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.
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 cross-sectional area is 6 mils and the other side is 3 mils, multiply 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.
Circular Mil
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 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 o a circle (area [A] = pi x radius squared = pi x r2 . 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 the diameter. Through substitution A = 3.1416 (12.5)2 . 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, by definition he 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, the formula A = pir2 is applied. Therefore, A = 3.1416 x (.5)2 . 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?
Solution:
a. A = D2 = (80.81)2 = 6,530 circular mils.
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 cross-sectional 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.
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.
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.
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?
p = 10.37 ohms/circular mil-foot
L = 1,000 feet
A = 10,400 circular mils
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.
Figure 12-4 shows some of the different types of wire and cable used in the military.
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.
The following factors determine the current rating of a wire:
Some of these factors affect the resistance of a wire carrying current.
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 (double-banked). 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 current-carrying 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 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.
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.
Marine cable insulation should be one of the following materials:
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 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.
Figure 12-10 shows an armored cable. Basket-weave 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.
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. 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.
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:
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.
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.
Splices should be located in an area that is easily accessible and inspect able. The splice should consist of the following components:
Continuity must be maintained between the armor covering and the vessel's hull at all times.
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:
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.
The following minimum precautions are necessary when preparing conductors for repair:
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.
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.
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.
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.
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.
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.
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.
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.
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 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.
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.
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 four-way 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
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 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 486-1975[25]) are the preferred method of compression.
The following discussion on basic soldering skills provides information needed when soldering wires to electrical connectors, splices, and terminals.
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.
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.
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.
Soldering Gun. The soldering gun (Figure 12-26) has gained popularity in recent years 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.
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.
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.
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.
