1900's. The 1931 Underground Systems Reference Book stated ... "satisfactory operation of installed cables is assured provided that it has suffered no mechanical in jury. "

Buller analyzed effects of duct curvature in bends in 1949 (4) and Rifenberg established a more exact engineering method in 1953(5). These formulae and definitions of the associated conditions were used to gain additional confidence in longer and more complex pulls. By the mid 1970's it was generally known that sorne of the factors limiting longer cable pulls, such as sidewall bearing pressure, were neither realistic nor based on a strict engineering analysis.

The time had come to update available information on pulling tension limits for extruded power cable. As a result, an Electric Power Research Institute (EPRI) project was funded to quantify the factors that influence the maximum pulling lengths for pipe-type cable. This was published as EL-2847 (6).

The advent of extruded dielectric cables and the lack of information on pulling factors for these newer designs of cable led to the funding of an additional EPRI project. These results have been published as EL-3333 (7). This last research project has demonstrated the ruggedness of modem extruded cable constructions and has shown the need to compile the information into a comprehensive pulling guide. Use of these recent research results will not only enhance underground system reliability, but also minimize cable system costs.

4.0 ECONOMIC CONSIDERATIONS

Determining the maximum safe pulling lengths for power cables is an essential element necessary for designing the most cost effective and reliable cable system. When there is no equipment or physical limitations, the number of cable splices and splicing chambers can be minimized. Also, cable damage can be avoided.

5.0 NOMENCLATURE

There are many terms unique to cable/duct installations. Before a subject of this type can be discussed in detail, the terminology must be completely understood. Therefore, the definitions used in this guide are listed as follows:

Cable Entrance

Duct section closest to cable feeding equipment. This is the lowest tension point of the cable in the duct section under examination.

Cable Exit

Duct section closest to the cable pulling equipment. This is the highest tension point of the cable in the duct section under examination.

Extruded lEncapsulating) Cable Jacket

Cable jacket that substantially encapsulates the metallic shield. Tubed or Sleeved Cable Jacket

Cable jacket that is loosely applied over the metallic shield. Usually a separator tape separates the jacket and the metallic shield.

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5.1 Definition of Symbols

T1 T2

R

Re Rel W

e

K L SWBP Wc D d d' cmil C

J

A1ead

Ra

Ri 1t

e

Cable entrance tension Cable exit tension

Inside radius of duct bend Inside bending radius of cable Duct centerline radius

Total weight per unit length of cables in duct Angle

Coefficient of friction Length of cable in section Sidewall bearing pressure Weight correction factor Inside diameter of duct

Nominal outside diameter of one cable 1.05 x d

Area of circle 1 mil (.001 ") in dia. Clearance

Jam ratio

Lead sheath cross-sectional area Lead sheath outside radius Lead sheath inside radius 3.1416 2.7183 Pounds Pounds Feet Feet Feet Pounds/Foot Radians or Degrees* Dimensionless Feet Pounds/Foot Dimensionless Inches Inches Inches Circular Mils Inches Dimensionless Square Inches Inches Inches Dimensionless Dimensionless F Factor for determining minimum bending radius Dimensionless

*(Radians = Degrees x 1t/180)

6.0 DESIGN CRITERIA AND PULLING LlMITS

6.1 Cable Diameters and Weights

Cable diameters and weights listed in manufacturers' catalogs' and specification sheets are generally approximate and subject to normal manufacturing tolerances. Possible variations in cable diameters are taken into consideration in the formulae for cable clearance and jam ratio. Catalog weights are generally adequate, except for marginal cable pulls for which more accurate weights should be requested from the cable manufacturer. Actual cable weights should be used if available.

Assemblies of triplexed cables will weigh more per foot than paralleled cable. The cable manufacturer should be contacted to determine the weight of triplexed cables. The weight of parallel cables, whether they are delivered on individual reels or on multiple reels, will be the cable weight per unit length times the number of cables.

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6.2 Jamming

The Jam ratio (J), is defined as the ratio of the inside diameter of the duct (D) to the cable diameter (d), Le.

J= Dtd 6-1

When this ratio is close to 3.0, one of the cables in a three-cable pull may slip between the other two cables causing the cables to jam in the duct. This is most likely to occur when the cables are pu lied around a bend. Jamming is not usually a problem for essentially straight cable pulls. The following guidelines are suggested to minimize the risk of such an occurrence during cable installation in a duct.

If J is between 2.8 and 3.0, jamming could occur and it is not recommended that cable pulls with this jam ratio be performed unless the conduit run is free of elbows or sharp bends. Recognized variations in cable and conduit diameter and ovality in conduit diameter at bends are taken into account in these limits. Triplexed cable assemblies tend to maintain a triangular configuration during a cable pull and therefore jamming is not likely to occur regardless of the calculated jam ratio.

Pull lines, especially synthetic or fiber ropes can wear a groove in conduit bends. Jamming can occur in these bends for single cable and parallel multiple cable pulls when the cable diameter is slightly larger than the diameter of the groove. This happens because of a tendency for the cable to wedge into the groove and is an especially acute problem for cables with tubed jackets. Therefore, the pulling line diameter should be greater than the cable diameter or at least 0.5 inches smaller in diameter than the cable diameter.

6.3 Cable Configuration in Duet

The relative position of three parallel cables pulled in a duct is important because it affects the weight distribution of the cables and hence the normal (perpendicular) force between the cable and the duct. When pulling three parallel cables, the configuration of the cables is govemed by the ratio of the inside diameter of the duct to the nominal diameter of the cable. This parameter was defined earlier as the jam ratio, J. Based on field experience, observations made in EPRI Project EL-3333 and on information presented in Rifenburg (5), the following general trends can be expected for cable configuration in duct.

If J< 2.4, cables are triangular

If 2.4 < J < 2.6, cables tend toward triangular

If 2.6 < J < 2.8, cables are either triangular or cradled If 2.8< J < 3.0, cables tend toward cradled

If J> 3.0, cables are cradled

Note: Jamming can occur when 2.8 ~ J ~3.0.

However, if the sidewall bearing pressure, SWBP, is greater than1000 Ib.lft. and if 2.6<J<3.0, the cables tend to form a cradled configuration in a bend. When it is not clear which cable configuration will be encountered, the cradled formation is more conservative and should be assumed. Triplexed cables are assumed to travel in a triangular configuration at ail times.

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Single Cable

6.4 Cable Clearance

Cable Configuration in Duct

Three Cables Triangular

Three Cab les Cradled

It is necessary to calculate the clearance, C, to help ensure that the cable(s) will physically fit in the conduit intended for installation. From information in References (1), (2), and (3), the minimum clearance is 0.5 inches. A lesser clearance, as low as 0.25 inches, may be acceptable for essentially straight pulls. The clearance should also be adequate to accommodate the pulling eye or cable grip that'will be employed for the cable pull. In some cases the National Electrical Code (NEC), or other codes, may have limitations that supersede the clearance criteria indicated in this guide and the user should determine if there are other goveming regulations. It is necessary only to calculate the clearance for the triangular configuration. Cradled cables change to a triangular configuration before the fit becomes too tight in a cradled configuration.

6.4.1 Clearance Formulas

a) Single Cable Pull

C = D - d' 6-2

b) Three Cable Pull (Based on Triangular Configuration)

C = D/2 - 1.366d' + 0.5(D-d') (1 - [d'/(D-d')] 2 ) 0.5 6-3

(Symbols are in Definition of Symbols, Section 5.1, on Page 5)

NOTE: ln order to allow for variations in cable and duct dimensions and ovality of the duct at bends, the nominal cable diameter (d) has been increased by five percent (d') in the above formula, i.e. d'= 1.05d.

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Single Cable

6.5 Minimum Bending Radius

Cable Clearance

Three Cab les Triangular

If a power cable is bent in a radius that is too severe, the cable structure may be damaged. Field experience and laboratory tests have been used to establish the minimum bending radii for various cable designs. The following information outlines the minimum bending radii that have been established by ICEA standards, and are generally accepted for commonly used unarmored power cables. These are values to which insulated cables may be bent for permanent training during installation. Although developed as minimum training radii, the results from EPRI project EL-3333 indicate that they are appropriate for minimum cable pulling radii for conduit bends and sheaves

if

tension and SWBP limits are not exceeded. They are not necessarily applicable for cable pulled over rollers. The cable manufacturer should be contacted when rollers are used in a cable pull. In ail cases the minimum radius specified refers to the inner surface of the cable and not to the axis of the cable.

For single cables, the minimum bending radius is a multiple factor "F" of the single cable overall diameter. For cable assemblies of either paralleled or multiplexed single cables, the minimum bending radius is a multiple F of the circumscribed diameter of the assembly. For three single cables paralleled or triplexed, the circumscribed diameter of the assembly is 2.155 times the diameter of a single cable. For four single cables paralleled or quadruplexed, the circumscribed diameter is 2.414 times the diameter of a single cable.

Therefore, for a single cable, the allowable minimum bending radius is: R min = F

x

0.0.

For an assembly of three single cables, the allowable minimum bending radius is:

R min

=

F X (2.155 X 0.0.)

For an assembly of four single cables, the allowable minimum bending radius is:

R min

=

F X (2.414 X 0.0.)

Where: R min

=

minimum allowable bending radius F

=

multiplication factor for the cable design 0.0. = overall single cable diameter

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F factors are given in the following tables 1A, 1 B, and 1 C for different cable configurations. The appropriate factor and formula must be applied, according to whether a single cable or a cable assembly is being pulled into a duct, of if the cable assembly is simply being trained as single cables at sorne terminal point.

Table 1A

F Factor for Cables Rated 600 V to 2 kV

Thickness of F for single cable F for cable assemblies of single cable cable insulation 0.0. of 0.0. 0.0. of 0.0. of O.D.

in mils 1" or less over1" 1" or less >1" to 2" over2"

155 or less 4 5 4 5 6

170 to 310 5 6 5 6 7

Table 18

F Factor for Cables Rated 5 kV to 35 kV with Metallic Shield

F for assemblies Type of Shield F for single cable of single cable

Concentric Neutral 8 5

Tape and LC Shield 12 7

Wlre Shield 8 5

Combination Tape & Wlre 12 7

Lead Sheath 12 7

Table 1C

F Factor for Cables Rated 46 kV to 138 kV with Metallic Shield Type of Shield F for single cable

Tape and LC Shield 20

Combination Tape and Wlre 20 Wlre and Flat Strap Shield 18

Lead Sheath 18

Smooth Aluminum 40

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6.6

Duct Size

It is critical that the inside diameter of the duct to be used is known for pulling calculations. Also, the inside radius of the bend must be known before SW8P calculations can be performed. The National Electrical Code specifies limitations with regard to cable and conduit size for installations under its jurisdiction. It requires that the cross-sectional area of the cables be not more than a certain percentage of the given duct. This is often referred to as a percent fill limitation. Other codes may have similar requirements and should be investigated before final calculations are made.

Many different duct types are used in the electric industry. Commonly employed ducts are made of PVC (polyvinyl chloride), PE (polyethylene), ABS (acrylonitrile butadiene styrene), fiber, transite, fiberglass, and steel. Sorne of the industry standards that cover the physical properties and dimensional characteristics of PVC duct are AS TM F512 and 01785, and NEMA TC6 and TC8. They are covered for ABS duct in ASTM 01788, 02282, and 02750. Many other industry standards are also applicable.

PVC and ABS duct thickness are covered by a range of categories. Schedule 40 and Schedule 80 conduit are categories that are commonly used for duct installed above and below ground. DB (Direct Buried) and EB (Encased Buried) are categories used for duct installed underground. The categories are listed in order of decreasing wall thickness.

Schedule 80 Schedule 40 DB - NEMA TC 6 DB-NEMATC8 EB - NEMATC 6 EB -NEMA TC 8

The inside diameter and cross-sectional area of a typical duct size (Schedule 40) are given in Table 2. The inside and centerline radii of Schedule 40 conduit bends are also given in the table. Manufacturers will supply tables for other conduit sizes or they may be obtained from applicable industry standards.

Table 2

Schedule 40 Conduit - 90° Bends or Sweeps

Duct Bend Centerline Radius (Inches) .

Size Duct ID Area 12 15 18 24 30 36 42 48

IPS* (Inchesl in2 _{Bend Inside Radius (feet)}

2 2.067 3.36 0.91 1.16 1.41 1.91 2.41 2.91 3.41 3.91 2-1/2 2.469 4.79 1.15 1.40 1.90 2.40 2.90 3.40 3.90 3 3.068 7.38 1.37 1.87 2.37 2.87 3.37 3.87 3-1/2 3.548 9.90 1.35 1.85 2.35 2.85 3.35 3.85 4 4.026 12.72 1.82 2.33 2.83 3.33 3.83 5 5.047 20.00 2.29 2.79 3.29 3.79 6 6.065 28.89 2.75 3.25 3.75

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6.7 Coefficient of Friction

The coefficient of friction, K, is an important aspect of cable pulling calculations and must be selected carefully. There are two types of friction coefficients, which must be considered. The static coefficient of friction dictates the fÇ>rce required to start a cable in motion. The dynamic coefficient of friction dictates the force required to keep a cable in motion. The static coefficient of friction is greater than the dynamic coefficient of friction. Most pulling calculations are made using the dynamic coefficient of friction because most cable pulls are continuous and it is the force on a cable during a pull that is of concern. If a pull is stopped before it is complete, a higher tension is required to restart the pull than is required to maintain the cable in motion. Cable pulls should be started (or restarted) slowly.

The coefficient of friction is a function of the materials that are in contact with each other and the pulling lubricant that is used. The coefficient of friction for both single and three cable pulls was measured experimentally under EPRI project EL-3333. The friction factor measured for three cables was not the same as that predicted by the commonly used Rifenberg relationships. Thus, for tension calculations performed in this guide, separate friction factors are provided for single cable and three-cable geometries. The three-cable geometry friction factors are based on the EL-3333 measured values. The weight correction factor, Wc, is used in this guide for three cable pulls to account for different cable/duct geometries.

Under the EPRI EL-3333 project it was observed that the normal force between the cable and conduit significantly affects the dynamic coefficient of friction in lubricated ducts. As the normal force increases, the friction factor decreases. This phenomenon occurs because the function of the lubricant changes. At low normal forces, the pulling lubricant layer between the cable and the conduit is relatively thick. Under this condition, shear forces in the compound must be overcome for this cable to move. However, when the force is high, the lubricant layer is very thin and the boundary layer phenomenon takes over, allowing the pulling compound to act as a more effective lubricant.

The data indicates that at the side wall bearing pressure SWBP of approximately 150 pounds per foot, the dynamic coefficient of friction is significantly reduced. In effect, this means that the lower coefficient of friction can be used when the cable is being pu lied around bends where the pulling tensions and radii are such that the sidewall bearing pressure is 150 pounds per foot or greater. The higher value of dynamic coefficient of friction should be used for essentially straight pulls and lead-in bends at the start of the pull and bends where the SWBP is less than 150 pounds per foot. A tabulation of dynamic coefficients of friction for various cable materials, ducts, and lubricants are listed in Tables 3 and 4. Values are shown for low bearing pressures and for bearing pressures of 150 pounds per foot and higher. The friction factor was not measured for ail cable/conduitllubricant combinations. If values for specifie combinations are not shown, the cable, conduit, or lubricant manufacturer should be consulted.

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Table 3

Recommended Dynamic Coefficients of Friction for Straight Pulls and Bends with SWBP <

150 Ib.lft. (soap and water based lubricants)

Duct Cable Outer One Cable Three Cables

Material Covering PerDuct Per Duct

Installation Temp. 75°F 20°F 75°F PVC XLPE 0.40 0.40 0.60 PE 0.40 0.35 0.45 PVC 0.50 0.25 0.60 N 0.90 0.55 1.50 CN 0.40 0.40

-Pb 0.25 0.25

--PE p<LPE 0.45 0.35 0.55 PE 0.25 0.20 0.85 PVC 0.30 0.20 0.45 N 0.65 0.45

--~N 0.20 0.20

--Pb 0.20 0.25

--FIBRE p<LPE 0.30 0.20 0.65 PE 0.25 0.35 0.60 PVC 0.40 0.20 0.45 N 0.40 0.30 0.55 ~N 0.40 0.35

-Pb

-

--

-~ONCRETE p<LPE 0.30

--

--PE 0.35

--

-PVC 0.55

--

-N 0.50

--

--~N

-

--

--Pb 0.55

--

--:TRANSITE XLPE 0.70

-

0.70 PE 0.70 0.35

--PVC 0.70 0.35 0.70 N 1.00 0.95 1.80 CN

-

--

--Pb

--

--STEEL XLPE 0.60 0.45 0.65 PE 0.50 0.50

--PVC 0.65 0.40

--N 1.05 0.70 1.75 CN 0.50 0.50

--Pb

--

--1

1

·

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Abbreviations for Tables 3 and 4 PVC - polyvinyl chloride

XLPE - crosslinked polyethylene Pb -Iead

PE - polyethylene N - neoprene

CN - bare concentric neutral

Fiberglass ducts were not commonly used when EPRI EL-3333 was conducted. Therefore, no information on fiberglass duct was generated. Limited data from EL-3333 indicates that at low sidewall bearing pressures, coefficients of friction for clay based lubricants are 20 to 250 % higher than for soap and water based lubricants.

Table 4

Recommended Dynamic Coefficients of Friction Single and Three Cable Pulls

Bends Where Sidewall Bearing Pressure is 150 Ib./ft. or Greater Soap and Water or Clay Based Lubricants

Cable Outer Covering Duct Material K

XLPE,PE,N PVC, PE, CONCRETE 0.15

PVC PVC, PE, CONCRETE 0.30

XLPE,PE,N STEEL 0.25

PVC STEEL 0.30

Pb STEEL 0.20

Consideration should also be given to the coefficient of friction for the pulling line. Coefficients of friction for pulling lines are often high and therefore can affect the maximum tension in the early part of the pull. This may govern the required capacity of the pulling equipment, sheaves or eyebolts as weil as the duct construction. Pulling li ne manufacturers should be contacted for this information.

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6.8 Weight Correction Factor

The Weight Correction Factor, Wc, is used to account for the weight distribution of the individual cables in a multiple-cable pull. This factor depends on the relative position of the individual cables in the duct. Both the cradled and triangular configurations produce a greater normal force between cables and conduit than would exist for a single cable. This can be viewed as an effective increase in the weight of the cables and leads to the development of the Weight Correction Factor.

Weight Correction Factors for three cable pulls in cradled and triangular configurations can be calculated as follows:

a) Three single cab les in cradled configuration:

b) Three single cables in triangular configuration:

c) Single cable in a conduit:

6.9 Sidewall Bearing Pressure

4[

d

J2

WC = 1 +

-3 D-d _6-4

6-5

Wc

=

1 6-6

Sidewall bearing pressure (SWBP) is a radial force per unit length exerted on a cable being pulled around a bend. For a single cable it is defined as:

SWBP

=

Tension in a cable at Bend Exit (Pounds) Inside Radius of Bend (ft.)

6-7

The remaining SWBP formulae for three different cable configurations are presented in Sec. 9. Exceeding the maximum allowable SWBP may subject the cable to crushing damage. For this reason, sidewall bearing pressure may be the most restrictive factor for those installations having bends and high tensions. High anticipated SWBP values could be lowered by increasing the duct radius to minimize the possibility of cable damage.

The inside radius of the duct bend should be used when calculating the SWBP. The values of inside radius for Schedule 40 duct bends are given in Table 2. For specially fabricated bends, the inside radius should be measured or calculated using the following expressions:

R = (Rel - 0.5D)/12 6-8

Where: R = Inside Radius of Bend (feet)

Rel

=

Centerline Radius of Bend (inches) D = Duct Inside Diameter (inches)

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7.0 DESIGN LlMITS

The maximum pulling tension that can be applied to a given cable system is dictated by the physical limitations of the cable (both tensile and crushing strengths), the method of cable attachment (pulling eyes or grips), and the design of the duct structure.

7.1 Tension Limits for Pulling Eyes and Bolts

The maximum recommended tensions for pulling eyes and pulling bolts are listed in Table 5. For

most pulls, standard compression pulling .eyes or bolts are adequate. Higher tensions can be

obtained when needed by using solder filled pulling eyes on copper conductors or by using pulling eyes filled with aluminum based epoxy on aluminum conductors.

Table 5

Recommended Maximum Pulling Tension Stress for Pulling Eyes or Bolts on Copper and Aluminulll Conductors

Maximum Stress for Pulling Eyes/Bolts

Aluminum Solder Filled Epoxy Filled

ComRression

Conductor psi Ib.lcmil psi Ib.lcmil psi Ib./cmil

Copper 14,000 0.011 16,000 0.013

-

-(annealed) Solid Aluminum 8,000 0.006 10,000 0.008 (1/2 to Full

-

-hard) Stranded Aluminum 10,000 0.008

-

- 14,000 0.011 (3/4 & Full hard)

NOTE: - Ib.lcmil values are approximate

- The conductor cross-sectional area is sum of the cross-sectional areas of the individual strands.

For three single conductor cables in a parallel or triplexed configuration, the allowable pulling tension should be based on two cables sharing the load. This recommendation is based on field experience and on the test performed in EPRI Project EL-3333.

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7.2 Tension Limits for Pulling Grips

When using steel wire basket grips, the maximum recommended pulling tensions are given in Table 6. Before installing the grip, the cable should be cleaned and wrapped with two ha If lapped layers of cloth friction tape. The back end of the grip should be secured with a steel band or equivalent device to aid in initially seating the grip and to prevent it from loosening should the pulling tension be relaxed during the pull. The pulling tension limits in Table 6 are applicable only if the grip is properlyapplied.

A grip placed on multiple cables does not seat as weil as a grip placed on a single cable. Higher tensions can be obtained by using one grip for each cable in a multiple cable pull. Cables under grips plus an additional two feet should be cut off after pulling is complete. Thus sufficient cable should be pulled to allow for this removal before terminating or splicing. If split grips are used for "slack pulling", the values in Table 6 are not applicable and the cable manufacturers should be contacted for appropriate values.

Table 6

Recommended Maximum Pulling Tension Limits for Pulling Grips - See Note 1 Maximum Tension -Ib.*

Triplexed and Paralleled Cables

Single One Grip on Three Grips

-CABLE CONSTRUCTION TYPE Cable Three One Grip per

Cables Cable

XLPE Insulation - 600 V Cable 2,000 2,000 4,000

EPR - Neoprene- 600 V Cable 2,000 2,000 4,000

PE & XLPE insulation, concentric wire shield,

with and without encapsulating jacket - ail 10,000 5,000 20,000

voltages

PE & XLPE insulation, LC Shield, LOPE jacket

15, 25 & 35 kV Cable 8,000 4,000 16,000

46 - 138 kV Cable 4,000 2,500 8,000

PE & XLPE insulation, concentric wire or tape

shield, LOPE & PVC sleeved jackets - ail 10,000 5,000 20,000

voltages

EPR insulation, concentric wire or tape shield,

10,000 10,000 20,000

LOPE & PVC sleeved jackets - ail voltages XLPE insulation, copper wire or ribbon shield,

18,000 9,000 36,000

MDPE sleeved jacket - ail voltages

* See NOTES for Tables 6,7 and 8 at the end of Table 8.

BE SURE THAT THE CONDUCTOR IS LARGE ENOUGH TO HANDLE THE TENSION VALUES LlSTED ABOVE

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When a grip is placed on lead sheathed cable, the maximum pulling stress is based on the cross sectional area of the lead sheath of an individual cable as indicated in Table 7. If the tension limit is exceeded, the lead sheath may be pulled off the cable.

As a precaution, ail the tension placed on any cable by a grip is assumed to be transferred to the conductor. Therefore, for sm ail conductor cables, the maximum conductor stress will limit the cable pulling tension. This procedure is allowed because the amount of tension that is distributed within the cable layers is very difficult to predict. A check of the stress placed on the conductor should be made to verity that conductor stress limits are not exceeded.

Table 7

Recommended Maximum Pulling Tension Stress Limits* For Pulling Grips on Lead Sheathed Cable

See Note 2

Maximum Stress - psi of lead sheath Triplexed and Paralleled Cables

Cable Construction Single Cable One Grip Three Grips

-With and without jackets On Three Cables One Grip per

Cable

XLPE insulation 16,000 16,000 32,000

EPR insulation 8,000 8,000 16,000

* See NOTES for Tables 6, 7 and 8 at end of Table 8.

7.3 Maximum Sidewall Bearing Pressure (SWBP)

The recommended maximum sidewall bearing pressure (SWBP) limits are given in Table 8. Although they are higher than previously published values, they do incorporate conservative safety

factors. However, because of the uncertainties involved in pulling cables, cables should not be

pulled at SWBP levels that are higher than the values listed. To do so may damage the cable. Remember not to pull the cable through a radius that is less than the recommended minimum bending radius of the cable given in Tables 1A .18. and 1C.

The sidewall bearing pressure limits in Table 8 may not be applicable for conduit that is not encased in concrete. They are higher than previously published values. Under these conditions the pulling line or cable could wear through plastic conduit. Also, the duct system could be damaged if it is not sufficiently anchored in the earth.

(21)

Table 8

Recommended Maximum Sidewall Bearing Pressure

Cable Construction Type Maximum SWBP

(Ib.lft.) XLPE Insulation - 600 V cable

1,200

EPR, Neoprene - 600 V Cable 1,000

PE & XLPE insulation, concentric wire shield:

Without Jacket 1,200 (3)

With encapsulating jacket 2,000

PE & XLPE insulation, LC shield, LOPE jacket 1,500

PE, XLPE, EPR insulation, concentric wire or tape shield, LOPE, & PVC

2,000 (4) sleeved jackets

Lead sheathed cable, with & without jackets:

XLPE insulation 2,000 (2)

EPR insulation 2,000 (2)

XLPE insulation, copper ribbon shield, MOPE sleeved jacket. 2,000

NOTES FOR TABLES 6, 7, AND 8:

1. When using a grip. the stress on the cable conductor should not exceed the following values: 16,000 psi (0.013 Ib.lcmil) for copper conductors (annealed)

14,000 psi (0.011 Ib.lcmil) for stranded aluminum conductors (1/2 through full hard)

10,000 psi (0.008 Ib.lcmil) for solid aluminum conductors (3/4 & full hard) For three single conductor cables in parallel or triplexed configuration, the allowable conductor stress should be based on two cables sharing the load.

2. The stress values are based on the cross-sectional area of one lead sheath.

3. For a three-cable pull (triplexed or parallel), a maximum SWBP limit of 750 Ib.lft. is recommended. 4. The recommended SWBP limit should be reduced to 1500 Ib.lft. wh en the jacket is not applied

tightly to the cable core.

5. The maximum SWBP limits included in this guide are higher than those previously published. They were developed by conducting pulls on a wide variety of cables. However, sorne cable

manufacturers may be unwilling to commit to these limits for sorne of their cable designs. Therefore, the user of this guide is encouraged to contact the cable manufacturer to verity that they are in agreement with the newly established SWBP limits.

(22)

8.0 PULLING TENSION FORMULAE

The following formulae can be employed to determine pulling tensions for a cable installation. Each equation applies to a specifie conduit configuration. In order to use the formulae, the cable pull should be subdivided into specifie sections. The configuration of each section should be identified with one of the graphical depictions accompanying the equations. Angles must be expressed in radians for use in eguations with exponentials.

The mathematical expression associated with each of the accompanying sketches will yield the cumulative tension, T2 , on the leading end of the cables(s) as it exits from a specifie section. T1 is the tension on the cable entering that section. The maximum tension obtained when pulling in one direction often differs from that obtained when pulling in the opposite direction due to the location of the bends and the slope of the pull. Therefore, the pulling tension should be calculated in both directions.

These expressions have been simplified to cover the most common situations. The simplifying assumptions are within 5% of the true value for the stated limitations. The more complex version of the tension expressions as weil as the vertical dip equation is included in the Appendix. If the calculated tension or SWBP is within 20 percent of the maximum allowable value, the more exact relationship should be used as a precaution.

(23)

Pulling Tension Formulae for Cable in Conduit

**Symbols are in Definition of Symbols section on page 5 **

8.1 Straight Pull

8.2 Siope - Upward Pull

8.3 Siope - Downward Pull

CG52005 L T2

=

T1 + LW(sin

e

+ KWc cos

e)

(9 in degrees) 8-1 8-2

NOTE: Angle

e

(in degrees)

measured from horizontal axis

T2 = T1 - LW(sin

e -

KWc cos 8) 8-3

(24)

8.4 BENDS

Simplified equations for feed-in bend sections will not produce significant errors even if the WRlT1 requirement is not satisfied. The Iimits of the WRfT1 requirement are identified next to each illustration.

T₂

=

T1e (k)(Wc)( 9) (Simplified) (9 in radians) 8-4

Equation (8-4) applies to each of the following Assumptions:

8.4.1 Assumption #1 - Horizontal Bends

R

Tz

WRfT1 < 0.5 0<9<n12

8.4.2 Assumption #2 - Vertical Convex Bend - Upward Pull

T

1

R

WRfT1 < 0.05 0<9<7t/2

Angle 9 measured from the vertical axis

(25)

8.4.3 Assumption # 3 - Vertical Convex Bend - Downward Pull

Tl

WRlT1 <0.1

0<8<1t/2

Angle 8 measured trom the vertical axis

8.4.4 Assumption # 4 - Vertical Concave Bend - Upward Pull

WRlT1 < 0.08

0<8<1t/2

Angle 8 measured

trom the vertical axis

8.4.5 Assumption # 5- Vertical Concave Bend - Downward Pull

Tl

WRlT1 < 0.05

0<8<n12

Angle 8 measured

(26)

For bends offset from the vertical axis, a slightly moditied procedure is necessary to separate the angle into two calculations. The tirst procedure is to draw a vertical line, L, through the bend such that it intersects the bend perpendicularly. The arc, A, of the bend may have to be extended to execute this procedure. The tension drop is then calculated for two arc sections, which can be added or subtracted to yield the tension in the desired offset bend. The resulting two arc sections

should be treated as normal convex or concave bends that have WRJT1 restrictions. As an

example, suppose the tension increase was to be calculated for the bend below.

For angle S offset from vertical axis by angle Sa

1E-1<--R--~~

---.

L T1---l~-~ T1---l~-A 1 1 1 1 1 1 1 1 1 1 1

,

1 1

(27)

9.0 SIDEWALL BEARING PRESSURE FORMULAE

The cable sidewall bearing pressure (SWBP) is the radial pressure experienced by the cable as it is pulled through a curved section. The pressure is caused by the tension and weight of the cable, which tends to force it against the conduit wall. The parameters, which influence the SWBP, are cable tension and the inside radius of curvature of the bend. For single cable in conduit pulls, the maximum SWBP in a bend is:

9-1 where T2 is the tension at the bend exit.

The SWBP for three cable pulls should be calculated for the cable which presses hardest against the conduit.

ln a cradled formation, the center cable presses hardest against the conduit and the SWBP for that cable is expressed as follows:

SWBP

=

(3Wc - 2) T2/(3R)

9-2

ln a triangular formation, the bottom two cables share the bearing load equally and

experience the greatest SWBP. For this condition the SWBP equation is:

SWBP

=

Wc T2/(2R)

9-3

(28)

10.0 CALCULA TION SEQUENCE

If the calculation sequence outlined below is foIl owed, cable duct runs can be designed with minimum effort and recalculation. This outline is a format for determining safe pulling lengths for cables installed in duct systems.

Procedure

1. Select cable; determine its outside diameter and its weight per foot. 2. Determine the duct type, and size it to handle the required number of

cables per duct.

3. Calculate the jam ratio, clearance factor and weight correction factor. 4. Look up the maximum allowable tension, SW8P and minimum bending

radius for the cable under consideration.

5. Look up both low and high SWBP friction factors for given cable, conduit, and lubrication types.

6. Consider accessibility and the limitations of pulling equipment and hardware. 7. Design the ductlmanhole system if an existing system is not being used.

Calculate the tension and SWBP for each section. As the calculation proceeds from one duct section to the next, the existing tension for a given section, T2 , becomes the entering tension T1 , for the next section. Check to see if the allowable tension or SW8P Iimit has been exceeded. 8. Angles are expressed in degrees when tension calculations involve

trigonometric functions (sin, cos), such as uphill or downhill slopes. Angles are expressed in radians wh en tension calculations involve exponential functions, su ch as conduit bends.

The following table shows the equivalency of degrees to radians:

Degrees Radians 3600 _27t 1800 _7t 900 _7t/2 450 _7t/4 Where 7t = 3.1416

(29)

11.0 SAMPLE CALCULA TIONS

These sample calculations are primarily intended to show the user of this guide how to perform cable-pulling calculations. They do not necessarily represent a typical cable/conduit system.

11.1 Sam pie Calculation NO.1

Cable: A single, 25kV, 1000 kcmil, 3/4 hard, stranded aluminum conductor, XLPE insulated,

concentric wire shielded cable with an extruded polyethylene jacket is to be pulled in the duct system shown below.

Conduit:

Cable weight, W = 2.41 Ib./ft.

Outside diameter, d = 2.3 inches Insulation thickness = 0.260 inches

3 inch PVC, schedule 40 Conduit I.D., D= 3.1 inches Lubricant: water based

Number of cables in conduit: 1

DuctiManhole Layout:

SIDE VIEW

NOTTOSCALE

5 2500 ft 1500 ft Note: 15 0 = n/12 radians B 2000 ft

(30)

Using the calculation sequence outlined in Section 10, the following values are determined.

Cable weight, W

=

2.41 Ib.lft.

Cable outside diameter, d = 2.3 inches Conduit inside diameter, D

=

3.1 inches

Jam Ratio, (not needed because there is only one cable per conduit) Weight Correction Factor, Wc

=

1 (1 cable per conduit from 6-6) Clearance, C

=

D -

1.05d

=

3.1 inches - (1.05) (2.3 inches) (from 6-2)

=

0.69 inches. C is greater than 0.5, so clearance should not be a problem. Tension Limits:

If pulling Eye is used,

T max

=

0.008 Ib.lcmil X 1,000,000 cmil

=

8000 lb. (Table 5)

If Pulling Grip is used:

T max = 10,000 lb. (Table 6)

Note:

From Note 1 for tables 6,7, and 8, the maximum allowable force on the conductor

must be calculated. For stranded aluminum conductor, the maximum stress is 14,000 psi or .011 Ib.lcmil. For a 1000 kcmil conductor, this is a force of 11,000 lb. Thus a pulling grip can be used to make the pull, but tension must be limited to 10,000 lb.

The maximum SWBP is 2000 Ib.lft. (Table 8)

The minimum bending radius is 8 x 2.3 = 18.4 inches (Table 1 B) The low SWBP friction factor = 0.40 (Table 3)

(31)

The Calculation sequence is performed twice to examine the tension and SWBP for each pull direction.

Calculation based on pulling from B to A

Conduit Section 1: Straight Pull

I~

_{2000 ft}

Assume the tension from the cable reel to the duct entrance is approximately 50 lb. From Equation 8-1

The low SWBP friction factor is used for straight sections.

T2

=

50 lb. + 2.41 Ib.lft. x 0.4 x 1 x 2000 ft., or

(32)

Conduit Section 2: Concave Bend - Upward Pull

From equation 8-4, Assumption #4

Check to see if simplified equation is valid,

WRrr

1= (2.41 Ib.lft.) (30 ft.) /1978 lb. = 0.037

WRrr1< 0.08, therefore the equation is valid

Assuming the low SWBP friction factor, T₂

=

(1978 lb.) e (O.4)(1)(n/12) T2

=

2196 lb.

SWBP

=

T2/R

=

21961b.l30 ft.

=

73 Ib.lft.

e

=1t/12

SWBP < 150 Ib.lft., therefore the low SWBP friction factor assumption was

(33)

Conduit Section 3: Siope - Upward Pull

1500ft

From equation 8-2

T2

=

T1 + LW (sin

e

+ K Wc cosa)

Using the low SWBP friction factors because this is a straight section;

T2

=

2196 lb. + (1500 ft.)(2.41 Ib./ft.) [(sin 15° + (0.4)(1)(cos 15°)], or T2 = 4528 lb.

Conduit Section 4: Convex Bend - Upward Pull

T~

1

T

,

R=6Oft

a

=

rr112

Check to see if the simplified equation is valid; WRfT1 = (2.41 Ib./ft.)(60 ft.) /4528 lb.

WRfT1 < 0.05, therefore the equation is valid Assuming the low SWBP friction factor;

T₂= 4528 x e (O.4)(1)(7t/12)

=

5028 lb. SW8P

=

T 2/R

=

5028 Ib./60 ft.

=

84 Ib./ft.

(34)

From equation 8-1 T2 = T1 + WKWcL

T2 = 5028 lb. + (2.41Ib./ft.)(0.4)(1)(2500 ft.), orT2

=

7,438 lb.

Conduit Section 6: Concave Bend - Upward Pull

T₂= T₁

e

KWc9

Check to see if simplified equation is valid.

WRlT1

=

(2.41 Ib./ft.)(3 ft.)n438

=

0.001

WRlT1 < 0.08, therefore the equation is valid.

Assuming the low SWBP friction factor,

T₂= (7438 lb.) e (0.4)(1)("'2) = 13,942 lb. SWBP

=

T2/R = (13942 Ib.)/(3ft.) = 4647 lb.

(35)

Thus, if the cable travels from B to A, the maximum calculated system tension and SWBP values are:

T maximum

=

9414 lb. At A

SWBP maximum

=

3138 Ib.lft.

The pulling tension T max is less than the maximum allowable value determined earlier; however the

SWBP is greater than the maximum allowable value determined earlier. Therefore, to see if the tension and SWBP will be lower if the pull is made in the opposite direction, the calculation will be performed assuming that the cable travels from

A

to B.

Calculation - Pulling from A to B

Conduit Section 6: Concave Bend - Downward Pull

Assume tension from cable reel to duct entrance is approximately 50 lb.

Check to see if the simplified equation is val id.

WRlT1 = (2.41 Ib.lft.)(3 ft.)/50 lb. = 0.14

WRIT1

= is not < 0.05. However the simplified equation is used because it yields a more

conservative answer in this case. Assuming the low SWBP friction factor,

T₂= (50 lb.)

e

(0.4)(1)("'2) = 94 lb.

SWBP

=

T2/R

=

941b.l3 ft.

=

31 Ib.lft.

(36)

T

₂----2-5-00-ft---.~

T

₁

From equation 8-1

Using the low SW8P friction factor because this is a straight section; T2

=

94 lb. + (2.41 Ib.lft.)(0.4)(1)(2500 ft.), or T2

=

2504 lb. Conduit Section 4: Convex Bend - Downward Pull

T1~T2

Check to see if the simplified equation is correct; WRfT1 = (2.41 Ib./ft.)(60ft.)/2504Ib. = 0.058

WRfT1 is <0.10, therefore the equation is valid.

Assuming the low SWBP friction factor, T₂

=

(2504 lb.)

e

(0.4)(1)(lt/12) or

T2

=

2780 lb.

SW8P

=

T2/R

=

2780 Ib.l60ft.

=

46 Ib.lft.

R=60ft 81i1'12

(37)

Conduit Section 3: Straight Pull- Downward Siope

----'---- T

_z

From equation 8-3

T2

=

T1 - LW (sinS - KWcCosS)

Using the low SWBP friction factor (straight pull)

T2

=

2780 lb. - (1500 ft.)(2.41 Ib./ft.)[(sin 15°) - (0.4)(1)(cos 15°)]

T2

=

3241 lb.

Conduit Section 2: Concave Bend - Downward Pull

From Equation 8-4, Assumption #5

WRIT

1 = (2.41 Ib./ft.)(30 ft.)/3241 lb. = 0.022

WRlT1 < 0.05, therefore the simplified equation is valid.

Assuming the low SWBP friction factor;

T₂

=

(3241 lb.) e (0.4)(1)("'12)

=

3599 lb.

SWBP = 3599 Ib./30 ft. = 120 Ib./ft.

Therefore the low SWBP friction factor assumption was correct.

(38)

---2-0-0-0-ft---.~

T

2

From Equation 8-1

Using the low SWBP friction factor (straight pull) T2 = 3599 lb. + (2.41 lb.! ft.)(0.4)(1 )(2000 ft.), or T2

=

5527 lb.

This tension is below the maximum allowable pulling tension for this cable and the SWBP limit has not been exceeded. Therefore, the pull can safely be made by pulling from A to B.

11.2 Sam pie Calculation No. 2

Cable: 35 kV, 1/0 AWG stranded, copper conductor, EPR in~ulated, lead sheathed with an XLPE jacket. Three cables are to be pulled in one duct in the system shown below.

Cable weight, W = 2.94 Ib.lft. per cable or 8.82 Ib.lft. for 3 cables. Cable outside diameter, d = 1.59 inches

Insulation thickness, 0.345 inches Jacket thickness, 0.20 inches Lead thickness, 0.040 inches Conduit: 4 inch steel, schedule 40

conduit inside diameter, D = 4.026 inches soap and water lubricant

(39)

DuctlManhole Layout: TOPVIEW B 91E/2 3 R=25ft

NOTTOSCALE

75ft 5 A

Using the calculation sequence outlined in Section 10, the following values are deterrnined: -Cable weight, W = 2.94 Ib.lft.

-Cable outside diameter d

=

1.59 inches -Conduit inside diameter, D = 4.026 inches

-Lead Sheath inside diameter (cable outside diameter minus twice the jacket and lead thickness) = 1.11 inches

-Lead sheath inside radius, Ri

=

0.555 inches -Lead sheath outside radius, Ro = 0.595 inches -Jam Ratio:

From 6-1, J

=

Dtd

J

=

4.026 inchest1.59 inches

=

2.53

J < 2.76, therefore jamming should not occur.

Since there are three cables in one conduit the cable configuration must be determined. Based on the discussion on this subject in Section 6, when 2.4 < J <2.6 the cables will form a triangular configuration if the SW8P at a bend is not above 1000 Ib.lft. Since J = 2.53, the cables are assumed to forrn a triangular configuration unless high SW8P's are calculated at any of the bends.

(40)

- Weight correction factor:

From 6-5, for a triangular configuration;

Wc = 1/ (1 - [d / (0-d)]2) 0.5

Wc = 1 / (1- [1.59 inches/(4.026 inches - 1.59 inches)] 2) 0.5 Wc= 1.32

- Clearance:

From section 6, for a triangular configuration

C = 0/2 - (1.366) d' + 0.5 (0 - d')(1 - [d' / (O-d')] 2 )0.5 Where d' = 1.05d = 1.67 inches

C = 4.026/2 - (1.366)(1.67)+ 1/2 (4.026-1.67)(1 - [1.67 / (4.026-1.67)] 2) 0.5 (Ali dimensions are in inches)

C = 0.56 inches

C > 0.5 inches, therefore the cables should not have a clearance problem.

- Tension Limits: From Table 5:

If a compression eye is used, and two cables carry the load, T max = 2(0.011 Ib./cmil) X (105,600 cmil) = 2323 lb., or T max = 2323 lb.

If a solder-filled eye is used, and two cables carry the load, T max = 2(0.013 Ib.lcmil) X (105,600 cmil) = 2746 lb.

(41)

From Table 7:

If ail three cables are placed in one grip;

T max

=

(A lead square inches)(8000 Ib./ square inch), or, where A lead

=

1t (Ro2- Ri2)

= 1t [ (0.595 inches)2 - (0.555 inches)2 ]

=

0.144 square inches 1 cable

T max = (0.144 square inches)(8000 Ib./square inches) = 1152 lb.

If one grip is used per cable

T max

=

(0.144 square inches)(16000 Ib./square inches)

=

2304 lb.

Note: From Note 1 for Tables 6, 7, and 8, the maximum conductor stress for copper is 0.013Ib./cmil or 1373 lb. Iftwo cables share the load, the maximum allowable tension is 2746 lb. Therefore, the pull can be made with a grip or an eye but the maximum allowable total tension is 2746 lb.

- The maximum SWBP is 2000 Ib./ft. (Table 8)

- The minimum bending radius is 7 x 1.59 inches = 11.13 inches (Table 1 B) - The low SWBP friction factor for 3 cables is

=

0.65 (Table 3)

- The high SWBP friction factor for 3 cables is

=

0.25 (Table 4)

- The calculation is performed twice to examine the maximum tension and SWBP that occurs for each pull direction.

(42)

Calculations based on pulling from B to A Conduit Section 1: Straight Pull (Horizontal)

Assume tension from cable reel to duct entrance is approximately 50 lb. From equation 8-1,

Using the low SWBP friction factor because this is a straight section,

T2 = 50 lb. + (8.82 Ib./ft.)(0.65)(1.32)(25 ft.) T2

=

239 lb.

Conduit Section 2: Horizontal Bend

e

= n/2

Check to see if simplified equation is valid.

WRlT1

=

(8.82 Ib./ft.) (3 ft.) /239 lb.

=

0.11

WRlT1 < 0.5, therefore the simplified equation is valid.

(43)

The SWBP for a triangular configuration is WcT2/(2R) (From Section 9.0) SWBP

=

(1.32) (920 lb.) /(2)(3ft.)

=

202 Ib./ft.

Since the SWBP is > 150 Ib./ft. the high SWBP friction factor should be used.

T

2 = 239 lb. e (0.25)(1.32)("'2) T2

=

401 lb.

SWBP

=

(1.32) (401 lb.) / [(2)(3 ft.)]

=

88 Ib./ft.

It is interesting to note that this tension yields a SWBP of 88 Ib./ft. which would indicate that the low SWBP friction factor should be used. Since one case indicates that the low SWBP friction factor should be used and the other case indicates the opposite, the conservative approach will be taken and the tension will be based on the low SWBP friction factor,

Le. T2 = 920 lb.

Conduit Section 3: Straight Pull (Horizontal):

From equation 8-1

Using the low SWBP friction factor because this is a straight section. T2

=

920 lb. + (8.82 Ib./ft.)(O.65)(1.32)(25 ft.), or

T2

=

1109 lb.

(44)

WRlT1 = (8.82 Ib.lft.)(25ft.)/1109 lb. = 0.2

WRlT1 = < 0.5, therefore the simplified equation is valid

T

2 = 1109 lb. e (O.65)(1.32)(7tl4) T2

=

2175 lb.

SWBP

=

(1.32)(2175Ib.)/(2)(25ft.)

=

57.4 Ib.lft.

SWBP < 150 Ib.lft.; therefore the low SWBP friction factor was correct.

Conduit Section 5: Straight Pull (Horizontal)

75 ft.

T

2

.~---From Equation 8-1,

T2

=

T1 + WKWcL

T2

=

2175 lb. + (8.82 Ib.lft.)(0.65)(1.32)(75 ft.)

T2

=

2743 lb.

The maximum allowable tension is 2746 lb., therefore pulling in this direction would be

questionable.

Calculation based on pulling from A to B

Conduit Section 5: Straight Pull (Horizontal)

75 ft.

---+~

T2

Assume tension from cable reel to duct entrance is approximately 50 lb. From equation 8-1

(45)

R=25 ft

WR/ T1

=

(8.82 Ib.lft.) (25 ft.) /618 lb.

=

0.36

WRI T1 < 0.5, therefore the simplified equation is valid.

T

2

=

618 lb. e (0.65)(1.32)("'4)

T2

=

1212 lb.

SWBP

=

(1.32) (1212Ib.) / (2)(25ft.)

=

321b.lft.

SWBP < 150 Ib.lft., therefore the low SWBP friction factor was correct.

From equation 8-1

T2= 1212 lb. + (8.82Ib.)(O.65)(1.32)(25tt.) or

(46)

T_{2 "}T₁

e

KWc6

Check to see if simplified equation is valid

WRfT1 = (8.82 Ib.lft.)(3 ft.)/1401 lb. = 0.02

WRfT1 < 0.5, therefore the simplified equation is valid.

T

2

=

1401 lb. e (O.65)(1.32)(7tl2) or

T2

=

5392 lb.

SWBP = (1.32) (5392 lb.) / (2) (3 ft.) = 1186 Ib.lft.

SWBP> 150 Ib.lft., therefore the high SWBP friction factor should have been used.

T₂

=

1401 lb. e (O.25)(1.32)(7tl2)

=

2352 lb.

A recheck of the SWBP yields;

SWBP = (1.32)(2352 Ib.)/(2)(3ft.) = 517 Ib.lft.

SWBP >150 Ib.lft., therefore the high SWBP friction factor is correct.

(47)

Although this tension is lower than B to A calculated tension, it is approximately 93 percent of the maximum allowable tension of 2746 lb. The system planner must use good engineering judgment and knowledge of the duct system to decide if the pull should be made without any system changes.

ln this case it might be desirable to recalculate the tensions around the bends using the exact horizontal bend equation. Also, it would be necessary to know if the straight sections really are absolutely straight and if the steel duct is new, clean, and smooth, or old, dirty, and rusty.

It is also important to consider the increased tensions that result from start-up and surging. These phenomena, which are discussed briefly in Section 12, are difficult to predict, and point out the desirability of having a maximum calculated pulling tension that is no more than 80 percent of the maximum allowable tension. After considering ail of these factors, it may be necessary to redesign part of the system in order to make a good, safe cable pull.

(48)

12.0 INSTALLATION CONSIDERATIONS

12.1

Pulling Lines and Duct Wear

There are a wide variety of pulling lines being used in the utility industry. Duct wear is strongly affected by the diameter of the line. As the diameter of the pulling line is reduced, the wear is increased. This occurs because the normal force per unit area of the line on the duct increases as the line diameter decreases. The surface condition of the pulling lines also affects duct wear. A coarsely braided fiber pulling line or stranded steel rope will wear through duct much more

rapidly than a nylon jacketed pulling line.

Also, some duct materials are much more susceptible to wear than others. PVC, polyethylene and bituminized fiber duct will wear at a much faster rate than transite, fiberglass or steel duct.

12.2

Surging

Surging is a complicated phenomenon, which can result in higher levels of pulling tension than would be expected from the analysis of a steady pulling condition. Almost ail cable pulls involve some degree of surging. Cable pulls with pulling lines that have a large amount of elasticity surge more readily than pulls with lines that have little elasticity, particularly at lower tensions. Also, cables with neoprene jackets have a greater tendency to surge than cables with other jacket types.

Based on observations made in EPRI project EL-3333, the dominant factor in cable surging is the difference in the static and dynamic coefficients of friction between cable and duct. As the difference increases, the cable will slide quickly, and then stop until the tension increases to a level that will overcome the static coefficient of friction. At this po·int the cable will slide again as the surging phenomenon continues. Cable engineers may want to review the dynamic and static coefficients of friction published in EL-3333 to determine the likelihood of surging for a given cable system design.

Predicting the amount of surging that will occur in a cable pull is virtually impossible. Therefore, the engineer must rely on experience, discussions with cable manufacturers, or increased pulling tension safety factors to allow for the surging phenomenon.

12.3

Slack Pulling

Slack pulling is a pulling technique where the pull is stopped and restarted to accumulate slack in the cable. It is usually performed in a manhole to accumulate extra cable for splicing. Most often, special split grips are employed which can be loosened and slid down the cable up to the lip of the duct. It is then retightened against the cable so that an additional length of cable can be pulled into the manhole.

Slack pulling is not a recommended pulling procedure. There is always the possibility that the slack-pulling grip will cause compressive damage to the cable. Since the section of the cable undemeath the slack-pulling grip cannot usually be removed, a cable failure may eventually

AEIC CG5 2005-Underground Extruded Power Cable Pulling Guide

AEIC

UNDERGROUND EXTRUDED POWER CABLE

PULLING GUIDE

TABLE OF CONTENTS

AEIC

UNDERGROUND EXTRUDED POWER CABLE

PULLING GUIDE

1.0

INTRODUCTION

2.0

CABLE REMOVAL

3.0

HISTORY

4.0

ECONOMIC CONSIDERATIONS

5.0

NOMENCLATURE

5.1

Definition of Symbols

R

e

J

Ra

e

6.0

DESIGN CRITERIA AND PULLING LlMITS

6.1

Cable Diameters and Weights

6.2

Jamming

6.3

Cable Configuration in Duet

6.4

Cable Clearance

Cable Configuration in Duct

6.4.1 Clearance Formulas

6.5

Minimum Bending Radius

Cable Clearance

if

x

=

=

=

=

6.6

6.7

Coefficient of Friction

-

--

--

--

--

--

-

--

--

-

-

--

--

--

--

--

1

·

6.8

Weight Correction Factor

6.9

Sidewall Bearing Pressure

4[

J2

=

=

=

7.0

DESIGN LlMITS

7.1

Tension Limits for Pulling Eyes and Bolts