(2) Example 2.
When the demand
a good grade of timber which will
load, the length of wire, and the size
last.
Good pressure treated Texas
of wire are known, the voltage drop
southern pine, for example, can be
can be determined as a check of the
expected to last 35 to 50 years.
On
the other hand, untreated local timber
Assuming a 1 2W
system's adequacy.
such as soft pine may last only a
120V system is being used having a
couple of years and may have a useful
certain section supplying a 5 KVA load
life
of
only
one
season
under
with a No. 2 wire, what is the voltage
unfavorable conditions.
However, the
drop in its 250 foot length?
fact that a pole is in good condition
is
not
proof
that
it
can
Solution: Using figure 7 enter 250
satisfactorily
support
the
line.
feet at the bottom right horizontal
Poles must withstand column loadings
scale; proceed vertically to the No. 2
as well as transverse loads from wind
curve; proceed horizontally to the
and turns in the line.
vertical line representing 5 KVA as
read from the lower left horizontal
b. Height and class. Table 2 shows
scale.
At this point, the voltage
the required heights and classes of
drop is read as 3.3 percent using the
poles for various types of loadings in
lower voltage drop figures.
This
military installations. The smaller a
voltage drop may, or may not, be
class number, the sturdier will be the
within specifications.
If it is not,
pole.
The class of a given pole
then a larger size of wire must be
depends on its length, diameter, and
used which will lower the drop to the
the material from which it is made.
predetermined specification.
Tables
correlating
class
with
dimensions for various pole materials
8.
POLE SELECTION
can be found in appropriate TM's. The
height is governed by restrictions
As previously stated, TO electric
both above and below the proposed
distribution systems are of the radial
conductors.
Near airfields, poles
type;
lines
radiate
from
the
must be kept low, yet high enough to
towards
the
loads,
provide
adequate
clearance
over
branching and rebranching until all
streets and roads.
Corner poles,
areas are served.
Lines are normally
transformer poles, and the like are
overhead, supported by poles or strung
usually one or more classes heavier
from building to building.
As a
and sometimes 5 feet higher than line
general rule, lines carrying over 240
poles.
Table 3 gives the required
volts are supported on poles while
classes for transformer poles.
those carrying 120 to 240 volts maybe
supported
on
masts
or
c. Vertical wire spacing. Vertical
attached to buildings.
Service wires
wire spacing is the clear distance
should be kept at least 3 feet from
between wires. 6-inch minimum spacing
windows and 8 inches above flat roofs.
is required for spans up to and
Since theater of operations electrical
including
200
feet
while
12-inch
distribution systems are exclusively
minimum spacing is required for spans
overhead distribution systems, wooden
over 200 feet.
A clear space is
poles
similar
to
those
used
in
always
left
at
the
top
of
an
civilian construction are used to
electrical pole.
This distance is
carry the conductors. Unlike civilian
equal to the wire spacing, that is, 6
construction, crossarms are not used
inches for 200 foot spans and less,
in the theater of operations since the
and 12 inches for spans greater than
size of the system does not warrant
200 feet.
them.
Conductors are hung on the
pole, one above the other.
d. Setting poles. Poles should be set
deep enough to develop their full bending
a. Poles.
The poles used in a
distribution system should consist of
2-14
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