TM
5-811-1/AFJMAN
32-1080
a. Construction.
factory, and is performed in accordance with indus-
(1) Wall thickness. Nonmetallic ducts are man-
try standards. The designer will specify the field
tests in accordance IEEE Std 400. Unless the cable
ufactured with thin-wall (type EB) and thick-wall
accessory manufacturers indicate higher voltages
(type DB) thicknesses. The thin-wall type is de-
are acceptable, test voltages will not exceed the
signed to be used with an added concrete encase-
recommendations of IEEE Std 404 for cable joints
ment. The thick-wall type is used without encase-
and IEEE Std 48 for cable terminations. Table 7-2
ment in concrete; this type of duct is installed with
compares the recommendations of various stan-
an earth fill separating the ducts in a bank, except
dards regarding test voltages for insulation, termi-
that under areas used for vehicular traffic concrete
nations, and joints. Test voltages for 28 kV and 35
encasement is necessary. Guidelines indicating
kV cable insulation should be in accordance with
where concrete encasement is necessary are pro-
the AEIC standards since ICEA/NEMA standards
vided in chapter 5.
(2) Shape. Most ducts have round exteriors
do not cover 133 percent insulation for these two
voltage classes. Tests for cables 600 volts and
with a round bore; however, octagonal and square
below will be specified in accordance with IEEE
exteriors are available, as are square bores.
Std 422.
Square or octagonal exteriors may make stacking
easier in some cases, but round bores are prefera-
7-3. Duct lines.
ble for cable pulling.
(3) Number. Ducts are available in a single
Excluding Alaska and other permafrost locations,
duct lines will be installed in accordance with the
raceway configuration. Although some ducts are
NESC regarding the frost line (i.e., below frost
available in multiple duct units with from two to
nine raceways in each length, this type will not be
lines or restrained with backfill, concrete encase-
ment, bracing, or other means to maintain its
used for electric power cables.
b. Systems. For new projects, duct lines will be
design position under stress of installation proce-
dures, cable pulling operations, settling, or frost
provided at the same time for both electric power
uplift). For permafrost locations, designs will incor-
and communication circuits. See TM 5-811-9 for
porate duct line installation methods which are
communications systems requirements. Communi-
standard for the base, post, or local utility. In clay
cation cables will be completely isolated from
electric power cables in accordance with ANSI C2,
soil, not less than a three-inch layer of sand will
by using separate conduits and access points, such
cover the bottom of the duct trench before ducts
are placed. Ducts will be covered with not less
as manholes. Separate electric and communication
conduits may be installed in the same duct bank,
than a six-inch layer of sand after they are placed.
Metallic conduit will not be used when concrete
however. For economy and space conservation,
electric power and communication ducts may be
encasement is provided. When tying into an exist-
ing asbestos composite duct bank, proper environ-
installed in the same trench and manholes may be
adjacent, when such arrangements suit the com-
mental protocol will be followed. The designer will
provide terrain profiles for all duct systems utiliz-
munication circuit requirements of the appropriate
agency. Power and communication cable separa-
ing manholes. Systems utilizing above ground
sectionalizing and pulling cabinets need profiles
tion will be in accordance with ANSI C2. Fiber
optic cable (and duct) spacing from power cable
only in hilly or congested areas.
will be the same as for conventional cable and
Table 7-2. Comparison of DC Test Voltages (kV)
duct because of the continued use of hybrid sys-
tems, and to provide an increased margin of
Joints
protection for the fiber optic cable.
Cable
on
c. Sizes. The nominal diameter of raceways for
Rated
Extruded
133 Percent
medium-voltage, communication, and other cables
Voltage
Termina-
Dielectric
Insulation
tions
Cable
(kV)
in ducts between manholes will be four inches,
with larger ducts provided where 15 kV cables
AEIC
IEEE
Phase
NEMA
IEEE
larger than No. 4/0 AWG are to be installed. The
to
WC7
CS5
No. 48
No. 404
Phase
WC8
CS6
communication service duct to any building will
-
-
2.5
25
40
not be less than three inches in diameter. Low-
5.0
25
-
50
25
voltage power ducts supplying building services
8.7
35
-
65
35
will be sized in accordance with the NEC. Exterior
15
65
-
75
55
loads supplied from a building such as multiple
25
100
-
105
75
28
-
125
115
85
lighting, control, or motor loads will be served
34.5
-
155
140
100
with not less than l-inch ducts. In general, sizes
7-6
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