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.

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