TM
5-811-1/AFJMAN
32-1080
CHAPTER 7
UNDERGROUND DISTRIBUTION LINES
7-1. General.
multiple conductor, metallic or non-metallic
sheathed. Nonmetallic sheathed cable 2,000 volt to
Underground lines will be provided only in those
5,000 volt rating shall be shielded.
areas as established in chapter 5. Underground
(b) Above 5,000 volts-copper, metallic, or
line installation will be coordinated with the in-
non-metallic sheathed, single or multiple conduc-
stallation master plan to avoid conflict with con-
tor, shielded cable.
struction of future facilities. Lines will normally
(c) Aluminum conductors of equivalent am-
be installed adjacent to roadways in urban, hous-
pacities may be used in lieu of No. 4 AWG copper
ing, or industrial plant areas, but may be routed
and larger copper conductors.
as required to meet the project objectives. A
(d) Aluminum conductors shall be termi-
careful study will be made of all underground
nated in compression type lugs or connectors tilled
utilities in order to ensure a minimum of interfer-
with high temperature, oxide inhibitor compound.
ence between electrical lines and other under-
(3) Cable installation. All primary under-
ground utilities, whether existing, being con-
ground cables shall be installed in concrete en-
structed, o r p r o p o s e d a s a d e f i n i t e f u t u r e
cased non-metallic ducts or concrete encased galva-
construction project. Electrical lines will be at
nized rigid steel conduits except as follows. Steel
least six feet from any steam or hot water lines,
conduit, which is not concrete encased, shall be
except at crossings where a one-foot separation
corrosion proof-coated (PVC, etc.) and shall be
from such lines is adequate.
employed only for short runs between buildings or
a. Symbols and codes. For uniformity, symbols
under paved areas. Direct buried ducts or direct
will comply with ANSI Y32.9. Installation will
buried cables may be employed for primary distri-
comply with the requirements of the NESC and
bution (above 600 V) feeders located in areas
the NEC as required. Where state safety rules are
which are remote to normal pedestrian and vehicle
predominantly accepted, such rules may be used
traffic. Consider using a direct buried, flexible
provided they are as stringent as those of the
polyethylene cable, duct type conduit with self
NESC.
contained cable to allow easier replacement. Con-
b. Construction and other conditions. Typical
crete markers shall be provided at approximately
underground construction details are shown later
200 foot intervals and at each change in direction
in this chapter. Service conditions are covered in
to indicate the location of underground cable
chapter 1. Grounding and surge protection are
route. Direct buried cable or cable duct, which is
covered in chapter 9. Other conditions will follow
not concrete and metallic conduit encased, should
the guidelines established by the NESC, the Rural
be marked with a metallic/magnetic warning tape
Electrification Authority (REA), or the local utility
buried six to twelve inches below the surface and
as applicable. Underground cable systems may
above the cable or duct. Cable warning tape shall
employ manholes or above-ground sectionalizing
be red or orange in color.
and pulling cabinets depending on local geological
conditions, potential for damage from vehicles and
7-2. Cable.
the required design life of the facility.
c. Air Force underground installations.
Conductor material and insulation type will be
(1) Secondary distribution. All secondary un-
specified. Restricting extensions of existing sys-
derground cables may be installed in either con-
tems to a specific conductor material and insula-
crete encased ducts or direct burial type ducts or
tion type in order to match an existing cable type
conduit, or they may be direct burial type cables.
is permitted only when a need has been estab-
Direct burial plastic conduit may be utilized when
lished. Neutral cables, where required, will be
cable temperature conditions are within the con-
installed with 600 V insulation unless concentric
duit rating and where the conduit is not under
neutral cable is used. In duct lines, neutrals will
large paved areas or building structures. Conduc-
be installed in the same conduit with associated
tors shall be copper, although aluminum may be
phase cables.
used in lieu of copper conductors No. 4 AWG and
a. Conductor material. Since underground con-
larger.
ductors are continuously supported, soft-drawn cop-
(2) Primary distribution.
per or aluminum alloy 5005 provides adequate
(a) 5,000 volts or less-copper, single or
strength. However, the selection of copper or alu-
7-1
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