NEETS Module 4 - Introduction to Electrical Conductors, Wiring Techniques, and Schematic Reading
Pages i - ix, 1-1 to 1-10,
1-11 to 1-20, 1-21 to 1-28,
2-1 to 2-10, 2-11 to 2-20,
2-21 to 2-30, 2-31 to 2-40,
2-41 to 2-53, 3-1 to 3-10,
3-11 to 3-20, 3-21 to 3-24, 4-1
to 4-10, 4-11 to 4-18, Index
Learning objectives are stated at the beginning of each chapter. These learning
objectives serve as a preview of the information you are expected to learn in the
chapter. The comprehensive check questions are based on the objectives. By successfully
completing the OCC-ECC, you indicate that you have met the objectives and have learned
the information. The learning objectives are listed below.
this chapter, you should be able to:
1. Recall the definitions
of unit size, mil-foot, square mil, and circular mil and the mathematical equations
and calculations for each.
2. Define specific resistance
and recall the three factors used to calculate it in ohms.
Describe the proper use of the American Wire Gauge when making wire measurements.
4. Recall the factors required in selecting proper size
5. State the advantages and disadvantages of copper
or aluminum as conductors.
6. Define insulation resistance
and dielectric strength including how the dielectric strength of an insulator is
7. Identify the safety precautions to be taken
when working with insulating materials.
8. Recall the
most common insulators used for extremely high voltages.
State the type of conductor protection normally used for shipboard wiring.
10. Recall the design and use of coaxial cable.
In the previous modules of this training series, you have learned
about various circuit components. These components provide the majority of the operating
characteristics of any electrical circuit. They are useless, however, if they are
not connected together. Conductors are the means used to tie these components together.
Many factors determine the type of electrical conductor used to connect
components. Some of these factors are the physical size of the conductor, its composition,
and its electrical characteristics. Other factors that can determine the choice
of a conductor are the weight, the cost, and the environment where the conductor
will be used.
To compare the resistance and size of one conductor with that of another,
we need to establish a standard or unit size. A convenient unit of measurement of
the diameter of a conductor is the mil (0.001, or one-thousandth of an inch). A
convenient unit of conductor length is the foot. The standard unit of size in most
cases is the MIL-FOOT. A wire will have a unit size if it has a diameter of 1 mil
and a length of 1 foot.
The square mil is a unit of measurement
used to determine the cross-sectional area of a square or rectangular conductor
(views A and B of figure 1-1). A square mil is defined as the area of a square,
the sides of which are each 1 mil. To obtain the cross-sectional area of a square
conductor, multiply the dimension of any side of the square by itself. For example,
assume that you have a square conductor with a side dimension of 3 mils. Multiply
3 mils by itself (3 mils x 3 mils). This gives you a cross-sectional area of 9 square
Figure 1-1. - Cross-sectional areas of conductors.
Q1. State the reason for the establishment
of a "unit size" for conductors.
the diameter in MILS of a conductor that has a diameter of 0.375 inch.
Q3. Define a mil-foot.
To determine the cross-sectional area of a rectangular conductor, multiply the
length times the width of the end face of the conductor (side is expressed in mils).
For example, assume that one side of the rectangular cross-sectional area is 6 mils
and the other side is 3 mils. Multiply 6 mils x 3 mils, which equals 18 square mils.
Here is another example. Assume that a conductor is 3/8 inch thick and 4 inches
wide. The 3/8 inch can be expressed in decimal form as 0.375 inch. Since 1 mil equals
0.001 inch, the thickness of the conductor will be 0.001 x 0.375, or 375 mils. Since
the width is 4 inches and there are 1,000 mils per inch, the width will be 4 x 1,000,
or 4,000 mils. To determine the cross-sectional area, multiply the length by the
width; or 375 mils x 4,000 mils. The area will be 1,500,000 square mils.
Q4. Define a square mil as it relates to a square conductor.
The circular mil is the standard
unit of measurement of a round wire cross-sectional area (view C of figure 1-1).
This unit of measurement is found in American and English wire tables. The diameter
of a round conductor (wire) used to conduct electricity may be only a fraction of
an inch. Therefore, it is convenient to express this diameter in mils to avoid using
decimals. For example, the diameter of a wire is expressed as 25 mils instead of
0.025 inch. A circular mil is the area of a circle having a diameter of 1 mil, as
shown in view B of figure 1-2. The area in circular mils of a round conductor is
obtained by squaring the diameter, measured in mils. Thus, a wire having a diameter
of 25 mils has an area of 252, or 625 circular mils. To determine
the number of square mils in the same conductor, apply the conventional formula
for determining the area of a circle (A = πr2). In this
formula, A (area) is the unknown and is equal to the cross-sectional area in square
mils, p is the constant 3.14, and r is the radius of the circle, or half the diameter
(D). Through substitution, A = 3.14, and (12.5)2; therefore, 3.14 x 156.25
square mils. The cross-sectional area of the wire has 625 circular mils but only
490.625 square mils. Therefore, a circular mil represents a smaller unit of area
than the square mil.
Figure 1-2. - A comparison of circular and square mils.
If a wire has a cross-sectional diameter of 1 mil, by definition,
the circular mil area (CMA) is A =
D2, or A = 12, or A
= 1 circular mil. To determine the square mil area of the same wire, apply the formula
A = πr2; therefore, A = 3.14 x (.5)2 (.5 representing
half the diameter). When A = 3.14 x .25, A = .7854 square mil. From this,
it can be concluded that 1 circular mil is equal to. 7854 square mil. This becomes
important when square (view A of figure 1-2) and round (view B) conductors are compared
as in view C of figure 1-2.
When the square mil area is given,
divide the area by 0.7854 to determine the circular mil area, or CMA. When the CMA
is given, multiply the area by 0.7854 to determine the square mil area. For example,
Problem: A 12-gauge wire has a diameter of 80.81 mils. What is (1)
its area in circular mils and (2) its area in square mils?
(1) A = D2 = 80.812 = 6,530 circular mils
(2) A = 0.7854 x 6,530 = 5,128.7 square mils
Problem: A rectangular conductor is 1.5 inches wide and 0.25 inch
thick. What is (1) its area in square mils and (2) in circular mils? What size of
round conductor is necessary to carry the same current as the rectangular bar?
(1) 1.5 inches = 1.5 inches x 1,000 mils per inch = 1,500 mils
0.25 inch = 0.25 inch x 1,000 mils per inch = 250 mils
A = 1,500 x 250 = 375,000 square mils
(2) To carry the same current, the cross-sectional area of the round conductor
must be equal.
There are more circular mils than square mils in this area. Therefore:
A wire in its usual form is a single slender rod or filament
of drawn metal. In large sizes, wire becomes difficult to handle. To increase its
flexibility, it is stranded. Strands are usually single wires twisted together in
sufficient numbers to make up the necessary cross-sectional area of the cable. The
total area of stranded wire in circular mils is determined by multiplying the area
in circular mils of one strand by the number of strands in the cable.
Q5. Define a circular mil.
What is the circular mil area of a 19-strand conductor if each strand is 0.004 inch?
A circular-mil-foot (figure 1-3)
is a unit of volume. It is a unit conductor 1 foot in length and has a cross-sectional
area of 1 circular mil. Because it is a unit conductor, the circular-mil-foot is
useful in making comparisons between wires consisting of different metals. For example,
a basis of comparison of the RESISTIVITY (to be discussed shortly) of various substances
may be made by determining the resistance of a circular-mil-foot of each of the
Figure 1-3. - Circular-mil-foot.
In working with square or rectangular conductors, such as ammeter
shunts and bus bars, you may sometimes find it more convenient to use a different
unit volume. A bus bar is a heavy copper strap or bar used to connect several circuits
together. Bus bars are used when a large current capacity is required. Unit volume
may be measured as the centimeter cube. Specific resistance, therefore, becomes
offered by a cube-shaped conductor 1centimeter in length and 1 square centimeter
in cross-sectional area. The unit of volume to be used is given in tables of specific
SPECIFIC RESISTANCE OR RESISTIVITY
Specific resistance, or resistivity, is the resistance in ohms offered
by a unit volume (the circular-mil- foot or the centimeter cube) of a substance
to the flow of electric current. Resistivity is the reciprocal of conductivity.
A substance that has a high resistivity will have a low conductivity, and vice versa.
Thus, the specific resistance of a substance is the resistance of a unit volume
of that substance.
Many tables of specific resistance are based on
the resistance in ohms of a volume of a substance 1 foot in length and 1 circular
mil in cross-sectional area. The temperature at which the resistance measurement
is made is also specified. If you know the kind of metal a conductor is made of,
you can obtain the specific resistance of the metal from a table. The specific resistances
of some common substances are given in table 1-1.
Table 1-1. - Specific Resistances of Common Substances
The resistance of a conductor of a uniform cross section varies directly
as the product of the length and the specific resistance of the conductor, and inversely
as the cross-sectional area of the conductor. Therefore, you can calculate the resistance
of a conductor if you know the length, cross-sectional area, and specific resistance
of the substance. Expressed as an equation, the "R" (resistance in ohms) of a conductor
ρ = (Greek rho) the specific resistance in ohms per circular-mil-foot
(refer to table 1-1)
L = length in feet
A = the cross-sectional area in circular mils
What is the resistance of 1,000 feet of copper
wire having a cross-sectional area of 10,400 circular
mils (No. 10 wire) at
a temperature of 20º C? Solution:
The specific resistance of copper (table 1-1) is 10.37 ohms. Substituting the
known values in the
preceding equation, the resistance, R, is determined as
Given: ρ = 10.37 ohms
L = 1,000 ft
A = 10,400 circular mils
= 1 ohm (approximately)
If R, ρ, and A are known, the length (L) can be determined by a simple
mathematical transposition. This has many valuable applications. For example, when
locating a ground in a telephone line, you will use special test equipment. This
equipment operates on the principle that the resistance of a line varies directly
with its length. Thus, the distance between the test point and a fault can be computed
Q7. Define specific resistance.
Q8. List the three factors used to calculate resistance of a particular
conductor in ohms.
The most common method for measuring
wire size in the Navy is by using the American Wire Gauge (AWG). An exception is
aircraft wiring, which varies slightly in size and flexibility from AWG standards.
For information concerning aircraft wire sizes, refer to the proper publications
for specific aircraft. Only AWG wire sizes are used in the following discussion.
Wire is manufactured in sizes numbered according to the AWG tables. The various
wires (solid or stranded) and the material they are made from (copper, aluminum,
and so forth) are published by the National Bureau of Standards. An AWG table for
copper wire is shown at table 1-2. The wire diameters become smaller as the gauge
numbers become larger. Numbers are rounded off for convenience but are accurate
for practical application. The largest wire size shown in the table is 0000 (read
"4 naught"), and the smallest is number 40. Larger and smaller sizes are manufactured,
but are not commonly used by the Navy. AWG tables show the diameter in mils, circular
mil area, and area in square inches of AWG wire sizes. They also show the resistance
(ohms) per thousand feet and per mile of wire sizes at specific temperatures. The
last column shows the weight of the wire per thousand feet. An example of the use
of table 1-2 is as follows.
Table 1-2. - Standard Solid Copper (American Wire Gauge)
Problem: You are required to run 2,000 feet of AWG 20 solid copper
wire for a new piece of equipment. The temperature where the wire is to be run is
25º C (77º F). How much resistance will the wire offer to current flow?
Solution: Under the gauge number column, find size AWG 20. Now read across the
columns until you reach the "ohms per 1,000 feet for 25º C (77º F)" column. You
will find that the wire will offer 10.4 ohms of resistance to current flow. Since
we are using 2,000 feet of wire, multiply by 2.
10.4 ohms x 2 = 20.8 ohms
An American Standard Wire Gauge (figure
1-4) is used to measure wires ranging in size from number 0 to number 36. To use
this gauge, insert the wire to be measured into the smallest slot that will just
accommodate the bare wire. The gauge number on that slot indicates the wire size.
The front part of the slot has parallel sides, and this is where the wire measurement
is taken. It should not be confused with the larger semicircular opening at the
rear of the slot. The rear opening simply permits the free movement of the wire
all the way through the slot.
Figure 1-4. - Wire gauge.
Q9. Using table 1-2, determine the resistance of
1,500 feet of AWG 20 wire at 25º C.
Q10. When using
an American Standard Wire Gauge to determine the size of a wire, where should you
place the wire in the gauge to get the correct measurement?
STRANDED WIRES AND CABLES
A wire is a
single slender rod or filament of drawn metal. This definition restricts the term
to what would ordinarily be understood as "solid wire." The word "slender" is used
because the length of a wire is usually large when compared to its diameter. If
a wire is covered with insulation, it is an insulated wire. Although the term "wire"
properly refers to the metal, it also includes the insulation.
conductor is a wire suitable for carrying an electric current.
A stranded conductor is a conductor composed of a group of wires or
of any combination of groups of wires. The wires in a stranded conductor are usually
twisted together and not insulated from each other.
is either a stranded conductor (single-conductor cable) or a combination of conductors
insulated from one another (multiple-conductor cable). The term "cable" is a general
one and usually applies only to the larger sizes of conductors. A small cable is
more often called a stranded wire or cord (such as that used for an iron or a lamp
cord). Cables may be bare or insulated. Insulated cables may be sheathed (covered)
with lead, or protective armor. Figure 1-5 shows different types of wire and cable
used in the Navy.
Figure 1-5. - Conductors.
Conductors are stranded mainly to increase their flexibility. The
wire strands in cables are arranged in the following order:
layer of strands around the center conductor is made up of six conductors. The second
layer is made up of 12 additional conductors. The third layer is made up of 18 additional
conductors, and so on. Thus, standard cables are composed of 7, 19, and 37 strands,
in continuing fixed increments. The overall flexibility can be increased by further
stranding of the individual strands.
Figure 1-6 shows a typical cross
section of a 37-strand cable. It also shows how the total circular-mil cross-sectional
area of a stranded cable is determined.
Figure 1-6. - Stranded conductor.
NEETS Table of Contents
- Introduction to Matter, Energy,
and Direct Current
- Introduction to Alternating Current and Transformers
- Introduction to Circuit Protection,
Control, and Measurement
- Introduction to Electrical Conductors, Wiring
Techniques, and Schematic Reading
- Introduction to Generators and Motors
- Introduction to Electronic Emission, Tubes,
and Power Supplies
- Introduction to Solid-State Devices and
- Introduction to Amplifiers
- Introduction to Wave-Generation and Wave-Shaping
- Introduction to Wave Propagation, Transmission
Lines, and Antennas
- Microwave Principles
- Modulation Principles
- Introduction to Number Systems and Logic Circuits
- Introduction to Microelectronics
- Principles of Synchros, Servos, and Gyros
- Introduction to Test Equipment
- Radio-Frequency Communications Principles
- Radar Principles
- The Technician's Handbook, Master Glossary
- Test Methods and Practices
- Introduction to Digital Computers
- Magnetic Recording
- Introduction to Fiber Optics