TM
5-811-1/AFJMAN
32-1080
CHAPTER 5
ELECTRIC DISTRIBUTION LINES
5-1. Selection.
tems installed directly in the ground. Cables may
be the direct-burial type cable assemblies in coil-
Criteria for electric distribution lines will be based
able plastic duct, or cable assemblies in metallic
on the requirements of agency criteria. Distribu-
armor (in direct-burial rated sheath). In this man-
tion lines will be sized to meet current demand
ual, the word duct will be used rather than
load, future loads, and line-loss factors.
conduit.
a. Aerial line requirements. Aerial lines will be
a . Requirements for medium-voltage lines.
used in all areas, except in the following instances:
Where underground systems are provided, the
(1) Where aerial lines would constitute haz-
following standards will be followed:
ards such as near flight lines (where poles must be
(1) In industrial and densely populated areas,
outside of the glide path) or near munitions build-
cables will be installed in underground duct lines
ings (where poles can be no closer than the length
with manholes. Ducts will be concrete encased.
of the lines between the poles which support the
(2) In lightly populated areas, cable may be
lines unless effective means is provided to assure
placed in non-concrete-encased duct or buried di-
that energized lines cannot, on breaking, come in
contact with the facility or its appurtenances).
rectly.
(3) The use of direct-burial cable will be lim-
(2) Where aerial lines would obstruct opera-
tions (e.g., interfere with crane-type, materials-
ited to long untapped runs in lightly populated
handling equipment).
areas where the reliability requirements are low;
(3) Where aerial lines would interfere with
or the facilities served by the cables have a
high-frequency communication or electronic equip-
short-term life; or for other reasons which would
ment. Aerial lines will not be within 250 feet of
justify the use of the more economical direct-burial
Communications-Electronic-Meterological (CEM)
installations.
b. Secondary distribution lines and service con-
Operation Buildings, not within 1,500 feet of
receiving antennas, and not within 1,000 feet of
ductors. Where underground systems are provided,
other antennas.
the following guidelines will be observed:
(4) Where aerial installations would conflict
(1) In industrial and densely populated areas,
with current policy for Family Housing Areas.
cables be installed in underground duct lines (with
(5) Where areas have such high load densities
manholes, handholes, or pullboxes as applicable).
that underground electric lines are economical.
Ducts will be concrete encased.
(2) In lightly populated areas, cable in non-
For Air Force projects, underground installation
must be approved by the HOST COMMAND.
concrete-encased duct or direct cable may be used.
(6) Where aerial lines would be incompatible
(3) Low-voltage direct-burial cable will be re-
stricted to applications where the load to be served
with the environment or architectural concept. For
Air Force projects, underground installation must
is not anticipated to be increased; the underground
be approved by the HOST COMMAND.
cable can be replaced easily upon failure; and the
b. Underground line requirements. Underground
cable system is not subject to disturbance or
physical damage. The designer will coordinate
distribution lines will be provided for the excep-
tions listed above, for minor extensions to existing
burial requirements with the using or maintaining
areas served by underground distribution lines,
organization.
and for medium-voltage or large low-voltage elec-
5-3. Types of Aerial lines.
tric services to buildings. When tying into an
Bare conductors will be used for medium-voltage
existing asbestos composite duct bank, proper envi-
circuits and insulated conductors will be used for
ronmental protocol will be followed.
low-voltage circuits.
5-2. Types of Underground lines.
a. Open wire medium-voltage construction. Bare
wires will be installed on pole lines using either
There are two methods of installing underground
armless or crossarm construction. Since armless
lines, In the first method, underground raceway
construction is more economical and presents a
systems (ducts) are installed below grade and then
more pleasing appearance, it will be provided for
cable is pulled through them. Ducts may or may
new lines, except where prohibited by technical
not be provided with concrete encasement. The
considerations, such as a line with many taps,
second method consists of underground cable sys-
5-1
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