FM 55-509-1
CHAPTER 12
ELECTRICAL CONDUCTORS
INTRODUCTION
Many factors determine the type of electrical
conductor to be used to connect components. Some
of these factors are the physical size of the conductor,
the type of material used for the conductor, and the
electrical characteristics of the insulation. Other factors that can determine the choice of a conductor are
the weight, the cost, and the environment where the
conductor is to be used.
CONDUCTOR SIZES
To compare the resistance and size of one
conductor with that of another, a standard or unit
must be established. A convenient unit of measurement for 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.
Square Mil
The square mil is a unit of measurement used
to determine the cross-sectional area of a square or
rectangular conductor (Figure 12-1 views A and B).
A square mil is the area of a square whose sides are
each 1 mil. To obtain the cross-sectional area of any
square conductor, multiply the dimensions of any
side of the conductor by itself. For example, with a
square conductor with a side dimension of 3 mils,
multiply 3 mils by itself (3 mils x 3 mils). This gives a
cross-sectional area of 9 square mils.
To determine the cross-sectional area of a
rectangular conductor, multiply the length times
the width of the end face of the conductor (in mils).
For example, if one side of the rectangular crosssectional area is 6 mils and the other side is 3 mils,
nultiply 6 mils x 3 mils. The cross-sectional area is
18 square mils.
The following is another example of how to
determine the cross-sectional area of a rectangular
conductor. Assume a bus bar is 3/8 inch thick and
4 inches wide. The 3/8 inch expressed in decimal
form is .375 inch. Since 1 mil equals .001 inch, the
thickness of the conductor is 375 mils. The width is
4 inches. Since there are 1,000 mils per inch, the
width is 4,000 mils.
To determine the cross-sectional area, multiply
the length by the width, or 375 mils x 4,000 mils. The
area (A) equals 1,500,00 square mils.
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Circular Mil
Solution:
The circular mil is the standard unit of measure
of the cross-sectional area of a wire. This unit of
measurement is found in American and English wire
tables. The diameter of a round conductor used to
conduct electricity may be only a fraction of an inch.
Therefore, it is convenient to express this fraction 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 whose
diameter is 1 mil, as shown in Figure 12-2 view B. The
area in circular mils of a round conductor is obtained
by squaring the diameter, which is measured in mils.
Thus 2a wire having a diameter of 25 mils has an area
of 25 or 625 circular mils.
a. A = D = (80.81) = 6,530 circular mils.
To determine the number of square mils in the
same conductor, apply the conventional formula for
determining the area o2 a circle (area [A] = pi x
radius squared = pi x r . In this formula, A is the
unknown. It equals the cross-sectional area in square
mils. Pi is the constant 3.1416. Letter r is the radius
of the circle, or half the2 diameter. Through substitution A = 3.1416 (12.5) . Therefore, 3.1416 x 156.25
= 490.625 square mils. The cross-sectional area of
the wire has been shown to have 625 circular mils,
while it has only 490.625 square mils. Therefore, a
circular mil represents a smaller unit of area than the
square mil. If a wire has a cross-sectional diameter
of 1 mil,2 by definition
he circular mil area (CMA) is
2
A = D ,or A = 1 , or A = 1 circular mil. To
determine thes2 are mil area of the same wire, the
form2 a A = pir is applied. Therefore, A = 3.1416
x (.5) . When the formula is carried forward, A =
3.1416 x .25, or A = .7854 square mils. From this, it
can be concluded that 1 circular mil equals .7854
square mil. This becomes important when square
conductors (Figure 12-2 view A) and round conductors (view B) are compared. View C shows the comparison. When the square mil area is given, divide
the area by 0.7854 to determine the circular mil area.
When the circular mil area is given, multiply the area
by 0.7854 to determine the square mil area.
Example: The American wire gauge (AWG)
No. 12 wire has a diameter of 80.81 mils:
a. What is the area in circular mils?
b. What is the area in square mils?
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2
2
b. A = 0.7854 x 6,530 = 5,128.7 square mils.
A wire in its usual form is a slender rod or
filament of drawn metal. In larger sizes, wire is difficult to handle. To increase flexibility, it is stranded.
Strands are usually single wires twisted together in
sufficient numbers to make up the necessary crosssectional area of the cable. The total area in circular
mils is determined by multiplying the area in circular
mils of one strand by the number of strands in the
cable.
FM 55-509-1
Circular Mil-Foot
A circular mil-foot is a unit of volume
(Figure 12-3). It is a unit conductor 1 foot in length
with across-sectional area of 1 circular mil. Because
it is considered a unit conductor, the circular mil-foot
is useful in making comparisons between wires that
are made of different metals. For example, a basis of
comparison of the resistivity (to be discussed later)
of various substances may be made by determining
the resistance of a circular mil-foot of each of the
substances.
In working with square or rectangular conductors, such as ammeter shunts and bus bars, it is
sometimes more convenient to use a different unit
volume. Bus bars are to be used when a large current
capacity is required. Accordingly, unit volume may
also be measured as the centimeter cube. Specific
resistance, therefore, becomes the resistance offered
by a cube-shaped conductor 1 centimeter in length
and 1 square centimeter in cross-sectional area. The
unit volume to be used is given in tables of specific
resistances.
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. If
the kind of metal of which a conductor is made is
known, the specific resistance of the metal may be
obtained from one of these tables. These tables also
specific the temperature at which the resistance measurement is made. Table 12-1 gives the specific
resistance of some 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, the resistance of a conductor may be calculated if the length, cross-sectional
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area, and specific resistance of the substance is
known. Expressed as an equation, the resistance (R)
in ohms of a conductor is —
R=pL
A
Where: p (Greek rho) = the specific resistance in ohms per circular mil-foot (refer to
Table 12-1)
L = the length in feet
A = the cross-sectional area in circular mils
Example: What is the resistance of 1,000 feet
of copper wire having a cross-sectional area of 10,400
circular mils (No. 10 AWG wire) at a temperature of
20C?
Given
p = 10.37 ohms/circular mil-foot
L = 1,000 feet
A = 10,400 circular mils
Solution:
R = p L = 10.37 (1,000) = 1 ohm (approximately)
10,400
A
RELATIONSHIP BETWEEN WIRE SIZES
Wires are manufactured in sizes numbered
according to a table known as the American wire
gauge (AWG). The National Bureau of Standards
publishes tables for various conductors either solid
or stranded and the material they are made from,
such as copper or aluminum. Table 12-2 is one
example of such a table. 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 in
Table 12-2 is 0000, and the smallest is number 22.
Larger and smaller sizes are manufactured but are
not commonly used by the Army. The tables show
the diameter, circular mil area, and area in square
inches of the different AWG wire sizes. It also
shows the resistance per thousand feet of the
various wire sizes at 25C.
12-4
STRANDED WIRES AND CABLES
A wire is a slender rod or filament of drawn
metal. The 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 in comparison with the diameter. If
a wire is covered by insulation, it is called an insulated wire. Although wire properly refers to the
metal, it is generally understood to include the
insulation.
A conductor is a wire suitable for carrying an
electric current. A stranded conductor is 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.
A cable is either a stranded conductor (a single
conductor cable) or a combination of conductors
insulated from one another (multiple conductor
cable). The term “cable” is a general one, and in
practice, it usually applies only to larger sizes of
conductors. A small cable is more often called a
stranded wire. The insulated cables may be sheathed
(covered) with lead or protective armor.
Figure 12-4 shows some of the different types
of wire and cable used in the military.
Conductors are stranded mainly to increase
their flexibility. The wire strands in cables are
arranged in the following order. The first layer of
strands around the center conductor are made of 6
conductors. The second layer is made up of 12 conductors. The third layer is made up of 18 conductors,
and so on. Thus, standard cables are composed of 7,
19, and 37 strands, in continuing fixed increments.
The overall flexibility may be increased by further stranding of the individual strands. All Army
marine electrical wires and cables will be of the
stranded type. The excessive vibration of a vessel
prohibits solid conductor wires.
Figure 12-5 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.
FM 55-509-1
SELECTION OF WIRE SIZE
Several factors must be considered in selecting the size wire to be used for transmitting and
distributing electric power. There are military
specifications that cover the installation of wiring of
ships and electrical/electronic equipment. These
specifications describe the technical requirements
for material which is to be purchased from manufacturers by the Department of Defense. One important reason for these specifications is to reduce the
danger of fires caused by the improper selection of
wire sizes. Wires can carry only a limited amount of
current safely. If the current flowing through a wire
exceeds the current-carrying capacity of the wire,
excess heat is generated. This heat may be great
enough to burn off the insulation around the wire and
continue to do much greater damage by starting a
fire.
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Some of these factors affect the resistance of a
wire carrying current.
An increase in the diameter or cross section of
a conductor decreases its resistance and increases its
capability to carry current. An increase in the
specific resistance of a conductor increases its resistance and decreases its capacity to carry current.
FACTORS AFFECTING THE CURRENT
RATING
The current rating of a cable or wire indicates
the current capacity that the wire or cable can safely
carry continuously. If this limit, or current rating, is
exceeded for a length of time, the heat generated may
bum the insulation. The current rating of a wire is
used to determine what size is needed for a given
electrical load.
The following factors determine the current
rating of a wire:
The conductor size.
The material of which the conductor is
made.
The location of the wire.
The type of insulation used.
Ambient temperature.
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The location of a conductor determines the
temperature under which it operates. A cable may
be located in a row of other cables (banked) or placed
alongside other cables in one of two rows (doublebanked). Therefore, it operates at a higher temperature than if it is open to the free air. The higher the
temperature under which a wire is operating, the
greater its resistance. Its capacity to carry current is
also lowered. In each case, the resistance of a wire
determines its current-carrying capacity. The
greater the resistance, the more power it dissipates
in the form of heat energy. Electrical conductors
may also be installed in locations where the ambient
temperature is relatively high. When this is the case,
the heat generated by external sources constitutes an
appreciable part of the total conductor heating. This
will be explained further under Temperature Coefficient. Due allowances must be made for the influence of external heating on the allowable
conductor current. Each case has its own specific
limitations. Table 12-3 gives the maximum currentcarrying capacity for distribution cable. Table 12-4
shows control cable ampacities. Table 12-5 specifies
the maximum allowable operating temperature of
insulated conductors. It varies with the type of conductor insulation being used.
The insulation of a wire does not affect its
resistance. It does, however, determine how much
heat is needed to burn the insulation. The limit of
current that an insulated conductor can withstand
depends on how hot the conductor can get before it
burns the insulation. Different types of insulation
will burn at different temperatures. Therefore, the
type of insulation used is a factor that determines the
current rating of a conductor. Polyvinyl chloride
(PVC) insulation will begin to deteriorate at relatively
low temperatures. Silicon rubber retains its insulating properties at much higher temperatures.
FM 55-509-1
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Copper has high conductivity. It is more ductile (can be drawn out). It has relatively high tensile
strength (the greatest stress a substance can bear
along its length without tearing apart). It can also
be easily soldered. US Army vessels use only soft
annealed copper wire. The copper conductor is
tinned or alloy-coated to ensure compatibility with
insulation.
TEMPERATURE COEFFICIENT
COPPER VERSUS ALUMINUM CONDUCTORS
Although silver is the best conductor, its cost
limits its use to special circuits. It is used where a
substance with high conductivity or low resistivity is
needed.
12-8
The resistance of pure metals, such as silver,
copper, and aluminum, increases as the temperature
increases. The resistance of some alloys, such as
constantan and manganin, changes very little as the
temperature changes. Measuring instruments use
these alloys because the resistance of the circuits
must remain constant to achieve accurate measurements. The amount of increase in the resistance of a
l-ohm sample of the conductor per degree rise in
temperature above 0C is called the temperature coefficient of resistance. For copper, the value is about
0.00427 ohm. This and more is taken into account
when designing the electrical distribution system of
the vessel. A wire is not just any wire. There is a
reason and a purpose for the entire electrical system.
The only changes in the electrical system should be
for expedient repairs and approved modifications.
Do not modify electrical systems without proper
authority.
FM 55-509-1
CONDUCTOR INSULATION
To be useful and safe, electric current must be
forced to flow only where it is needed. It must be
channeled from the power source to a useful load. In
general, current-carrying conductors must not be
allowed to come in contact with one another, their
supporting hardware, or personnel working near
them. To accomplish this, conductors are coated or
wrapped with various materials. These materials
have such a high resistance that they are, for all
purposes, nonconductors. They are generally
referred to as insulators or insulating material.
Only the necessary minimum of insulation is
applied to any particular type of conductor designed
to do a particular job. This is done because of several
factors. The expense, stiffening effect, and variety of
physical and electrical conditions under which the
conductors are operated must be considered.
Therefore, a wide variety of insulated conductors is
available to meet the requirements of any job.
Two fundamental properties of insulating
materials, such as rubber, glass, asbestos, and plastic, are insulation resistance and dielectric strength.
These are two entirely different and distinct
properties.
Insulation resistance is the resistance to current
leakage through the insulation materials. Insulation resistance can be measured by means of a megger without damaging the insulation. Information
so obtained serves as a useful guide in appraising
the general condition of the insulation. Clean, dry
insulation having cracks or other faults may show a
high resistance but would not be suitable for use.
Megger testing does not damage the cable. This is
one form of nondestructive testing.
Dielectric strength is the ability of the insulation to withstand potential difference. It is usually
expressed in terms of the voltage at which the insulation fails because of the electrostatic stress. Maximum dielectric strength values can be measured only
by raising the voltage of a test sample until the insulation breaks down. When the dielectric strength is
tested, the cable insulation is damaged. This is an
example of destructive testing.
Figure 12-6 shows two types of insulated wire.
One is a single, solid conductor. The other is a
two-conductor cable with each stranded conductor
covered with a rubber-type insulation. In each case,
the rubber serves the same purpose: to confide the
current to its conductor.
Materials
Marine cable insulation should be one of the
following materials:
Polyvinyl chloride (designated T). This is
the most common type of insulation currently used on modern vessels. It is a form
of polymerized vinyl compound, resin, or
plastic. The maximum conductor
temperature that the insulation can handle
is 75C. The voltage range is a maximum
of 600 volts. The maximum allowable
ambient temperature is 50C. It is of thermoplastic construction. This means it
becomes soft when heated and rigid when
cooled and cured. Polyvinyl chlorideprotected cable provides a nonmetallic
rigid sheathed cable. It is commonly called
PVC.
WARNING
This product produces extremely
toxic vapors when ignited. When
selecting this type of cable, a designation of “LS” (low smoke) indicates insulation modifications
have been made to reduce these
toxic gases.
Ethylene propylene rubber (designated
E). This insulation is thermosetting (not
reshapeable). The maximum conductor
temperature that the insulation can handle
is 90C. The maximum allowable ambient
temperature is 60C. It is normally used for
up to 2,000 volts. For special applications,
a maximum of 5,000 volts may be used.
Cross-linked polyethylene (designated X).
This insulation is thermosetting. The maximum conductor temperature it can handle is 90C. The maximum allowable
ambient temperature is 60C. It is used
from 2,001 to 5,000 volts.
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Mineral (MI) (designated M). This is a
refactory material made of magnesium
oxide that is highly compressed to provide
the properties needed for insulation. The
maximum conductor temperature that the
insulation can handle is 85C. The ambient
temperature is specified by distinct design
only. Some special applications allow a
maximum conductor temperature of 250C.
Care must be taken when considering
cable end fittings. See IEEE Standard 45,
Table A-21, for ampacity.
Silicon rubber (designated S). The maximum conductor temperature that the insulation can handle is 100C. The
maximum allowable ambient temperature
is 70C. It may be used in specific applications up to 5,000 volts.
Impregnated glass, varnished cloth (designated GTV). The outside covering consists of a composite wall of glass or
varnished cloth layers. Figure 12-7 shows
the insulation helically wound around the
cable. The maximum conductor temperature that the insulation can handle is 100C.
The maximum allowable ambient
temperature is 70C.
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Moisture-Resistant Jackets
An additional cable identification designation
of I will be displayed on all cables with a moistureresistant jacket. The jacket will be composed of one
of the following:
Thermoplastic type T.
Thermoplastic type T covered with a nylon
coating, which changes the designator to
type N.
Thermosetting chlorosulfonated
polyethylene (type CP).
Separators and Fillers
Separators may be provided inside the insulation to allow free stripping of cable conductors.
Fillers eliminate air spaces in the cable (Figure 12-8).
Marine cables will not permit the passage of water
along the inside of a cable, nor will they support
conductor oxidation.
Additional insulating coding and specifications may be found in the Recommended Practice
for Electrical Installations on Shipboard, the
FM 55-509-1
Institute of Electrical and Electronics Engineers,
Inc. (IEEE Standard 45).
Enamel Coating
The wire used on the coils of meters, relays,
small transformers, motor windings, and so forth is
called magnetic wire. It is insulated with an enamel
coating. The enamel is a synthetic compound of
cellulose acetate (wood pulp and magnesium). In the
manufacturing process, the bare wire is passed
through a solution of hot enamel and then cooled.
This process is repeated until the wire acquires from
6 to 10 coatings. Enamel has a higher dielectric
strength than rubber, thickness for thickness. It is not
practical for large wires because of the expense and
because the insulation is readily fractured when large
wires are bent. Do not handle any enamel-covered
conductors in a rough manner. Never set a disassembled component down on its enamel-coated
wires.
Figure 12-9 shows an enamel-coated wire.
Enamel is the thinnest insulating coating that can be
applied to wires. Hence, enamel-insulated magnetic
wire makes smaller coils. Enameled wire is sometimes covered with one or more layers of cotton to
protect the enamel from nicks, cuts, or abrasions.
CONDUCTOR PROTECTION
Wires and cables are generally subject to
abuse. The type and amount of abuse depends on
how and where they are installed and the manner in
which they are used. Generally, except for overhead
transmission lines, wires or cables are protected by
some form of covering. The covering may be some
type of insulator like rubber or plastic. Over this, an
outer covering of fibrous braid may be applied. If
conditions require, a metallic outer covering may be
used. The type of outer covering used depends on
how and where the wire or cable is to be used.
Metallic armor provides a tough protective
covering for wires or cables. The type, thickness, and
kind of metal used to make armor depends on three
factors:
The use of the conductors.
The circumstances under which the conductors are to be used.
The amount of rough treatment that is
expected.
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For additional information and specifications, refer
to IEEE Standard 45-1983, Section 20.2, and Code
of Federal Regulations, Title 46, Subpart 111.05-7.
Cables should not be painted. Only when
cables carry a potential of 5,000 volts or greater is
yellow color-coding permissible.
For general use, polyvinyl chloride-protected
cable is replacing armor cable.
WIRING TECHNIQUES
Figure 12-10 shows an armored cable. Basketweave wire-braid armor is used wherever a light and
flexible protection is needed. In the past, this type of
armor covering has been used almost exclusively onboard ships. Wire braid is still used for special purposes in the engineering spaces. The individual wires
that are woven together to form the braid are made
out of aluminum or bronze. Besides mechanical
protection, the wire braid also provides a static
shield. This is important in radio work aboard ship
to prevent interference from stray magnetic fields.
Wire connections should be made inside the
electrical component or inside watertight feeder,
branch, or connection boxes. These boxes are
generally brass or bronze. Watertight integrity is
maintained by using stuffing tubes and gaskets. All
the wire ends should be provided with lugs for connecting to bus terminals or for bolting and insulating
individual wires together. During the course of normal electrical servicing, splicing wires is not
authorized.
Electrical cables must be continuous between
the terminals except as outlined below:
Component subassemblies may be spliced
together. Splices may not be made to the
subassembly power supply cables or
branch circuits.
Cables may be spliced to extend a circuit when a vessel is receiving
authorized alterations.
An extremely long cable may be spliced to
allow its proper and efficient installation as
explained above.
CAUTION
The armor braid must be
grounded directly or indirectly to
the hull
In this situation, grounding does not mean that
current will be carried through the armor braid under
normal conditions. Rather, it means that an electrical path will be provided to the hull should an abnormal electrical fault cause current to flow in the armor.
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Splicing is authorized for repair of
damaged cables if the remainder of the
cable is in good mechanical and electrical
condition. The cable must be replaced in
its entirety at the most opportune time.
When electrical casualty requires expedient
repairs, it is absolutely necessary that the repairs be
made properly. A poor repair can prevent the operation of emergency equipment or develop into a tire.
Any electric circuit is only as good as its weakest link.
The basic requirement of any splice or connection is
that it is both mechanically and electrically sound.
FM 55-509-1
Quality workmanship and materials must be used to
ensure lasting electrical contact, physical strength,
and proper insulation. The most common methods
of making splices and connections in electrical cables
are explained below.
Splicing
Splices should be located in an area that is
easily accessible and inspect able. The splice should
consist of the following components:
A conductor connector (terminal lugs,
splice bolts, or splices) (Figure 12-11).
A replacement jacket for the insulation.
A shunt or suitable conductor to maintain
the electrical continuity between two
severed pieces of the armor braid.
WARNING
Continuity must be maintained between the armor covering and the
vessel’s hull at all times.
Removing Insulation
The preferred method of removing insulation
is with the use of a wire-stripping tool. The calibrated
hand wire stripper in Figure 12-12 is excellent for
even the most intricate electrical wire work.
Hand Wire Stripper. The procedure for stripping wire with the hand wire stripper is as follows:
Confirm the stripper’s operation. Use a
spare wire to ensure the conductor is not
marred by the cutting blades of the wire
stripper. If the wire strippers have been
used or abused, consult the manufacturer’s
instructions to adjust the depth of the insulation cut.
Insert the wire into the exact center of the
correct cutting slot for the wire size to be
stripped (Figure 12-13). Keep the wire
perpendicular to the stripper.
Close the handles together until the wire is
held firmly by the jaws of the wire stripper.
Continue to close the handles only until the
insulation starts to separate. Do not use
the stripper to pull the insulation from the
conductor.
Remove the wire from the wire stripper.
Gently unwind the insulation so that the
natural lay of the conductors is not
disturbed.
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emergencies, the following procedure will keep conductor damage to a minimum.
A sharp knife can be used to strip the insulation
from a conductor. The procedure is much the same
as sharpening a pencil. The knife should be held at
a 60-degree angle to the conductor. Use extreme
care to avoid cutting into the conductor. This procedure produces a taper on the cut insulation
(Figure 12-14). Should the connection require
solder, the tapered insulation will fuse more
readily to the conductor. This fusion increases
mechanical strength at the weak point and
prevents the entrance of moisture.
WIRE STRIPPING CAUTIONS
The following minimum precautions are necessary when preparing conductors for repair:
Ensure power is off before connecting or
removing wires.
NOTE: If you are going to solder the
conductor, do not touch the conductor
with your hands. The contamination
from your hands will start to oxidize
the conductor, and proper tinning and
soldering will become more difficult.
Knife Stripping. This is not recommended because it cuts into the conductor and effectively
reduces the circular mil area at the fault. Nicks and
cuts also reduce the mechanical strength of the conductor at a point that is naturally weaker then the
section of cable where it is protected by insulation.
However, the very nature of a vessel requires it
to be placed in situations that must be satisfactorily
managed until a more advantageous time arrives. In
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Do not touch any conductor you intend to
solder.
Ensure the wire strippers are held perpendicular to the wire.
Make sure the insulation is clean-cut with
no frayed or ragged edges. Trim if necessary. This is particularly important when
stripping armored cable. If the frayed
armor insulation is allowed to chafe the
replacement insulation at the spliced area,
current may accidentally energize the exterior armor of the cable. This can be
deadly.
Ensure all insulation is removed from the
conductor. Some wires are provided with
a transparent layer between the conductor
and the insulation. This must be removed.
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Figure 12-16 shows how a two-conductor cable
is joined to a similar size cable by means of staggering
the splices. Take care to ensure that a short wire
from one side of the cable is connected to a long wire
from the other side of the cable being spliced. Then
clamp the sharp ends firmly down on the conductor.
Figure 12-16 shows a Western Union splice being
staggered. Each conductor is insulated separately.
WESTERN UNION SPLICE
Figure 12-15 shows the steps to make a Western
Union splice. First, prepare the wires for splicing.
Remove enough insulation to make the splice and
then clean the conductor. Next, bring the wires to a
crossed position and make a long twist or bend in
each wire. Then wrap one of the wire ends four or
five times around the straight portion of the wire.
Wrap the other end of the wire in a similar manner.
Finally, press the ends of the wire down as close as
possible to the straight portion of the wire. This
prevents the sharp ends from puncturing the tape
covering that is wrapped over the splice.
SPLICE INSULATION
The splices discussed above are those usually
insulated with tape. The tape used to insulate a splice
should be centered over the splice and should overlap the existing insulation by at least 2 inches on each
side. The characteristics of rubber, friction, and
plastic electrical tape are described below.
Rubber Tape
Latex (rubber) tape is a splicing compound. It
is used where the original insulation was of a rubber
compound. The tape is applied to the splice with a
light tension so that each layer presses tightly against
the one beneath it. This pressure causes the rubber
tape to blend into a solid mass. Care must be taken
to keep the spliced area watertight. Upon completion, insulation similar to the original is restored.
WARNING
STAGGERED SPLICES
Joining small, multiconductor cables together
presents somewhat of a problem. Each conductor
must be spliced and taped. If the splices are contiguous to each other, the size of the joint becomes
large and bulky. A smoother and less bulky joint may
be made by staggering the splices.
Some rubber tapes are made for
special applications. These types
are semiconducting and will pass
current that presents a shock
hazard. These types of tape are
packaged similar to the latex rubber tape. Take care to insulate
splices only with latex rubber insulation tape.
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FM 55-509-1
In roll form, a layer of paper or treated cloth is
between each layer of rubber tape. This layer
prevents the latex from fusing while still on the roll.
The paper or cloth is peeled off and discarded before
the tape is applied to the splice.
furnished by friction tape. Plastic electrical tape
usually has a certain amount of stretch so that it easily
conforms to the contour of the splice.
Apply the rubber splicing tape smoothly and
under tension so that no air exists between the layers.
Start the first layer near the middle of the joint
instead of the end. The diameter of the completed
insulated joint should be somewhat greater than
the overall diameter of the original wire, including
the insulation.
Since marine cables are stranded, it is necessary to use terminal lugs to hold the stranded wires
together to help fasten the wires to terminal studs
(Figure 12-17). This is the preferred method for
connecting wires to terminals or to other wire ends.
Generally, distribution system cable connectors will
not use solder. The terminals used in electrical
wiring are either of the soldered or crimped type.
Terminals used in repair work must be of the size and
type specified on the electrical wiring diagram for the
particular equipment.
Friction Tape
Putting rubber over the splice means that the
insulation has been restored to a great degree. It is
also necessary to restore the protective covering.
Friction tape is used for this purpose.
WARNING
Some friction tapes may conduct
electrical current.
Friction tape is a cotton cloth that has been
treated with a sticky rubber compound. It comes in
rolls similar to rubber tape, except that no paper or
cloth separator is used. Friction tape is applied to
rubber; however, it does not stretch.
Start the friction tape slightly back on the
original insulation. Wind the tape so each turn overlaps the one before it. Extend the tape over onto the
insulation at the other end of the splice. From this
point, wind a second layer back along the splice until
the original starting point is reached. To complete
the job, cut the tape and firmly press down the end.
Plastic Electrical Tape
Plastic electric tape has come into wide use in
recent years. It has certain advantages over rubber
and friction tape. For example, it withstands higher
voltages for a given thickness. Single layers of certain
plastic tapes will withstand several thousand volts
without breaking down. In practice, however, several
layers of tape are used to equal or slightly exceed the
original thickness of the insulation. Additional layers
of plastic electrical tape add the protection normally
12-16
TERMINAL LUGS
FM 55-509-1
The increased use of crimp on terminals is a
result of the limitations of soldered terminals. The
quality of soldered connections depends mostly on
the operator’s skill. Other factors, such as temperature, flux, cleanliness, oxides, and insulation damage
due to heat, also add to defective connections.
An advantage of crimp on solderless terminal
lugs is that they require relatively little operator skill
to use. Another advantage is that the only tool
needed is the crimping tool. This allows terminal
lugs to be applied with a minimum of time and effort.
The connections are made more rapidly and are
cleaner and more uniform in construction. Because
of the pressures exerted and the material used, the
crimped connection or splice, properly made, is both
mechanically and electrically sound. Figure 12-17
shows some of the basic types of terminals. There are
several variations of these basic types, such as the use
of a slot instead of a terminal hole, three- and fourway splice type connectors, and insulation covering.
Figure 12-18 shows how to determine the
amount of insulation to remove from the wire.
Solderless terminals may be of the insulated
type. The barrel of the terminal or splice is enclosed
in an insulating material. The insulation is compressed along with the terminal barrel when it is
crimped, but it is not damaged in the process
(Figure 12-19).
Aluminum Terminals and Splices
Do not use aluminum terminals, connectors, or
wires interchangeably with copper wires and connectors. Copper and aluminum expand at different rates
and will become loose over a period of time.
Electrolysis also takes place. The two dissimilar metals and the salt air will create a chemical reaction
that will eat away the materials. Also, never use an
aluminum crimping tool for compressing copper
hardware.
Preinsulated Terminal Lugs
The use of preinsulated terminal lugs and
splices has become the most common method for
copper wire termination and splicing in recent years.
It is by far the best and easiest method. Many tools
are used for crimping terminal lugs and splices.
Small diameter copper wires are terminated
with solderless, preinsulated copper terminal lugs.
As Figure 12-20 shows, the insulation is part of the
terminal lug. It extends beyond the barrel so that it
covers a portion of the wire insulation. This makes
the use of spaghetti or heat shrink tubing unnecessary. Preinsulated terminal lugs also have an insulation support (a metal reinforced sleeve) beneath the
insulation for extra supporting strength of the wire
insulation. Some preinsulated terminals fit more
than one size of wire. The insulation is color-coded,
and the range of wire sizes is marked on the tongue.
This identifies the wire sizes that can be terminated
with each of the terminal lug sizes (Table 12-6).
For crimping small copper terminal lugs, the
MS90413 hand crimping tool is used for wire sizes
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FM 55-509-1
AWG 26 to 14. The MS3316 tool is used for wire sizes
12 and 10. Figure 12-21 shows these tools. These
hand crimping tools have a self-locking ratchet that
prevents the tool from opening until the crimp is
completed. These and other one-cycle compression
tools (as outlined in ANSI/UL 4S6-1975[25]) are the
preferred method of compression.
After completing the compression, visually inspect the terminal or splice. Check for the following
conditions:
Soldering Tools
Many types of soldering tools are in use today.
Some of the more common types are the soldering
iron, soldering gun, resistance soldering set, and pencil iron. The main concern when selecting a soldering tool is the selection of the wattage. Table 12-7
provides a guide for determining the correct wattage
for the size wire.
Indent centered on the terminal barrel.
Indent in line with the barrel.
Terminal lug not cracked.
Terminal lug insulation not cracked.
Insulation grip crimped.
SOLDERING
The following discussion on basic soldering
skills provides information needed when soldering
wires to electrical connectors, splices, and terminals.
12-18
Soldering Iron. Figure 12-22 shows some types
of common soldering irons. All high-quality soldering irons operate in the temperature range of 500 to
600F. Even the little 25-watt midget irons produce
this temperature. The important difference in iron
sizes is not the temperature, but the wattage. The
wattage, or thermal inertia, is the capacity of the iron
to generate and maintain a satisfactory temperature
while giving up heat to the joint to be soldered.
Although it is not practical to solder large conductors
with a 25-watt iron, this iron is suitable for replacing
a half-watt resistor in an electronic circuit or soldering a miniature connector. One advantage of using a
small iron for small work is that it is light and easy to
handle and has a small tip which is easily used in close
places. Even though its temperature is high enough,
it does not have the thermal energy to solder a large
conductor.
FM 55-509-1
A well-designed iron is self-regulating. The
resistance of its element increases with rising
temperature, thus limiting the flow of current. Figure
12-23 shows some tip shapes of the soldering irons in
common use in the Army.
An iron is always tinned prior to soldering a
component in a circuit. After extended use, the tip
tends to become pitted due to oxidation. Pitting
indicates the need for retinning, as shown in Figure
12-24. Melt a piece of clean solder dipped in rosin
flux over the soldering iron tip until all cavities are
gone and the tip is completely shiny and silver-coated
Figure 12-25). Use a lint-free paper towel to wipe
the solder away. Do not shake the solder off.
The larger soldering irons used exclusively for
large conductors may require the tip to be filed first.
The tip must then be tinned.
Never clean the tip of an iron by dipping it into
the flux container. All this does is contaminate the
flux and add impurities to the next soldering project.
Soldering Gun. The soldering gun (Figure
12-26) has gained popularity in recent years
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FM 55-509-1
because it heats and cools rapidly. It is especially
well-adapted to maintenance and troubleshooting
work where only a small part of the technician’s time
is spent soldering.
damage the soldering gun. The gun is operated by a
finger switch. The gun heats only while the switch is
depressed. For most jobs, depress the trigger for no
more than 10 seconds. Regulate the tip temperature
by pulsating the gun on and off with the trigger.
A transformer in the soldering gun supplies
about a volt at high current to a loop of copper, which
acts as the soldering tip. It heats to soldering
temperature in 3 to 5 seconds. However, it may
overheat to the point of incandescence if left on more
than 30 seconds. This should be avoided because
excess heat will burn the insulation of the wiring and
The gun or iron should always be kept tinned
to permit proper heat transfer to the connection to
be soldered. Tinning also provides adequate control of the heat to prevent solder from building up
on the tip. This reduces the chance of the solder
spilling over to nearby components and causing
short circuits.
12-20
FM 55-509-1
or acid flux cleans and prevents oxidation of the
materials to be soldered. Never use acid core solder
in soldering electronic components. Acid core flux
causes corrosion and leads to shorts or open conditions. Avoid acid core solder whenever possible
when soldering electrical components. Rosin is the
only acceptable electrical soldering flux.
Soldering Process
When the soldering gun or iron is used, heating
and cooling cycles tend to loosen the nuts or screws
that hold the replaceable tips. When the nut on the
gun becomes loose, the resistance of the tip increases. The temperature of the connection is increased and reduces the heat at the tip. Continued
loosening may even cause an open circuit. Therefore, the tip should be tightened before and during
operations as needed.
CAUTION
Never use soldering guns to solder
solid state electronic components,
such as resistors, capacitors,
and transistors, because the
heat generated can destroy the
components.
Cleanliness is necessary for efficient, effective
soldering. Solder will not adhere to dirty, greasy, or
oxidized surfaces. Heated metals tend to oxidize
rapidly, so the oxide must be removed before soldering. Remove oxides, scale, and dirt by mechanical
means, such as scraping and cutting with an abrasive
cloth, or by chemical means. Remove grease or oil
films with a suitable solvent (alcohol). Clean the
connections to be soldered just before the actual
soldering operation.
Items to be soldered should normally be tinned
before making a mechanical connection. Tinning is
coating the material to be soldered with a light coat
of solder (Figure 12-27). When the surface has been
properly cleaned, place a thin, even coating of flux
over the surface to be tinned. This will prevent oxidation while the part is being heated to soldering
temperature. Rosin core solder is usually preferred
in electrical work. However, a separate rosin flux
may be used instead. Separate rosin flux is often used
when wires in cable fabrication are tinned.
Solder
Ordinary soft solder is a fusible alloy consisting
chiefly of tin and lead. It is used to join together two
or more metals at temperatures below their melting
point, A good general solder for electrical work is
60/40 solder; 60/40 represents the tin-to-lead ratio
(percentage) of the solder. Eutectic solder (63/37) is
an ideal solder combination. Eutectic solder goes
from a solid to a liquid state without entering a mushy
condition. The eutectic solder is used in electrical
and electronic soldering processes.
Solder comes on rolls. Many times the solder
is hollow and contains a rosin or acid core. The rosin
Placing a very thin coating of solder on the
copper conductor is called tinning the wire. This is
done before soldering the conductor to a terminal or
other component. First, strip the insulation from the
wire. Do not touch the copper conductor with your
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FM 55-509-1
hands or any other oily objects. Apply heat from the
soldering iron under the copper conductor. Then
apply the solder to the top of the copper conductor.
Starting at either the insulation end of the conductor or at the bitter end of the conductor, move the
soldering iron as the solder melts and coat the entire
length of the conductor. There is to be a complete,
yet very fine solder coating. Every layer of the conductor should be easily defined underneath the
bright solder coating.
If the tinned lead is to be connected to a shaped
device, such as a turret or post, then form the tinned
portion to exactly match the shape. Ensure no open
space is between the tinned wire and the point of
connection. Figure 12-28 shows the exact relationship the stripped conductor maintains with the terminal post. The insulation is stripped back far
enough to be one conductor diameter from the post.
The bitter end of the conductor never goes farther
around the post terminal than its widest point. This
is the only way to ensure the best current flow.
In earlier years, tinning was not extended to the
insulation because the wire was thought to provide
better performance when it was allowed to flex at
the insulation. However, actual performance is
improved when this weakened portion is not allowed
to flex. Proper strength is maintained when there
is a slight fusing between the soldier and the insulation. This gives more protection from vibration
and maintains the conductor more steadily in application. Tinning to the insulation eliminates the
12-22
small movement that causes the conductor to break
near the insulation.
Do not tin wires that are to be crimped to
solderless terminals or splices.
Once the conductor has been tinned, start to
prepare the terminal for soldering. Clean all oils and
foreign material from the terminal. Remove all
remaining solder and any leftover broken conductors. Use a soldering wick to remove old solder
expeditiously. Place the wick on the old solder and
the soldering tool on top of the wick (Figure 12-29).
Capillary action draws the old solder off the terminals and into the wick. Clean the area with denatured alcohol and a white pencil-type typist eraser.
Connect the tinned conductor to the terminal
without placing your hands on either prepared surface. Apply heat and solder. Never apply the solder
to the tip of the soldering iron. Always apply solder
to the opposite side of the component lead. There
should be just enough solder to penetrate and surround the terminal and conductor connection.
There should not be so much solder that there is a
bulge at the connection.
If the conductor or terminal is moved while the
solder is solidifying, a cold solder joint will result.
This poor joint has a dull, grainy appearance. If there
is any bridging, dimples, or holes, then the joint has
been improperly made and must be remade. A
properly soldered joint will be bright and shiny.