UNITED STATES DEPARTMENT OF AGRICULTURE

Rural Utilities Service

BULLETIN 1751F-643

SUBJECT: Underground Plant Design

TO: All Telecommunications Borrowers

 RUS Telecommunications Staff

EFFECTIVE DATE: August 28, 2002

OFFICE OF PRIMARY INTEREST: Outside Plant Branch,

Telecommunications Standards Division

INSTRUCTIONS: This bulletin supersedes RUS Bulletin 1751F-643,

“Underground Plant Design,” issued May 14, 1998.

AVAILABILITY: This bulletin is available on the Internet at

http://www.usda.gov/rus/telecom/publications/bulletins.htm

PURPOSE: This bulletin provides RUS borrowers, consulting

engineers, contractors and other interested parties with

information on the design and construction of underground plant

facilities. This bulletin also provides information on the

design of poured-in-place underground cable vaults.

Roberta D. Purcell 8/28/02

Roberta D. Purcell Date

Assistant Administrator

Telecommunications Program
Bulletin 1751F-643

Page 2

TABLE OF CONTENTS

 Abbreviations ........................................... 4

 Definitions ............................................. 4

 1. General ................................................. 7

 2. Application ............................................. 8

 3. Construction Materials ................................. 11

 4. Conduit Types and Usage ................................ 11

 5. Conduit Formations and Curves .......................... 13

 6. Conduit Section Lengths ................................ 14

 7. Conduit Protection ..................................... 15

 8. UCV Types .............................................. 15

 9. Cable Racks, Rack Supports, and Pulling Irons .......... 22

10. UCV and Conduit Locations .............................. 24

11. Determination of Installation Length and Pulling

 Tension of Filled Copper Cables ........................ 25

12. Determination of Installation Length and Pulling

 Tension of Filled Fiber Optic Cables ................... 28

13. Design Considerations .................................. 29

TABLES & FIGURES

Table 1 Conduit Formations ................................. 14

Table 2 Minimum Headroom - Poured-In-Place UCVs ............ 16

Table 3 Floor Thicknesses and Reinforcing Rod Spacings

 Rectangular Poured-In-Place UCVs in Fluid Soils .... 17

Table 4 Floor Thicknesses and Reinforcing Rod Spacings

 Type UM-V Poured-In-Place UCVs in Fluid Soils ...... 17

Table 5 Wall Construction – Poured-In-Place UCVs 

 in Firm Soils ...................................... 19

Table 6 Wall Construction – Poured-In-Place UCVs 

 in Fluid Soils ..................................... 20

Table 7 Roof Thicknesses and Reinforcing Rod Diameters

 Rectangular Poured-In-Place UCVs ................... 21

Table 8 Roof Thicknesses and Reinforcing Rod Diameters

 Type UM-V Poured-In-Place UCVs ..................... 21

Table 9 UCV Opening Forms for UCV Frame Types and Sizes

 Poured-In-Place UCVs ............................... 21

Table 10 Number and Size of Cable Racks Versus Headroom

 Poured-In-Place UCVs ............................... 23

Table 11 Coefficient of Friction

 Conduit Versus Cable Jacket Material ............... 25
Bulletin 1751F-643

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TABLES & FIGURES - Continued

Table 12 Cross-sectional Area of Copper Conductors ........ 27

Figure 1 Overall Design Layout – Poured-In-Place UCVs ..... 31

Figure 2 Floor Construction

 Poured-In-Place UCVs In Fluid Soils .............. 32

Figure 3 Sump and Drain Construction

 Poured-In-Place UCVs ............................. 33

Figure 4 Wall Construction – Poured-In-Place UCVs ......... 34

Figure 5 Roof Construction – Poured-In-Place UCVs ......... 35

Figure 6 Conduit Entrances and Recesses

 Poured-In-Place UCVs ............................. 36

Figure 7 Frame and Cover Types – Poured-In-Place UCVs ..... 37

Figure 8 Brick Collar Construction – Poured-In-Place UCVs . 38

Figure 9 Typical Installation of Cable Racks

 Poured-In-Place UCVs ............................. 39

Figure 10 Arrangement and Installation of Pulling-In Irons

 Poured-In-Place UCVs ............................. 40

Figure 11 Example of Underground Cable Vault and Conduit

 Route Construction Drawing ....................... 41

Figure 12 Underground Plant Construction Abbreviations

 and Symbols ...................................... 42

Figure 12A Underground Plant Construction Abbreviations

 and Symbols (Continued) .......................... 43

Figure 12B Underground Plant Construction Abbreviations

 and Symbols (Continued) .......................... 44

Figure 12C Underground Plant Construction Abbreviations

 and Symbols (Continued) .......................... 45

Figure 13 Example of Underground Filled Copper Cable

 Installation in a Conduit and UCV System

 Containing Multiple Bends ........................ 46

Figure 13A Example of Underground Filled Copper Cable

 Installation in a Conduit and UCV System

 Containing Multiple Bends (Continued) ............ 47
Bulletin 1751F-643

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TABLES & FIGURES - Continued

Figure 13B Example of Underground Filled Copper Cable

 Installation in a Conduit and UCV System

 Containing Multiple Bends (Continued) ............ 48

Figure 14 Example of Determining the Cable Sidewall Pressure

 at Conduit Bends in a Conduit and UCV System ..... 49

Figure 14A Example of Determining the Cable Sidewall Pressure

 at Conduit Bends in a Conduit and UCV System

 (Continued) ...................................... 50

INDEX:

Outside Plant

Design, Underground

Telecommunications

ABBREVIATIONS

AWG American Wire Gauge

CFR Code of Federal Regulations

cm Centimeters

ft Foot or Feet

in. Inches

kgs Kilograms

kgs/m Kilograms per meter

kgs/mm2 Kilograms per square millimeter

lbs Pounds

lbs/ft Pounds per foot

LD Loan Design

m Meter

mm Millimeters

mm2 Square Millimeters

R/W Rights-of-way

RUS Rural Utilities Service

SAVE Serving Area Value Engineering

TE&CM Telecommunications Engineering and Construction Manual

UCV Underground Cable Vault

UCVS Underground Cable Vaults

UM Underground Manhole

DEFINITIONS

Cable Rack: A device usually secured to the wall of an

underground cable vault, cable raceway, or building to provide

support for cables.

Conduit: A tubular raceway for holding wires or cables, which is

designed expressly for, and used solely for, this purpose. 
Bulletin 1751F-643

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DEFINITIONS - Continued

Construction Drawings: The drawings developed through the

staking process and used to guide the construction of outside

plant facilities.

Eligible Country: Any country that applies with respect to the

United States an agreement ensuring reciprocal access for United

States products and services and United States suppliers to the

market of that country, as determined by the United States Trade

Representative.

Loan Design: A comprehensive engineering plan for the project

supporting a loan application to RUS.

Underground Cable Vault (UCV): A subsurface chamber, large

enough for a person to enter for the purpose of installing cables

and other devices, and for making connections and tests.

Underground Cable Vault (UCV) Cover: A removable lid which

closes the opening to an underground cable vault or similar

subsurface enclosure.

Underground Cable Vault (UCV) Frame: A structure that caps the

roof opening of the underground cable vault at ground level and

supports the cover.

Pulling Iron: An anchor secured in the wall, ceiling, or floor

of an underground cable vault to attach rigging used to pull

cable.

Resident: The qualified representative of the Engineer who is

delegated full time "on site" Construction Administration

responsibilities of the Engineer.

Right-of-way: The strip of land over which facilities such as

highways, railroads, power lines, other utilities, or

telecommunication lines are constructed.
Bulletin 1751F-643

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DEFINITIONS - Continued

RUS Accepted (Material and Equipment): Equipment which RUS has

reviewed and determined that:

a. Final assembly or manufacture of the equipment is

 completed in the United States, its territories and

 possessions, or in an eligible country;

b. The cost of components within the material or equipment

 manufactured in the United States, its territories and

 possessions, or in an eligible country is more than 50

 percent of the total cost of all components used in the

 material or equipment; and

c. The material or equipment is suitable for use on

 systems of RUS telecommunications borrowers.

RUS Technically Accepted (Material and Equipment): Equipment

which RUS has reviewed and determined that the material or

equipment is suitable for use on systems of RUS

telecommunications borrowers but the material or equipment does

not satisfy both paragraph (a) and (b) of this definition:

a. Final assembly or manufacture of the equipment is

 not completed in the United States, its territories and

 possessions, or in an eligible country; and

b. The cost of components within the material or equipment

 manufactured in the United States, its territories and

 possessions, or in an eligible country is 50 percent or

 less than the total cost of all components used in the

 material or equipment.
Bulletin 1751F-643

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1. GENERAL

1.1 This bulletin discusses in particular the design of

underground plant using filled copper or fiber optic cables.

This bulletin also provides information on the design and

construction of poured-in-place underground cable vaults (UCVS).

The information and recommendations in this bulletin are

advisory.

1.2 Underground plant consists of a conduit and an underground

cable vault (UCV) system and the telecommunications copper or

fiber optic cables installed in the conduit and UCV system.

1.3 Additional information for the use in the design and

construction of underground plant facilities can be found in the

following Rural Utilities Service (RUS) documents:

a. Telecommunications Engineering and Construction Manual

(TE&CM) Section 116, Plant Engineering and Record

System;

b. TE&CM Section 204, Telephone System Design;

c. TE&CM Section 210, Telephone System Design - Sizing

Criteria;

d. TE&CM Section 218, Plant Annual Cost Data for System

Design Purposes;

e. TE&CM Section 219, Present Worth of Annual Charge

Studies for System Design;

f. TE&CM Section 230, General Principles of FeederDistribution Cable Engineering [Serving Area Value

Engineering (SAVE)];

g. TE&CM Section 231, Design Techniques of FeederDistribution Cable Engineering (SAVE);

h. TE&CM Section 232, Transmission Design and Costs

Considerations of Feeder-Distribution Cable

Engineering (SAVE);

i. TE&CM Section 628, Plastic-Insulated Cable Plant Layout;

j. TE&CM Section 629, Cable Plant Layout - Serving Area

Value Concept for Rural Systems;

k. RUS Bulletin 344-3, "Buy American" Requirement, as

amended by the Notice published in the Federal Register,

Volume 66, Number 222, dated November 16, 2001;

l. RUS Bulletin 1751F-644, Underground Plant Construction;
Bulletin 1751F-643

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m. RUS Bulletin 1751F-670, Outside Plant Corrosion

Considerations;

n. RUS Bulletin 1751F-801, Electrical Protection

Fundamentals;

o. RUS Bulletin 1751F-815, Electrical Protection of Outside

Plant;

p. RUS Bulletin 1751H-601, Lightwave Fundamentals, Systems,

 and Application;

q. RUS Bulletin 1753F-151, Specifications and Drawings for

Construction of Underground Plant, Form 515b;

r. RUS Bulletin 1753F-205(PE-39), RUS Specification for

Filled Telephone Cables;

s. RUS Bulletin 1753F-208(PE-89), RUS Specification for

Filled Telephone Cables with Expanded Insulation;

t. RUS Bulletin 1753F-402(PC-2), RUS Standard for Splicing

Copper and Fiber Optic Cables;

u. RUS Bulletin 1753F-601(PE-90), RUS Specification for

Filled Fiber Optic Cables;

 v. 7 Code of Federal Regulations (CFR) § 1753.6,

Standards, specifications, and general requirements.

2. APPLICATION

2.1 Underground plant facilities should be considered for the

following conditions:

a. In urban or suburban areas;

b. Where public authorities object to aerial plant

construction;

c. Where direct buried plant is inadvisable;

d. Where alternative aerial or buried routes are not

practicable;

e. Where the appearance of an aerial entrance cable to a

central office building would be objectionable; or

f. Where department of transportation officials prohibit

aerial crossings over roads, streets, highways, etc.
Bulletin 1751F-643

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2.2 Since underground plant facilities are usually exposed to

less physical damage than aerial or buried facilities, fewer

cable faults should be associated with underground plant

facilities. This lower fault rate should result in long periods

of maintenance-free operation and greater subscriber

satisfaction. Underground plant facilities could be used in

areas susceptible to frequent ice storms and/or high wind

velocities.

2.3 Permission from appropriate authorities needs to be obtained

before installing underground plant facilities, which include

conduits, UCVs, and copper or fiber optic cables, in public

rights-of-way (R/W) or travel lanes of roadways, streets, etc.

In most circumstances cooperation with appropriate authorities

can be obtained when underground plant construction techniques

are fully explained. A comprehensive investigation of the public

R/W or travel portions of roadways, streets, etc. should be made

as to possible roadway, street, etc. improvements, such as

widening or changing grade elevations, before the installation of

underground plant facilities to eliminate costly future

rearrangements and rerouting. When installing underground plant

facilities at railroad crossings, on bridges, and the crossing of

facilities owned by other utilities, the need for permits should

be thoroughly investigated.

2.4 In urban and suburban areas, other buried or underground

facilities such as water lines, gas lines, sewer lines, Community

Antenna Television lines, electric and/or telecommunications

lines present obstacles to the installation of the conduit and

UCV components of underground plant facilities. When the above

buried or underground obstacles are anticipated, detailed

information should be obtained from the other utilities on

present and proposed buried or underground facilities locations.

Where it is known that buried or underground obstacles will be

encountered during construction of the conduit and UCV components

of underground plant facilities, those areas should be identified

separately in the Loan Design (LD).

2.5 The physical location of the conduit and UCV components of

the underground plant facilities should be determined in

conjunction with federal, state, county, and local authorities,

as applicable. Special attention should also be given to the

type and size of equipment that will be required to perform the

conduit, UCV, and cable installation of underground plant

facilities.

2.6 Some of the considerations that can affect the physical

locations of the conduit and UCV components of underground plant

facilities are:

a. Width of road shoulders;

b. Type of road and road surface;
Bulletin 1751F-643

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c. Easements;

d. Required number of conduits and UCVs;

e. Public R/W;

f. Railroad crossings;

g. Bridge crossings;

h. Buried or underground obstacles of other utilities; and

i. Environmentally sensitive areas.

2.7 When conduit and UCV components of underground plant

facilities are to be installed in public R/W or travel lanes of

roadways, streets, etc. discussions with department of

transportation authorities should be held to inform the

authorities as to locations and depths of conduits and UCVs in

the R/W or travel lanes of roadways, streets, etc.

2.8 The same procedure as outlined in paragraph 2.7 of this

section should be followed in coordination with town and

development areas where conduit and UCV components of underground

plant facilities are to be installed. Assistance should be given

to town planning authorities, land, industrial, and commercial

developers by either identifying on existing construction

drawings or in the preparation of new construction drawings, the

locations and depths of all conduits and UCVs. Here again, these

organizations should be informed that the borrower will provide

identification and location of its existing underground plant

facilities when requested.

2.9 Proposed underground plant construction activities should be

coordinated with other utilities. This coordination is necessary

to properly develop a working relationship among the various

utilities on future planning of all utility plant. Construction

drawings of underground plant to be constructed should be

provided to each of the utility companies whose plant is, or may

be placed, within the area of the borrower's underground plant

facilities.

2.10 RUS Bulletin 1753F-151, Specifications and Drawings for

Construction of Underground Plant, RUS Form 515b, provides

assistance to either the resident, consulting engineer, or

borrower’s engineering staff in the design and construction of

underground plant facilities. The resident, consulting engineer,

or borrower’s engineering staff should check with Federal, State,

county, and local authorities in regard to any existing and

proposed land development programs which would require special

design considerations to be taken into account in the design and

construction of underground plant facilities.
Bulletin 1751F-643

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2.11 In the design of conduit and UCV systems of underground

plant facilities, more conduits than initially needed should be

provided to eliminate the added expense of reopening trenches and

adding additional conduits to existing conduit systems.

2.12 The resident, consulting engineer, or borrower’s

engineering staff should keep in mind during the design of

underground filled copper or fiber optic cable components of

underground plant facilities that flexibility of underground

cable components depend to a great extent on the physical

arrangement of the cables and associated equipment in the conduit

and UCV systems of underground plant facilities. Correct

assignments and arrangements of underground copper or fiber optic

cables should provide economical advantages by decreasing the

cost of future rearrangements.

2.13 Prior to the design layout of the underground filled copper

or fiber optic cables, the resident, consulting engineer, or

borrower’s engineering staff should make an on-site field survey

of all UCVs, riser poles, building conduits, etc.

2.14 Where underground filled copper or fiber optic cables are

pulled through UCVs without splicing, duct selections should be

the same at both ends of UCVs. Changes in duct selections,

especially in elevations, should be considered carefully to avoid

damage to underground cable sheaths and to keep the pulling

tensions as low as possible.

3. CONSTRUCTION MATERIALS

3.1 For all underground plant construction projects financed

with RUS loan funds, RUS regulation 7 CFR 1753.6 requires that

only RUS accepted materials be used.

3.2 RUS technically accepted, nondomestic manufactured

materials, may also be used on underground plant construction

projects. Before technically accepted materials can be used on

underground plant construction projects, permission is required

from the RUS borrower. In addition, borrower's are required to

ensure that the cost of the technically accepted materials are at

least 6 percent less than the cost of the RUS accepted materials,

as specified in RUS Bulletin 344-3, "Buy American" Requirement,

as amended by the Notice published in the Federal Register,

Volume 66, Number 222, dated November 16, 2001.

4. CONDUIT TYPES AND USAGE

4.1 Conduit types most frequently used in underground plant

facilities are as follows:

a. Multi-duct concrete;
Bulletin 1751F-643

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b. Multi-duct plastic;

c. Fiberglass/epoxy;

d. Plastic;

e. Flexible plastic; and

f. Plastic fiber optic cable duct.

4.2 Multi-duct concrete conduits are available in 3.5 inch (in.)

[89 millimeters (mm)] and 4 in. (102 mm) bore sizes and two

design types. The first design consists of a bell and spigot

with joint seals utilizing rubber gaskets and/or joint sealing

compound. The second design utilizes a polyethylene sleeve with

dowel pins for alignment. Conduits are available in 4-way, 6-

way, and 9-way configurations. Short joints, miters, split

conduits, and special adapters; such as concrete-to-steel,

concrete-to-concrete, and concrete-to-plastic, are standard stock

items for both designs which are available from manufacturers.

4.3 Multi-duct plastic conduits are available in bore sizes of

4 in. (102 mm) in bell and spigot designs. Joint seals are made

using rubber gaskets and spring clips. Conduits are available in

4-way, 6-way, and 9-way configurations supplied in 42 in.

[107 centimeters (cm)] sections. Miters, double bells, spigots,

etc. are standard stock items available from manufacturers.

Short joint sections can be made in the field by cutting conduits

with a hacksaw.

4.4 Fiberglass/epoxy conduits are available in 2 in. (51 mm),

3 in. (76 mm), 4 in. (102 mm), and 6 in. (152 mm) bore sizes and

three design types. The first design is a bell and spigot with

joint seals utilizing joint sealing compound. The second design

is a bell and spigot with joint seals utilizing joint sealing

compound and a mechanical thread to ensure proper make up. The

last design uses threaded mechanical connectors to ensure

positive joint seals. Sweeps, elbows, bends, expansion joints,

etc. are standard stock items available from manufacturers.

4.5 Plastic conduits are available in bore sizes of 2 in.

(51 mm), 3 in. (76 mm), 4 in. (102 mm), and 6 in. (152 mm) and

five design types. The five design types are B, C, D, EB, and

DB. Design types B and EB are used for concrete encasement.

Types C and DB are used for direct burial. Type D is used for

exposed installations, such as on bridges, viaducts, etc.

Jointing is accomplished with plastic couplings and solvent

cements. Sweeps, elbows, adapters, plugs, bell ends, etc. are

standard stock items available from manufacturers. Conduits can

be cut with a hacksaw in the field to provide short joint

sections. Plastic spacers are available for building conduit

formations. The conduits are usually supplied in 20 foot (ft)
Bulletin 1751F-643

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[6 meter (m)] lengths. Because plastic conduits are somewhat

flexible, gradual changes in widths and heights of conduit banks

can be made without special miter fittings. Plastic conduits

also have low coefficients of friction which allow longer conduit

runs.

4.6 Flexible plastic conduits are available in bore sizes of

2 in. (51 mm), 3 in. (76 mm), 3.5 in. (89 mm), 4 in. (102 mm),

and 6 in. (152 mm) and two design types. The first type is a

smooth wall finish. The second design is a corrugated wall

finish. Jointing is accomplished by heat fusion with caps, ells,

etc. which are standard stock items from manufacturers. The

conduits are usually supplied in 500 ft (152 m) coiled lengths.

Because of the conduit’s flexibility, the conduit may be bent in

radii as sharp as 4 ft (1.2 m) without kinking. Special fittings

necessary to make turns, dips, and bends are not usually needed

with flexible plastic conduits. Flexible plastic conduits can be

installed either by trenching or plowing. Smooth wall flexible

conduits may be installed in multiple configurations. Corrugated

wall flexible conduits should only be used on subsidiary conduit

routes. Corrugated wall flexible conduits should only be placed

on horizontal planes (side by side).

4.7 Plastic fiber optic cable ducts are small diameter flexible

plastic conduits available in bore sizes of 0.5 in. (13 mm),

1 in. (25 mm), and 1.5 in. (38 mm). These type ducts are

available in either smooth or corrugated wall finishes. The

ducts can be supplied in 1650 ft (502 m) coiled lengths. The

primary usage of this type duct is for installation in existing

conduits. This duct may also be used for direct burial.

Placement of filled fiber optic cables in these type ducts

provide clean, low friction, low hazard environment for fiber

optic cable installations.

4.8 Steel pipes may be used as conduits where special conditions

prohibit the use of conduit types listed in paragraph 4.1 of this

section. Such conditions may include locations where trenching

could not be performed or where pipe pushes are required. Steel

pipes are available in all commercial sizes and can be obtained

with threaded ends and couplings or with plain ends.

4.9 Sewer tiles, which are available in all commercial sizes,

should be used as drains or sumps for poured-in-place UCVs.

5. CONDUIT FORMATIONS AND CURVES

5.1 Conduit formations consisting of four or more ducts should

not be less than two, nor greater than four, ducts wide. Table 1

of this section lists various conduit formations based on trench

sizes and UCV vault racking diagrams.
Bulletin 1751F-643

Page 14

TABLE 1

Conduit Formations

Number of Ducts Duct Width

4, 6, 8, and 10 ducts 2 wide

9, 12, 15, and 18 ducts 3 wide

5.2 Conduit sections containing curves should be avoided

whenever possible. When curves are required in conduit sections,

angles of curves should be less than 90 degrees. Curves with

angles greater than 90 degrees or curves with extremely small

radii should not be used because of possible damage to conduits

or cables.

6. CONDUIT SECTION LENGTHS

6.1 Conduit section lengths should depend on diameters and types

of conduits and types of filled copper or fiber optic cables to

be installed in conduits. Conduit sections should be as long as

practicable to reduce the number of UCVs, splices, and set-ups

for cable pulling.

6.2 Conduit sections should not exceed 1500 ft (457 m). Typical

conduit sections range from 600 to 1,000 ft (183 to 305 m).

Achieving the 600 to 1,000 ft (183 to 305 m) distance may be

restricted by junction points, offsets, or degrees of curvature

in conduit sections. An offset is defined as the displacement of

one portion of a straight conduit section run relative to the

remainder of the conduit run. Displacements (offsets) may occur

in any plane. If offset distances are 5 ft (1.5 m) or less,

offsets may be disregarded in determining maximum section

lengths. If offsets in otherwise straight conduit sections are

greater than 5 ft (1.5 m) or radii of sweeps in offsets are less

than 1200 ft (366 m), conduit section lengths should be shortened

proportionately (up to one-third for extreme conditions).

Conduit sections containing offsets should normally not exceed

600 ft (183 m).

6.3 Bends, twists, or curves in conduit runs should be used to

change the general direction of conduit runs at street corners or

to avoid underground obstructions. For conduit sections

containing bends or curves, maximum conduit section lengths

should be determined by angles between straight conduit runs on

each side of curves and radii of curves.

6.4 Actual lengths of conduit runs should be obtained and

verified from existing conduit system records to ensure that

cable lengths will be correct. Distances, such as UCV wall-towall, UCV-to-riser pole, or UCV-to-building, should be measured

and added to the length of cable required in each UCV for

pulling-in, setting-up, splicing, and testing. The sum of the
Bulletin 1751F-643

Page 15

distance measurements mentioned above and the cable lengths for

pulling-in, setting-up, splicing, and testing is known as the

cable “cut lengths.” Cable “cut lengths” should be indicated on

construction drawings. The excess length of copper cable

normally required for splicing, testing, and pulling usually

amounts to 3 ft (one meter) per cable or 6 ft (2 m) per section.

The excess length of fiber optic cable normally required for

splicing, testing, and pulling should be based on the LD.

7. CONDUIT PROTECTION

7.1 Conduits should be protected against mechanical damage due

to settlement of the ducts, excavating equipment, etc. by use of

either treated wood planks or concrete. Protection may consist

of top protection only, base only, top and sides only, or

complete encasement. The type of protection should depend on the

particular circumstances of the project. Where no hazards are

likely to be encountered, protection of conduits should not be

required.

7.2 Top protection should be provided where conduits are to be

installed under roadways which may be paved at later dates.

Concrete bases should be used whenever the ground is spongy or

yielding, such as swamps or marshlands, or where bases are

desirable as leveling mediums under conditions where sand base

trenches are subjected to washing out. Conduits within railroad

R/W should be protected if required by regulations.

7.3 Concrete encasement of conduits should be considered for the

following conditions:

a. Road or street crossings having earth covers that are

equal to or less than 30 in. (76 cm);

b. Railroad crossings;

c. Earth covers parallel to and within street, highway, or

road travel areas that are less than 30 in. (76 cm);

d. Stream crossings, storm canals, ditches, etc.; and

e. Bend angles of 20° or greater in conduit lengths equal

to or greater than 550 ft (168 m).

8. UCV TYPES

8.1 Precast and poured-in-place UCVs should be designed to

provide sufficient and suitable space for installation of cables

and associated equipment. Precast and poured-in-place UCV types

and sizes that are normally used within the RUS program are shown

in the construction guide drawings of RUS Bulletin 1753F-151,
Bulletin 1751F-643

Page 16

Specifications and Drawings for Construction of Underground

Plant, Form 515b, listed below:

a. Guide drawing Underground Manhole (UM) UM-A,-L,-T,-J;

b. Guide drawing UM-V; and

c. Guide drawing UM-X,-Y.

8.2 Headroom, floor, wall, and roof thicknesses, openings, wall

recesses, frames, covers, collars, sumps, or drains of precast

UCVs should be designed in accordance with the manufacturers’

recommendations.

8.3 Headroom, floor, wall, and roof thicknesses, openings, wall

recesses, frames, covers, collars, sumps, or drains of poured-inplace UCVs should be designed in accordance with the engineer’s

specifications. Figure 1 of this bulletin provides an example of

the poured-in-place UCV overall design layout.

8.4 Sufficient headroom should be provided in poured-in-place

UCVs to allow for racking of cables plus a clear space of 12 in.

(305 mm) just above the floor and below the UCV roof. Table 2 of

this section indicates the minimum headroom recommended for the

various types of poured-in-place UCVs.

TABLE 2

Minimum Headroom

Poured-In-Place UCVs

UCV Types

Minimum Headroom

in. (cm)

UM-X 42 (107)

UM-Y 48 (122)

UM-A 66 (168)

UM-L 66 (168)

UM-T 66 (168)

UM-V 66 (168)

UM-J 66 (168)

8.5 Unreinforced concrete should be used for floors of pouredin-place UCVs that are to be constructed in firm soils. Floor

thicknesses should be 4 in. (102 mm) for poured-in-place UCVs

with unreinforced concrete walls. For poured-in-place UCVs with

reinforced concrete walls, floors should be 6 in. (152 mm) thick.

8.6 Reinforced concrete should be used for floors of poured-inplace UCVs that are to be constructed in fluid soils. The

reinforcing members should be 5/8 in. (16 mm) diameter deformed

bars. Table 3 of this section and Figure 2 of this bulletin

provide design information on the floor thicknesses and

reinforcing bar spacings of all rectangular poured-in-place UCVs
Bulletin 1751F-643

Page 17

in fluid soils. Floor thicknesses of UM-V type poured-in-place

UCVs vary with depths of finished floors below grade. Table 4 of

this section and Figure 2 of this bulletin provide design

information on the floor thicknesses and reinforcing bar spacings

of UM-V type UCVs in fluid soil.

TABLE 3

Floor Thicknesses and Reinforcing Rod Spacings

Rectangular Poured-In-Place UCVs in Fluid Soils

Depth of UCV Floor

Below Grade

Ft (m)

Floor Thickness

In. (mm)

Spacing of

Reinforcing Rods

in. (mm)(See Note)

8 (2.4) 6 (152) 8 (203)

9 (2.7) 6 (152) 8 (203)

10 (3.0) 6 (152) 8 (203)

11 (3.4) 6 (152) 8 (203)

12 (3.7) 6 (152) 8 (203)

13 (4.0) 6 (152) 8 (203)

14 (4.3) 6 (152) 7 (178)

15 (4.6) 6 (152) 6.5 (165)

16 (4.9) 6 (152) 6.5 (165)

TABLE 4

Floor Thicknesses and Reinforcing Rod Spacings

Type UM-V Poured-In-Place UCVs in Fluid Soils

Depth of UCV

Floor Below Grade

Floor

Thickness

Spacing of Reinforcing Rods

In. (mm)(See Note)

ft (m) in. (mm) Spacing A Spacing B Spacing C

8 (2.4) 6 (152) 8 (203) 9 (229) 11 (279)

9 (2.7) 6 (152) 8 (203) 9 (229) 11 (279)

10 (3.0) 6 (152) 8 (203) 9 (229) 11 (279)

11 (3.4) 6 (152) 8 (203) 9 (229) 11 (279)

12 (3.7) 6 (152) 8 (203) 9 (229) 11 (279)

13 (4.0) 6 (152) 8 (203) 9 (229) 11 (279)

14 (4.3) 6.5 (165) 7 (178) 8 (203) 10 (254)

15 (4.6) 7 (178) 6.5 (165) 8 (203) 10 (254)

16 (4.9) 7 (178) 6.5 (165) 8 (203) 10 (254)

Note: See Figure 2 of this bulletin for diagrams of reinforcing

 rod spacings.

8.7 Sumps or drains for poured-in-place UCVs should be located

directly under UCV covers and at depths such that finished floor

surfaces should be graded toward the sumps or drains. Sumps or

drains should be at least 8 in. (203 mm) in diameter or 8 in.

(203 mm) square. Sumps or drains should be constructed using
Bulletin 1751F-643

Page 18

12 in. (305 mm) lengths of 8 in. (203 mm) sewer tiles. Figure 3

of this bulletin provides information on two types of sump or

drain designs.

8.8 Table 5 of this section and Figure 4 of this bulletin

provide design information on wall thicknesses and vertical

reinforcement spacings, if any, for concrete walls of poured-inplace UCVs in firm soils. Table 6 of this section and Figure 4

of this bulletin provide design information on wall thicknesses

and vertical reinforcement spacings, if any, for concrete walls

of poured-in-place UCVs in fluid soils. It should be noted in

Tables 5 and 6 of this section that the variations in concrete

wall thickness and the quantity of vertical reinforcement

required varies only with the depths of the poured-in-place UCV

walls. Vertical reinforcing bars should be of sufficient lengths

to extend 1 in. (25 mm) from the bottoms of concrete floors to

within 1 in. (25 mm) of the tops of UCV roofs. In addition to

vertical reinforcement, UCV walls should also contain horizontal

reinforcement. For UCV walls that are 12 in. (305 mm) or less in

thickness, horizontal reinforcement should consist of 3/8 in.

(10 mm) or larger diameter round bars spaced 12 in. (305 mm) on

centers. UCV walls that are greater than 12 in. (305 mm) thick

should be horizontally reinforced using 5/8 in. (16 mm) or larger

diameter round bars spaced 12 in (305 mm) on centers. Horizontal

reinforcing bars should be of sufficient lengths to extend within

1 in. (25.4 mm) of outside surfaces of the two adjacent walls.
Bulletin 1751F-643

Page 19

TABLE 5

Wall Construction

Poured-In-Place UCVs in Firm Soils

Reinforced Walls

Plain

Walls

Vertical Reinforcement Using

Deformed Bars

Category

Depth of

UCV Floor

Below

Surface

of Street

in. (cm)

Wall

Thickness

in. (mm)

Wall

Thickness

in. (mm)

Size

In. (mm)

Deformed

Bar Types

Spacing On

Center

in. (mm)

8.0

96 (244)

or less 6 (152) 6 (152) None None None

8.5 102 (259) 7 (178) 6 (152) 3/8 (10)

1⁄2 (13)

Round

Round

8 (203)

12 (305)

9.0 108 (274) 7.5 (190) 6 (152) 3/8 (10)

1⁄2 (13)

Round

Round

6 (152)

10 (254)

9.5 114 (289) 8 (203) 6 (152) 3/8 (10)

1⁄2 (13)

Round

Round

5.5 (140)

9 (229)

10.0 120 (305) 8.5 (216) 6 (152) 3/8 (10)

1⁄2 (13)

Round

Round

4 (102)

8 (203)

10.5 126 (320) 9 (229) 6 (152) 3/8 (10)

1⁄2 (13)

Round

Round

4 (102)

7 (178)

11.0 132 (335) 10 (254) 6 (152) 1⁄2 (13) Round 6 (152)

11.5 138 (350) 10.5 (267) 6 (152) 1⁄2 (13) Round 5 (127)

12.0 144 (366) 11 (279) 6.5 (165) 1⁄2 (13) Round 5 (127)

12.5 150 (381) 12 (305) 6.5 (165) 1⁄2 (13)

5/8 (16)

Round

Round

4.5 (114)

7 (178)

13.0 156 (396) 12.5 (317) 7 (178) 1⁄2 (13)

5/8 (16)

Round

Round

4.5 (114)

7 (178)

13.5 162 (411) 13 (330) 7 (178) 5/8 (16) Round 6.5 (165)

14.0 168 (427) 14 (356) 7.5 (190) 5/8 (16) Round 6.5 (165)

14.5 174 (442) 14.5 (368) 7.5 (190) 5/8 (10)

3⁄4 (19)

Round

Round

5.5 (140)

8 (203)

15.0 180 (457) 15 (381) 8 (203) 5/8 (10)

3⁄4 (19)

Round

Round

5.5 (140)

8 (203)

16.0 192 (488) Reinforcement

Required

9 (229) 5/8 (10)

3⁄4 (19)

Round

Round

5.5 (140)

7.5 (190)

17.0 204 (518) Reinforcement

Required

9.5 (241) 5/8 (10)

3⁄4 (19)

Round

Round

4.5 (114)

7 (178)

18.0 216 (549) Reinforcement

Required

10 (254) 5/8 (10)

3⁄4 (19)

Round

Round

4 (102)

6 (152)

19.0 228 (579) Reinforcement

Required

11 (279) 5/8 (10)

3⁄4 (19)

Round

Round

4 (102)

5.5 (140)

20.0 240 (610) Reinforcement

Required

11.5

(292)

5/8 (10)

3⁄4 (19)

Round

Round

3.5 (89)

5.5 (140)
Bulletin 1751F-643

Page 20

TABLE 6

Wall Construction

Poured-In-Place UCVs in Fluid Soils

Reinforced Walls

Plain

Walls

Vertical Reinforcement Using

Deformed Bars

Category

Depth of

UCV Floor

Below

Surface

of Street

In. (cm)

Wall

Thickness

in. (mm)

Wall

Thickness

In. (mm)

Size

In. (mm)

Deformed

Bar Types

Spacing

On Center

in. (mm)

8.0A

96 (244)

or less 9 (229) 6 (152) 1⁄2 (13) Round 6 (152)

8.5A 102 (259) 10 (254) 6.5 (165) 1⁄2 (13) Round 6 (152)

9.0A 108 (274) 11 (279) 7 (178) 1⁄2 (13) Round 5.5 (140)

9.5A 114 (289) 12 (305) 7.5 (190) 1⁄2 (13) Round 5 (127)

10.0A 120 (305) 13 (330) 8 (203) 1⁄2 (13) Round 4.5 (114)

10.5A 126 (320) 14 (356) 8.5 (216) 1⁄2 (13) Round 4 (102)

11.0A 132 (335) 15 (381) 9 (229) 5/8 (10) Round 6 (152)

11.5A 138 (350) 16 (406) 9.5 (241) 5/8 (16) Round 5.5 (140)

12.0A 144 (366) Reinforcement

Required

10 (254) 5/8 (16) Round 5.5 (140)

12.5A 150 (381) Reinforcement

Required

10.5

(267)

5/8 (16) Round 5 (127)

13.0A 156 (396) Reinforcement

Required

11 (279) 5/8 (16)

3⁄4 (19)

Round

Round

4.5 (114)

6 (152)

13.5A 162 (411) Reinforcement

Required

11.5

(292)

5/8 (16)

3⁄4 (19)

Round

Round

4 (102)

6.5 (165)

14.0A 168 (427) Reinforcement

Required

12 (305) 3⁄4 (19) Round 5.5 (140)

14.5A 174 (442) Reinforcement

Required

12.5

(317)

3⁄4 (13) Round 5.5 (140)

15.0A 180 (457) Reinforcement

Required

13 (330) 3⁄4 (13) Round 5 (127)

16.0A 192 (488) Reinforcement

Required

14 (356) 3⁄4 (13) Round 4.5 (114)

8.9 Roofs of poured-in-place UCVs should be designed using

reinforced concrete. Table 7 of this section provides design

information on UCV roofs regarding thicknesses and reinforcing

rod diameters for all rectangular poured-in-place UCVs. Table 8

of this section provides design information on UCV roofs

regarding thicknesses and reinforcing rod diameters for UM-V type

poured-in-place UCVs. Figure 5 of this bulletin provides design

information on center-to-center spacings of reinforcing rods for

both rectangular and UM-V type poured-in-place UCV roofs.
Bulletin 1751F-643

Page 21

TABLE 7

Roof Thicknesses and Reinforcing Rod Diameters

Rectangular Poured-In-Place UCVs

Inside Width of UCV

in. (cm)

Roof Thickness

In. (mm)

Reinforcing

Rod Diameter

in. (mm)

36 (91) 6 (152) 1/2 (13)

42 (107) 6 (152) 1/2 (13)

48 (122) 7 (178) 1/2 (13)

54 (137) 7.5 (190) 1/2 (13)

TABLE 8

Roof Thicknesses and Reinforcing Rod Diameters

Type UM-V Poured-In-Place UCVs

Inside Width or

Length of UCV

Whichever is Greater

ft (m)

Roof Thickness

In. (mm)

Reinforcing

Rod Diameter

in. (mm)

8 (2.4) 11 (279) 3/4 (19)

9 (2.7) 11.5 (292) 3/4 (19)

10 (3.0) 12 (305) 3/4 (19)

11 (3.3) 12.5 (317) 3/4 (19)

12 (3.7) 13 (330) 3/4 (19)

8.10 Openings of poured-in-place UCVs should be determined by

the types and sizes of UCV frames. Diameters of frame openings

at bases of frames should be equal to diameters of UCV roof

openings. Table 9 of this section provides design information on

the sizes of UCV opening forms to be used with various types and

sizes of UCV frames.

TABLE 9

UCV Opening Forms

for UCV Frame Types and Sizes

Poured-In-Place UCVs

Frame Types

Frame Sizes

In. (cm)

UCV Opening

Form Sizes

in. (cm)

B 24 (61) 35 (89)

B 27 (69) 38 (96)

B 30 (76) 41 (104)

R 27 (69) 28 (71)

R 30 (76) 31 (79)

8.11 When poured-in-place UCVs require wall recesses at conduit

entrances, concrete should be placed between the faces of the

recesses and the excavation walls to provide watertight bonds.

Concrete thicknesses should be equal to at least one-half of UCV
Bulletin 1751F-643

Page 22

wall thicknesses. Figure 6 of this bulletin provides additional

design information concerning wall recesses. When conduits

entering UCVs are splayed, recessing of conduit banks should not

be required.

8.12 Type B frames should be used with poured-in-place UCVs

installed in areas subjected to vehicular traffic. Type R frames

should be used with poured-in-place UCVs that are installed in

areas not subjected to vehicular traffic such as sidewalks,

median strips. etc. Figure 7 of this bulletin provides

additional design information on Types B and R frames. Thirty

inch (30 in.) (76 cm) frames should be used in areas subjected to

vehicular traffic. Twenty-seven inch (27 in.) (69 cm) frames

should be used in areas not subjected to vehicular traffic.

Poured-in-place UM-X and UM-Y type UCVs normally use 24 in.

(61 cm) frames.

8.13 Frames of poured-in-place UCVs should be supported with

either brick or concrete collars. Brick or concrete collars

should be of sufficient heights to bring UCV covers flush with

grades of travel surfaces. Figure 8 of this bulletin provides

additional design information on brick collars.

8.14 Solid covers should be used with all types of poured-inplace UCVs except when light and ventilation of UCVs are

required. Heavy duty covers should be used on all types of

poured-in-place UCVs that are subjected to vehicular traffic.

9. CABLE RACKS, RACK SUPPORTS, and PULLING IRONS

9.1 The number of cable racks and rack supports, spacing of

cable racks, and placement of pulling irons for precast UCVs

should be designed in accordance with manufacturers’

recommendations.

9.2 For poured-in-place UCVs, the number and size of vertical

cable racks should be based on headroom within UCVs as shown in

Table 10 of this section. Additional design information that may

be used to assist in the installation of cable racks within

poured-in-place UCVs may be found in Figure 9 of this bulletin.
Bulletin 1751F-643

Page 23

TABLE 10

Number and Size of Cable Racks Versus Headroom

Poured-In-Place UCVs

Number of Racks

Headroom

In. (cm)

14 Hole

Rack Size

18 Hole

Rack Size

37 Hole

Rack Size

60 (152)(1) 1 1 1

66 (168)(2) - 2 1

72 (183)(2) - 2 1

78 (198) 3 - - 84 (213) 2 1 - 90 (229) 1 2 - 96 (244) - 3 - 102 (259) 4 - - 108 (274) 3 1 -

114 (290) 2 2 - 120 (305) 1 3 - 126 (320) 4 1 - 132 (335) 4 1 -

Note (1): UCVs may contain either one 14 and one 18 hole rack or

one 14 and one 37 hole rack

Note (2): UCVs may contain either two 18 hole racks or one 37

hole rack.

9.3 UCV racking diagrams for poured-in-place UCVs should include

all hardware items needed for supporting cables within UCVs along

with all additional work such as drilling holes, etc.

9.4 The spacing of cable racks within poured-in-place UCVs

should be specified on the construction drawings.

9.5 Cable racking diagrams should be made for central office

vaults, cable troughs, and central office building walls showing

locations of all existing cables, stubs, load coil cases, if any,

and ducts in use. Using this information vacant ducts should be

selected that would allow new cables to be installed without

crossing over existing cables or blocking other vacant ducts.

Trunk and special cables, which require protection, should be

assigned lower conduits because these type cables usually do not

require replacement. Cable duct assignments should be the same

in each UCV, whenever possible. However, each duct should be

chosen section-by-section, since the same relative position in

each UCV may not always provide the best cable arrangement.

9.6 Cables and stubs should be arranged in UCVs to allow

accessibility to cable splices. The number of branch cables from

ends of mainline splices should be limited to two. Stub cables

may be necessary to maintain this arrangement. At locations

requiring cable stubs, racking of main cables should be based on
Bulletin 1751F-643

Page 24

pair counts, gauge sizes, and intended use stub cables. Where

stub cables are used to connect branch feeder or distribution

cables entering UCVs from branch circuits, terminal ends of stub

cables should be racked on UCV walls opposite branch conduits.

9.7 For pull-through UCVs (no splice), cable lengths necessary

for racking should be added to wall-to-wall lengths of conduit

sections on both sides of UCVs.

9.8 Figure 10 of this bulletin should be used in the design of

pulling irons for poured-in-place UCVs.

10. UCV AND CONDUIT LOCATIONS

10.1 Locations of UCVs and conduit routes between UCVs should be

indicated on construction drawings. An example of a construction

drawing indicating UCV locations and conduit routes is shown in

Figure 11 of this bulletin. The construction symbols and

abbreviations used in preparing construction drawings are shown

in Figures 12 through 12C of this bulletin. Poured-in-place or

precast UCVs should be located to avoid unnecessary hazards and

to ensure that future work in UCVs will cause minimum

interference with the normal flow of vehicular traffic. Pouredin-place or precast UCVs should be located in public R/W wherever

possible. When poured-in-place or precast UCVs are located in

the travel portion of roads, streets, etc., intersection

locations should be avoid wherever possible. In congested areas

where the possibility of undisclosed subsurface conditions exist,

test holes may be required to verify that construction can be

carried out as planned to avoid useless excavation. Test holes

should be dug 12 in. (305 mm) deeper than the depth of the

proposed UCV excavation. If foreign pipes or other structures

are encountered in the test holes, steps should be taken to

determine ownership of the foreign structures. If the structures

can not be readily removed, a decision should be made as to the

practicability of either shifting the UCV, arranging with the

owning company for a change in the route of their plant, or

include the structure in the UCV. Penalties or costs associated

with each of the above mentioned alternatives should be carefully

considered before reaching a decision. Poured-in-place or

precast UCVs should not be located near springs or underground

streams.

10.2 Main conduits entering poured-in-place or precast UCVs

should be located equidistant between the floors and roofs and

centered between end walls. For UM-L, UM-J, and UM-T poured-inplace or precast UCVs, conduits entering side walls should be

located 4 in. (102 mm) from end walls which are located farthest

from central offices. Clearances of 12 in. (305 mm) should be

maintained between main conduit formations and roofs or floors of

UCVs. Unless indicated on the construction drawings, wall
Bulletin 1751F-643

Page 25

recesses should be provided at conduit entrances. Subsidiary

conduits entering UCVs should be located to provide clearances of

4 in. (102 mm) from roofs and adjacent walls.

11. DETERMINATION OF INSTALLATION LENGTH AND PULLING TENSION

 OF FILLED COPPER CABLES

11.1 Mechanical stresses placed on copper cables during

installations should be held to absolute minimums to avoid

excessive twisting, stretching, or flexing of cables. Therefore

to avoid excessive twisting , stretching, or flexing of copper

cables during installations, maximum pulling tensions for copper

cables should be calculated to ensure that copper cables can be

placed in conduit and UCV systems without damage.

11.2 One of the factors to consider in determining copper cable

pulling tensions is the coefficient of friction between various

conduit and cable jacket materials. Table 11 of this section

provides both dry and lubricated coefficient of friction values

between various conduit and cable jacket materials which may be

used in determining pulling tensions of cables. The information

in Table 11 of this section is based on conduit interior surfaces

being free of dirt, gravel, etc. Interior conduit surfaces that

are considered dirty, could result in increased coefficient of

friction values. Therefore interior conduit surfaces should be

cleaned before installation of cables.

TABLE 11

Coefficient of Friction

Conduit Versus Cable Jacket Materials

Coefficient of Friction (Sliding)

High Density

Polyethylene 

Low Density

Polyethylene

Conduit Material Dry Lubricated Dry Lubricated

Polyvinyl Chloride 0.31 0.13 0.36 0.16

Concrete 0.48 0.37 0.57 0.41

Corrugated Plastic 0.22 0.13 0.40 0.13

As the table indicates lubricated coefficient of friction values

are generally one-half or less of the dry coefficient of friction

values. The lower lubricated coefficient of friction values

should allow for lower cable pulling tensions which in turn would

allow the installation of longer cable lengths in conduit and UCV

systems. For purposes of safety, maximum pulling tension

calculations for copper cables should be computed using the dry

coefficient of friction values. The effectiveness of lubricants

is dependent to a large extent on the thoroughness of its

application to copper cables. In addition, conduit surface

conditions can contribute significantly to the actual coefficient

of friction.
Bulletin 1751F-643

Page 26

11.3 The factors that should be used in determining the lengths

of copper cables that may be safely installed in conduit systems

are as follows:

a. Cable size and conductor gauge;

b. Maximum pulling strain allowed on conductors;

c. Cable pulling method (pulling eye or basket weave grip);

d. Number of bends in conduit system;

e. Bend radii;

f. Bend angles;

g. Coefficient of friction; and

h. Cables pulled dry or lubricated.

11.4 Equation #1 of this section listed below should be used for

calculating maximum pulling tensions of copper cables equipped

with factory installed pulling eyes:

T = ANK (Equation #1)

Where:

T = Maximum allowable pulling tension in pounds

(lbs) [kilograms (kgs)].

A = Cross-sectional area of copper conductors in

 circular mils [square millimeters (mm2)].

N = Number of conductors terminated in factory

 installed pulling eyes. Typical all conductors

 in cables are terminated in factory installed

 pulling eyes.

K = 0.008 lbs/circular mils (7.2 kgs/mm2)

 for copper.

The cross-sectional areas of copper conductors designated by the

letter “A” in equation #1 of this section are listed in Table 12

of this section:
Bulletin 1751F-643

Page 27

TABLE 12

Cross-sectional Area of Copper Conductors

Conductor Size Cross-sectional Area

American Wire Gauge (AWG) Circular Mils (mm2)

19 1290 (0.653)

22 642 (0.324)

24 404 (0.205)

26 254 (0.128)

11.5 After calculating maximum pulling tensions of copper

cables, maximum allowable pulling lengths of cables should be

calculated using equation #2 of this section listed below:

L = T/Wf (Equation #2)

Where:

L = Maximum allowable pulling length in ft (m).

T = Maximum allowable pulling tension in lbs (kgs).

W = Cable weight in lbs/ft (kgs/m).

f = Coefficient of friction for type of conduit

 used in installation.

11.6 Another factor that should be considered in calculating

cable pulling tensions is the number and effects of bends in

conduit systems. For conduit systems containing bends, pulling

tensions of copper cables should be calculated on cumulative

basis using equations #3 and #4 of this section listed below:

T = LWf (Equation #3) T = T1efa (Equation #4)

Where:

T = Pulling tension in lbs (kgs).

L = Cable length in ft (m).

W = Cable weight in lbs/ft (kgs/m).

T1 = Accumulated tension to start of bend

in lbs (kgs).

e = Naperian logarithm equal to 2.718.
Bulletin 1751F-643

Page 28

f = Coefficient of friction for type of conduit

 used in installation.

a = Bend angle in radians (1 radian = 57.3 degrees).

11.7 Another factor that should be considered is the maximum

allowable sidewall pressure. Sidewall pressures are radial

forces exerted on cable sheaths at bend points when cables are

under tension. Sidewall pressures at bend points should be

calculated using equation #5 of this section listed below:

P = T/R (Equation #5)

Where:

P = Sidewall Pressure in lbs/ft (kgs/m).

T = Pulling tension in lbs (kgs).

R = Bend radius in ft (m).

The maximum allowable sidewall pressure for copper cables should

not exceed 100 lbs/ft (149 kgs/m). In conduit systems containing

multiple bends, the last bend in the system would develop the

greatest sidewall pressure on the cable. Therefore during the

design of conduit systems, bends with the severest radii should

be installed at the start of conduit systems.

11.8 Figures 13 through 13B of this bulletin provide an example

of an underground filled copper cable installation that may be

used for calculating maximum cable pulling tensions for conduit

and UCV systems containing multiple conduit bends. Figures 14

and 14A of this bulletin provide an example of an underground

filled copper cable installation that may be used for calculating

the maximum sidewall pressures for conduit and UCV systems

containing multiple conduit bends.

12. DETERMINATION OF INSTALLATION LENGTH AND PULLING TENSION

 OF FILLED FIBER OPTIC CABLES

12.1 Factors that should be considered in determining the

installation lengths and pulling tensions of fiber optic cables

to ensure that fiber optic cables can be placed in conduit and

UCV systems without damage are listed below:

a. Maximum allowable pulling tension;

a. Minimum allowable bending radius during installation;

b. Minimum allowable bending radius after installation;

c. Construction and dimensions of pulling end;
Bulletin 1751F-643

Page 29

d. Cable weight and diameter;

e. Supply length of cable; and

f. Excess cable length at each splice location.

12.2 Fiber optic cable manufacturers should be contacted to

determine the appropriate values for the items listed in

paragraph 12.1 of this section to ensure that fiber optic cables

can be placed in conduit and UCV systems without damage.

12.3 In addition to the factors listed in paragraph 12.1 of this

section, fiber optic cable manufacturers should also be contacted

to determine the maximum allowable sidewall pressure that may be

exerted on fiber optic cable sheaths at bend points when cables

are under tension.

13. DESIGN CONSIDERATIONS

13.1 For outside plant underground facilities using copper

cables, circuit requirements should be thoroughly evaluated to

provide sufficient margin in the sizing of underground feeder

cables. The sizing of the copper cables should be based on the

recommendations listed in RUS TE&CM Section 210, Telephone System

Design - Sizing Criteria. The assigning of cable pairs should be

based on the recommendations included in RUS TE&CM Section 629,

Cable Plant Layout - Serving Area Value Engineering for Rural

Systems.

13.2 For outside plant underground facilities using fiber optic

cables, circuit requirements should be thoroughly evaluated to

provide sufficient margin in the sizing of the cables. The

sizing of the fiber optic cables should be based on circuit

requirements of the LD plus the number of circuits needed for

future growth.

13.3 Local characteristics of the project area should be

thoroughly studied to determine whether normal or extraordinary

construction conditions exist before making comparative cost

studies. In addition, the determination should be made as to

whether or not protection of conduits would be required so that

the increase in the incremental cost associated with the

protection of the conduits can be evaluated.

13.4 For copper underground plant, the design should provide for

the optimum use of fine gauge cable (22 and 24 American Wire

Gauge (AWG) conductors). Care should be exercised in the design

to ensure that transmission requirements will be met.

13.5 For copper underground plant, the choice of the proper

gauge conductors should depend on the transmission and signaling

requirements. Care should be exercised in selecting the cables
Bulletin 1751F-643

Page 30

since initial cost differentials between the various cable types,

sizes, and conductor gauges can be appreciable. Where economic

costs indicate the use of digital carrier and other types of

electronic equipment over physical circuits, the electronic

equipment should be designed.

13.6 For optical fiber underground plant, the design should

provide for the use of either dispersion-unshifted single mode

fibers (operate at both the 1310 and 1550 nanometer windows) or

dispersion-shifted single mode fibers (operate at the 1550

nanometer window). The choice as to which single mode fiber to

use should depend on the optical system's length and the

transmission and signaling requirements.

13.7 The optical fiber design should be prepared in accordance

with the recommendations in RUS Bulletin 1751H-601, Lightwave

Fundamentals, Systems, and Application.