lights
to
burn
dimly,
reduces
(1) Example
1.
The
maximum
efficiency, and may damage motors when
allowable voltage drop for a circuit
excessive.
As defined, voltage drop
is known to be 3 percent. The demand
is the difference in voltage between
load is 20 KVA and the length of the
the input and output ends of a
circuit is 450 feet.
The circuit is
It is caused by
3 4W 127/220 volt type. Using figure
the resistance of the wire and is also
9, determine the smallest permissible
known as "line loss." The percentage
wire size and the resulting voltage
of line loss or voltage drop may be
drop.
expressed as either a percentage of
the voltage required at the receiving
Solution: Figure 9 is used because it
end or as a percentage of the voltage
is based on the proposed 3 4W 127/220
impressed or generated by the source
volt
circuit.
To
determine
the
to the line.
For example, figure 6
required wire size enter the demand
shows the voltage at the motor to be
load of 20 KVA on the upper horizontal
220 volts. The generator is producing
scale and follow vertically downward
231 volts.
Therefore, the line loss
to the intersection with the diagonal
of voltage drop is 11 volts.
The
line representing 3 percent voltage
voltage drop expressed as a percentage
drop pertaining to the upper KVA
of the voltage at the receiving end is
scale.
Project from this point of
or
5
percent;
expressed
as
a
intersection a horizontal line to the
right until intersection is made with
percentage
of
the
generated
voltage,
the vertical line representing 450
it
is
or
4.76
percent.
feet
as
read
from
the
bottom
horizontal scale.
This second point
In civilian practice, the percent
of intersection falls between the
voltage drop is usually taken as a
curves for wire sizes No. 2 and No.
percentage of the voltage required at
1/0.
In such a case, select the
the receiving apparatus.
In military
larger
wire--the
No.
1/0.
To
practice, the percent voltage drop is
determine the actual voltage drop
taken as a percentage of the voltage
using the 1/0 wire, enter 450 feet on
at the generator.
For typical TO
the bottom scale; follow upwards to
installations, the voltage drop is
the
curve
representing
1/0
wire;
usually kept within a 5 percent limit
follow horizontally to the vertical
from the generator to the last or
line representing 20 KVA as read from
farthest load.
Figures 7, 8, and 9
the upper horizontal scale.
At this
will be used in this course for
point, the voltage drop is read as 2.4
computing
voltage
drops
and
wire
percent. From table 1 it can be seen
sizes.
These figures are nomographs
that a 1/0 wire is capable of carrying
which correlate the several factors
the 20 KVA load and from the above
involved.
Their use is discussed in
procedure, the voltage drop is under
the following examples.
the 3 percent specification.
Figure 6.
Line voltage drop.
2-10
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