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Tools and Equipment

The electrical apparatus and materials that an electrician is required to install and maintain are different from other building materials. Their installation and maintenance require the use of special hand-tools.

Section I. Interior Wiring

This section describes the tools normally used by an Army electrician in interior wiring. Appendix A shows some of these tools. For additional information on proper tool usage, see TM 9-243.


Pliers have either insulated or uninsulated handles. Long-nosed pliers are used for close work in panels or boxes. Side-cutter pliers are used to cut wire and cable to size. Slip-joint pliers are used to tighten locknuts, small nuts on devices, and conduit bushings and fittings. Round-nosed pliers are used for making screw loops and working in limited-space areas.


The fuse puller shown in Figure A-7(1) is designed to eliminate the danger of pulling and replacing cartridge fuses by hand. It is also used for bending fuse clips, adjusting loose cutout clips, and handling live electrical parts.

The second type of fuse puller (Figure A-7[2]) has the same general configuration but is made of molded plastic. Encased in the handle is an electrical circuit that is similar to a voltmeter, except that the indicating device is a neon-glow tube. Test probes are attached to the handle and may be used to determine if voltage is present in a circuit.


Screwdrivers come in many sizes and tip shapes. Those used by electricians should have insulated handles. Electricians generally use screwdrivers to attach electrical devices to boxes and attach wires to terminals. One variation of the screwdriver is the screwdriver bit, which is held in a brace and used for heavy-duty work. For safe and efficient operation, select a screwdriver that matches the screw slot and keep the tips square and properly tapered.


Adjustable open-end wrenches (commonly called crescent wrenches) and open-end, closed-end, and socket wrenches are used on hexagonal and square fittings such as machine bolts, hexagon nuts, or conduit unions. Pipe wrenches are used for pipe and conduit work and should not be used where crescent, open-end, closed-end, or socket wrenches can be used. Pipe-wrench construction will not permit application of heavy pressure on square or hexagonal material, and continued misuse of a pipe wrench will deform the teeth on the jaw faces and mar the surface of the material.


Electricians use a standard soldering kit consisting of--

  • Soldering irons (electric/nonelectric).
  • A blowtorch (for heating a nonelectric soldering wire and pipe or wire joints). An alcohol or propane torch can be used in place of a blowtorch.
  • A spool or solid tin-lead wire solder or flux-core solder. Acid-core solder should never be used in electrical wiring.
  • Soldering paste.


Drilling equipment is required to drill holes in building structures for the passage of conduit or wire in new or modified construction. High-speed drills are used to drill holes in sheet-metal cabinets and boxes. Carbide drills are used for tile or concrete work. Electric power drills aid in this phase of an electrician's work.

Standard drilling equipment consists of--

  • A brace.
  • A joist-drilling fixture.
  • An extension bit for drilling into and through deep cavities.
  • An adjustable wood bit.
  • A standard wood bit.
  • A ship auger, which is used with an electric drill.


Electricians use wood chisels and cross-cut or keyhole saws to remove wooden structural members that are obstructing a wire or conduit run and to notch studs and joists for conduit, cable, or box-mounting brackets. They are also used to construct wood-panel mounting brackets. The keyhole saw may be used to cut openings in walls of existing buildings where boxes need to be added.


Electricians use cold chisels and center punches, as well as several other types of metalworking tools, when working on steel panels. The knockout punch is used to make or enlarge a hole in a steel cabinet or outlet box. The hacksaw is usually used for cutting conduit, cable, or wire that is too large for wire cutters. A light, steady stroke of about 40 to 50 times a minute is best. Always insert a new blade with the teeth pointing away from the handle and tighten the tension wing nut until the blade is rigid. Hacksaw blades have 14, 18, 24, or 32 teeth per inch. The best blade for general use is one having 18 teeth per inch. A blade with 32 teeth per inch is best for cutting thin material. The mill file is used to file the sharp ends of cutoffs as a precaution against short circuits.


An electrician should have several sizes of masonry drills in his tool kit. These drills, which are normally carbide-tipped, are used for drilling holes in brick or concrete walls to anchor apparatus with expansion screws or allow the passage of conduit or cable.


Rigid conduit is normally threaded for installation. The tapered pipe reamer is used to ream the inside edge of the conduit as a precaution against wire damage. The thin-wall conduit cutter has a tapered-blade attachment for reaming the conduit ends.


Knives and patented wire strippers are used to bare the wire of insulation before making connections. Scissors are used to cut the insulation and the tape. The multipurpose tool is designed to cut and skin wires, attach terminals, gauge wire, and cut small bolts. The cable cutter may be used instead of a hacksaw to remove the armor from electrical conductors at the box entry or when cutting cable to length.


Hammers are used with other tools, such as chisels, or for nailing equipment to building supports. Electricians can use a carpenter's claw hammer and a machinist's ball-peen hammer advantageously.


Various types of tape are used to replace insulation and wire coverings.


Friction tape is made of cotton and impregnated with an insulating adhesive compound. It provides weather resistance and limited mechanical protection to a splice that is already insulated.


Rubber or varnished cambric tape may be used as an insulator when replacing wire covering.


Plastic electrical tape has adhesive on one face. It has replaced friction and rubber tape in the field for 120- to 600-volt circuits. Because it serves a dual purpose in taping joints, it is preferred over the former methods.


Fish tape is used primarily to pull wire through conduit. Many pulls are quite difficult and require a fish-tape grip or pull to obtain adequate force on the wire. Fish tape is made of tempered spring steel, is about 1/4-inch wide, and is available in different lengths to suit requirements. It is stiff enough to preclude bending under normal operation but can easily be pushed or pulled around bends or conduit elbows.


When pulling wire and cable in existing buildings, an electrician will normally employ a fish wire or a drop chain between studs. A drop chain consists of small chain links attached to a lead or iron weight. It is used only to feed through wall openings in a vertical plane.


Each electrician should keep a folding rule and a steel tape on hand so he can cut conduit to the proper size and determine the quantity of material required for each job.


A wire grip (Figure A-8) is an invaluable aid for pulling wire through conduit and for pulling open-wire installations tight. It has been designed so that the harder the pull on the wire, the tighter the wire will be gripped. A splicing clamp (Figure A-8) is used to twist the wire pairs into a uniform, tight joint when making splices.


An extension light normally includes a long extension cord and is used by electricians when normal building lighting has not been installed or is not functioning.


When an electrician uses indenter-type couplings and connectors with thin-wall conduit, an impinger (Figure A-9) must be used to permanently attach the fitting to the conduit. An impinger forms indentations in the fitting, pressing it into the outside wall of the conduit. The use of slip-on fittings and an impinger reduces the installation time required and thus reduces the cost of thin-wall conduit installations.


Tape with identifying numbers of nomenclature is available to permanently identify wires and equipment. The markers are particularly valuable to identify wires in complicated wiring circuits, fuse circuit-breaker panels, or junction boxes.



An indicating voltmeter or a test lamp is useful when determining the system voltage, locating the ground lead, and testing the circuit continuity through the power source. They both have a light that glows when voltage is present.


A tester is used to test 120- to 600-volt AC circuits, as well as the polarity of DC circuits.


A modern method of measuring current flow in a circuit uses the clamp-on tester of a multimeter (Figure A-10), which does not need to be hooked into the circuit. When measuring AC amperage, electricians clamp only one wire at a time. The multimeter is capable of measuring voltage, current, resistance, and continuity.

The basic unit of measurement for electric power is the watt. In the power ratings of electric devices used by domestic consumers of electricity, the term watt signifies that the apparatus will use electricity at the specified rate when energized at the normal line voltage. In AC circuits, power is the product of three quantities: the potential (volt), the current (amperage), and the power factor (percent).

Power is measured by a multimeter (Figure 2-1). This instrument is connected so that the current in the measured circuit flows through the stationary field coils in the multimeter and the voltage across the measured circuit is impressed upon the multimeter-armature circuit, which includes movablecoils and a fixed resistor. The power factor is automatically included in the measurement because the torque developed in the multimeter is always proportional to the product of the instantaneous value of current and voltage. Consequently, the instrument gives a true indication of the power, or rate, at which energy is being used.

Section II. Wiring Materials

Many different wiring systems currently in use vary in complexity from the simple-to-install open wiring to the more complex conduit systems. These systems contain common components.



A single conductor is an individual wire that is usually sheathed with insulating material, but a ground wire may be bare. American Wire Gauge (AWG) numbers assigned to electrical wires indicate the diameter of the metal conductor only; they do not include the insulation. Conductors are shielded from one another by material that does not carry current, color-coded thermoplastic. White or gray insulation indicates neutral wires, green indicates ground wires, and all other colors are used to identify hot wires.

Although copper is the best and most commonly used metal for conductors, aluminum and copper-clad aluminum are also used. Because aluminum is not as efficient a conductor as copper, aluminum or copper-clad aluminum wire must be larger than copper wire to conduct the same amount of electricity. To assure a good connection when using No 6 or larger aluminum conductors, electricians smear an oxide inhibitor on the end of the conductor first, then tighten the terminal. They go back the next day and tighten the terminal once again. Electrical codes take the guesswork out of conductor selection by prescribing wire use.

Wires or conductors are initially classified by the type of insulation applied and the wire gauge. The various types of insulation, in turn, are subdivided according to their maximum operating temperatures and the nature of use. Table B-2 lists the common trade classification of wires and compares them as to type, temperature rating, and recommended use.


Wire sizes are denoted by the use of the AWG standards. The largest gauge size is No 4/0. Wires larger than this are classified in size by their circular mil cross-sectional area. One circular mil is the area of a circle with a diameter of 1/1,000 inch. Thus, if a wire has a diameter of 0.1 inch or 100 mil, the cross-sectional area is 100 by 100, or 10,000 circular mils. The most common wire sizes used in interior wiring are 14, 12, and 10; and they are usually of solid construction. Table B-23 lists some characteristics of specific wire sizes.

  • The size of the wire decreases as the numbers become larger.
  • The sizes normally used have even numbers, such as 14, 12, and 10.
  • No 8 and 6 wires, which are furnished either solid or stranded, are normally used for heavy-power circuits or as service-entrance leads to buildings. Wire sizes larger than these are used for extremely heavy loads and for pole-line distribution. Tables B-3 through B-6 show the allowable current-carrying capacity for copper and aluminum conductors. Table B-7 shows the percent of reduction in current capacity if more than three conductors are in a cable or a raceway.


In many types of electrical wiring installation, the use of individual conductors spaced and supported side by side becomes an inefficient and hazardous practice. Multiconductor cables have been designed and manufactured for such installation. Multiconductor cables consist of the individual conductors as outlined above, arranged in groups of two or more. An additional insulating or protective shield is formed or wound around the group of conductors. The individual conductors are color-coded for proper identification. The description and use of each type are given below.

Armored Cable

Armored (Type BX) cable can be supplied either in two- or three-wire types and with or without a lead sheath. The wires in armored cable, matched with a bare bonding wire, are initially twisted together. This grouping, totaling two or three wires with the bonding wire, is then wrapped in coated paper and a formed self-locking steel armor. The cable without a lead sheath is widely used for interior wiring under dry conditions. The lead sheath is required for installation in wet locations and through masonry or concrete building partitions where added protection for the copper conductor wires is required. Metal-clad (Type MC) resembles armored cable and has a ground wire enclosed in the cable. It is used primarily in industrial wiring.

Nonmetallic-Sheathed Cable

Nonmetallic-sheathed cable is manufactured in two types--NM and NMC. Type NM consists of two or three thermoplastic-insulated wires, each covered with a jute-type filler material that acts as a protective insulation against mishandling. The cable is lightweight, simple to install, and comparatively inexpensive. It is used quite extensively in interior wiring but is not approved for use in damp locations.

Type NMC is a dual-purpose, plastic-sheathed cable with solid copper conductors and can be used outdoors or indoors. It needs no conduit, and its flat shape and gray or ivory color make it ideal for surface wiring. It resists moisture, acid, and corrosion and can be run through masonry, between studding, or in damp locations.

Plastic-Sheathed Cable

Type UF (underground feeder) is similar to Type NMC except that it has added water protection and is designed for direct ground contact.

Lead-Covered Cable

Lead-covered cable consists of two or more rubber-covered conductors surrounded by a lead sheathing that has been extruded around it to permit its installation in wet and underground locations. Lead-covered cable can also be immersed in water or installed in areas where the presence of liquid or gaseous vapors would attack the insulation on other types of cable.

Service-Entrance Cable

Service-entrance (Type SEC) cable normally has three wires, with two insulated and braided conductors laid parallel and wound with a bare conductor. Protection against damage for this assembly is obtained by encasing the wires in heavy tape or armor, which serves as an inner cushion, and covering the whole assembly with braid. Though the cable normally serves as a power carrier from the exterior service drop to the service equipment of a building, it may also be used in interior wiring circuits to supply power to electric ranges and water heaters at voltages not exceeding 150 volts to ground if the outer covering is armor. It may also be used as a feeder to other buildings on the same premises and under the same conditions, if the bare conductor is used as an equipment grounding conductor from a main distribution center located near the main service switch.


Many items using electrical power are pendant, portable, or vibration type. In such cases, the use of flexible cords is authorized for power delivery. These cords can be grouped and designated as lamp, heater, or heavy-duty power cords. Lamp cords are supplied in many forms. The most common types are the single-paired, rubber-insulated, and twisted-paired cords. Flexible cord is used to connect appliances, lamps, and portable tools to outlets. It can never be used as a permanent extension of fixed wiring.


Outlet boxes are simply connection points for joining wires or connecting to outside devices such as receptacles, switches, and fixtures. They bind the elements of a conduit or a cable system into a continuous grounded system. Electrical boxes provide a means of holding the conduit in position, space for mounting such devices as switches and receptacles, protection for the device, and space for making splices and connections. Regardless of general trade terminology, most boxes are used interchangeably. For example, with appropriate contents and covers, the same box could be used as an outlet box, a junction box, or a switch box.


The variety of sizes and shapes corresponds to variations in wiring methods, the type and number of devices attached to the box, and the number of wires entering it. One important factor is that boxes come in both metallic and nonmetallic versions. Outlet boxes are manufactured in sheet steel, porcelain, bakelite, or fiberglass and are round, square, octagonal, or rectangular. The fabricated steel box is available in a number of different designs. For example, some boxes are of the sectional or gang variety, while others have integral brackets for mounting on studs and joists. Moreover, some boxes have been designed to receive special cover plates so that switches, receptacles, or lighting fixtures can be installed easier. Other designs facilitate installation in plastered surfaces.

Each box has a certain volume in cubic inches that determines how many wires of a certain size may be brought into it. Regardless of the design or material, they all should have sufficient interior volume to allow for splicing conductors or making connections. For this reason, the allowable minimum depth of outlet boxes is limited to 1 1/2 inches in all cases except where building-supporting members would have to be cut. In this case, the minimum depth can be reduced to 1/2 inch.


The selection of boxes in an electrical system should be made according to Tables B-9 and B-10 which list the maximum allowable conductor capacity for each type of box. In these tables, a conductor running through the box is counted along with each conductor terminating in the box. For example, one conductor running through a box and two terminating in the box would equal three conductors in the box. Consequently, any of the boxes listed would be satisfactory. The tables apply for boxes that do not contain receptacles, switches, or similar devices. Each of these items mounted in a box will reduce the maximum number of conductors allowable by one, as shown in the tables.


Steel outlet boxes (Figure A-11) are generally used with rigid and thin-wall conduit or armored cable. The steel boxes are either zinc- or enamel-coated; the zinc coating is preferred for conduit installation in wet locations. All steel boxes have knockouts. These knockouts are indentations in the side, top, or back of an outlet box, sized to fit the standard diameters of conduit fittings or cable connectors. They can usually be removed with a small cold chisel or punch to facilitate entry into the box of the conduit or cable. Boxes designed specifically for armored cable use also have integral screw clamps located in the space immediately inside the knockouts and thus eliminate the need for cable connectors. This reduces the cost and labor of installation. Box covers are normally required when it is necessary to reduce the box openings, provide mounting lugs for electrical devices, or cover the box when it is to be used as a junction. The antishort bushing is inserted between the wires and the armor to protect the wire from the sharp edges of the cut armor when it is cut with a hacksaw or cable cutter.



Steel boxes are also used for nonmetallic cable and surface or open wiring. However, the methods of box entry are different from those for conduit and armored-cable wiring, because the electrical conductor wires are not protected by a hard surface. The connectors and interior box clamps used in nonmetallic and surface or open wiring are formed to provide a smooth surface for securing the cable rather than being the sharp-edged type of closure normally used.


Nonmetallic outlet boxes are made of plastic or fiberglass, and they are used with open and nonmetallic-sheathed wiring. Cable or wire entry is generally made by removing the knockouts of preformed, weakened blanks in the boxes.


When adding an outlet or doing remodeling work, it is sometimes necessary to install an outlet box in a finished wall. Boxes with beveled corners and internal cable clamps simplify the procedure (Figure A-12).


Outlet boxes that do not have brackets are supported by wooden cleats or bar hangers as shown in Figure A-13.

Wooden Cleats

Wooden cleats are first cut to size and nailed between two wooden members. The boxes are nailed or screwed to these cleats through holes provided in their backplates.

Mounting Straps

If the outlet box is to be mounted between studs, mounting straps are necessary. The ready-made straps are handy and accommodate not only a single box but also a 2-, 3-, 4-, or 5-gang box.

Bar Hangers

Bar hangers are prefabricated to span the normal 16- and 24-inch joist and stud spacings and are obtainable for surface or recessed box installation. They are nailed to the joist or stud-exposed faces. The supports for recessed boxes are normally called offset bar hangers.

Patented Supports

When boxes have to be installed in walls that are already plastered, several patented supports can be used for mounting. These obviate the need for installing the boxes on wooden members and thus eliminate extensive chipping and replastering.


Safety codes and regulations require that conductors be spliced or joined with approved splicing devices (Figure A-14) or be brazed, welded, or soldered with a fusible metal or alloy. Splices must be -mechanically and electrically secure before they are soldered. Using soldering and splicing devices provides added protection. To assure a high-quality connection, electricians must select the proper size connector relative to the number and size of wires.



All conduits and cables must be attached to the structural members of a building in a manner that will preclude sagging. The cables must be supported at least every 4 1/2 feet for either a vertical or horizontal run and must have a support in the form of a strap or staple (Figure A-15) within 12 inches of every outlet box. Conduit-support spacings vary with the size and rigidity of the conduit. See Table B-11 for support of rigid nonmetallic conduit and Table B-17 for support of rigid metal conduit.


Use cable staples for a very simple, effective method of supporting armored cables on wooden members.


Bell or signal wires are normally installed in pairs in signal systems. The operating voltage and energy potential is so low in these installations (12 to 24 volts) that protective coverings such as conduit or loom are not required. To avoid any possibility of shorting in the circuit, they are normally supported on wood studs or joists by insulated staples.


Conduit and cable straps (Figure A-15) are supplied as either one-hole or two-hole supports and are formed to fit the contour of the specific material for which they are designed. The conduit and cable straps are attached to building materials by anchors designed to suit each type of supporting material. For example, a strap is attached to a wood stud or joist by nails or screws. Expanding anchors are used for box or strap attachment to cement or brick structures and also to plaster or plaster-substitute surfaces. Toggle and molly bolts are used where the surface wall is thin and has a concealed air space that will allow for the release of the toggle or expanding sleeve.



Portable appliances and devices are readily connected to an electrical supply circuit by means of an outlet called a receptacle (Figure A-16). For interior wiring, these outlets are installed either as single or duplex receptacles. Safety standards require that all receptacles for 15- or 20-amp, 120-volt branch circuits (most of the circuits in homes) must be the grounding type. Receptacles previously installed, as well as their replacements in the same box, may be two-wire receptacles. All others must be the three-wire type. The third wire on the three-wire receptacle is used to provide a ground lead to the equipment that receives power from the receptacle. This guards against dangers from current leakage due to faulty insulation or exposed wiring and helps prevent accidental shock. To eliminate the possibility of plugging a 120-volt appliance into a 240-volt receptacle, higher-voltage circuits use special receptacles and matching attachment plugs. The receptacles are constructed to receive plug prongs either by a straight push action or by a twist-and-turn push action. Fixtures are similar to receptacles but are used to connect the electrical supply circuit directly to lamps inserted in their sockets.


Like switches, all receptacles are rated for a specific amperage and voltage. Receptacles marked CO-ALR can be used with either copper or aluminum wire. Unmarked receptacles and those marked CU-AL may be used with copper wire only. The receptacles commonly used with conduit and cable installations are constructed with yokes to facilitate their installation in outlet boxes. In this case, they are attached to the boxes by metal screws through the yokes. Wire connections are made at the receptacle terminals by screws that are an integral part of the outlet. Receptacle covers made of brass, steel, or nonmetallic materials are then attached to box and receptacle installations to afford complete closure at the outlets.


Surface metal raceways (Figure A-17) provide a quick, inexpensive electrical wiring installation method, since they are installed on the wall surface instead of inside the wall. Surface metal raceways are basically either one- or two-piece construction. Electricians working with the one-piece construction type install the metal raceway like conduit, then pull the wires to make the necessary electrical connections. When working with the two-piece construction type, electricians install the base piece along the wiring run, lay wiring in the base piece, and hold the wiring in place with clamps. After the wires are laid, they snap on the capping and the job is complete.

A multioutlet system, with ground inserts if desired, has outlets spaced every few inches so that several tools or pieces of equipment can be used simultaneously. An over-floor metal raceway system handles telephone and signal or power and light wiring where the circuits must be brought to locations in the middle of the floor area. These systems are all designed so that they can either be installed independently of other wiring systems or economically connected to existing systems.



Portable appliances and devices that are to be connected to receptacles have electrical cords that are equipped with plugs (Figure A-18). These plugs, called male plugs, have prongs that mate with the slots in the outlet receptacles. A three-prong plug can fit into a two-prong receptacle by using an adapter. If the electrical conductors connected to the outlet have a ground system, the plug on the lead wire of the adapter is connected to the center screw holding the receptacle cover to the box. Many of these plugs are permanently molded to the attached cords. Other types of cord grips hold the cord firmly to the plug. Twist-lock plugs have patented prongs that catch and are firmly held to a mating receptacle when the plugs are inserted into the receptacle slots and twisted. When the plugs do not have cord grips, the cords should be tied with an Underwriter's knot (Figure 2-2) at the plug entry. This knot eliminates tension on the terminal connections when the cord is connected and disconnected from the outlet receptacle.


In some operating conditions, a cord must be connected to a portable receptacle. This type of receptacle, called a cord-connector body or a female plug, is attached to the cord in a manner similar to the attachment of a male plug.



A switch is used to connect and disconnect an electrical circuit from the source of power. Switches may be either one-pole or two-pole for ordinary lighting or receptacle circuits (Figure A-19). If they are of the one-pole type, they must be connected to break the hot or ungrounded conductor of the circuit. If they are of the two-pole type, the hot and ground connection can be connected to either pole on the line side of the switch. Switches are also available that can be operated in combinations of two, three, or more in one circuit. These switches are called three-way and four-way switches and are discussed fully in Chapter 3.


Switches used for exposed wiring and nonmetallic-sheathed cable wiring are usually of the tumbler type with the switch and cover in one piece. Other less-common ones are the rotary-snap and push-button types. These switches are generally nonmetallic in composition.


The tumbler switch and cover plates normally used for outlet-box installation are mounted in a manner similar to that for box-type receptacles and covers and are in two pieces. Foreign installations may still use push-button switches.


At every power-line entry to a building, a switch-and-fuse combination or a circuit-breaker switch must be installed at the service entrance (Figure A-20). This switch must be rated to disconnect the building load while in use at the system voltage. Entrance, or service switches as they are commonly called, consist of one knife switch blade for every hot wire of the power supplied. The switch is generally enclosed and sealed in a sheet-steel cabinet. When connecting or disconnecting the building circuit, the blades are operated simultaneously through an exterior handle by the rotation of a common shaft holding the blades. The neutral or grounded conductor is not switched but is connected at a neutral terminal within the box. Many entrance switches are equipped with integral fuse blocks or circuit breakers that protect the building load. The circuit-breaker type of entrance switch is preferred, particularly in field installations, because it is easier to reset after the overload condition in the circuit has been cleared.



The device for automatically opening a circuit when the current rises beyond the safety limit is technically called a cutout but is more commonly called a fuse. All circuits and electrical apparatus must be protected from short circuits or dangerous overcurrent conditions through correctly rated fuses.


The cartridge-type fuse is used for current rating above 30 amperes in interior wiring systems. The ordinary plug or screw-type fuse is satisfactory for incandescent lighting or heating-appliance circuits.


Whenever motors are connected on branch circuits, time-lag fuses should be used instead of the standard plug or cartridge-type fuse. These fuses have self-compensating elements that maintain and hold the circuit in line during a momentary heavy ampere drain, yet cut out the circuit under normal short-circuit conditions. The heavy ampere demand normally occurs in motor circuits when the motor is started. Examples of such circuits are the ones used to power oil burners or air conditioners.


As a general rule, the fusing of circuits is concentrated at centrally located fusing or distribution panels. These panels are normally located at the service-entrance switch in small buildings or installed in several power centers in large buildings. The number of service centers, or fuse boxes in the latter case, is determined by the connected power load.


Circuit breakers (Figure A-21) are devices resembling switches that trip or cut out the circuit in case of overamperage. They perform the same function as fuses and can be obtained with time-lag opening features similar to the special fuses discussed earlier. Based on their operation, they may be classified as thermal, magnetic, or thermal-magnetic reaction.

A thermal circuit breaker has a bimetallic element integrally built within the breaker that responds only to fluctuations in temperature within the circuit. The element is made by bonding together two strips of dissimilar metal, each of which has a different coefficient of expansion. When a current is flowing in the circuit, the heat created by the resistance of the bimetallic element expands each metal at a different rate, causing the strip to bend. The element acts as a latch in the circuit as the breaker mechanism is adjusted so that the element bends just far enough under a specified current to trip the breaker and open the circuit.

A magnetic circuit breaker responds to changes in the magnitude of current flow. In operation, an increased current flow will create enough magnetic force to pull up an armature, opening the circuit. The magnetic circuit breaker is usually used in motor circuits for closer adjustment to motor rating, while the circuit conductors are protected, as usual, by another circuit breaker.

The thermal-magnetic circuit breaker combines the features of the thermal and magnetic types. Practically all of the molded-case circuit breakers used in lighting panel boards are of this type. The thermal element protects against overcurrents in the lower range, and the magnetic element protects against the higher range usually occurring from short circuits.

Circuit breakers are preferred over fuses because they can be manually reset after tripping, and fuses must be replaced. Also, fuses can easily be replaced with higher-capacity ones that do not protect the circuit, but this is difficult to do with circuit breakers. Circuit breakers combine the functions of a fuse and a switch. When tripped by overloads or short circuits, all of the ungrounded conductors of a circuit are opened simultaneously. Each branch circuit must have a fuse or circuit breaker protecting each ungrounded conductor. Some installations may or may not have a main breaker that disconnects everything.

As a guide during installation, a main breaker or switch is not required ahead of the branch circuit breaker if less than six movements of the hand are required to open all the branch circuit breakers. However, if more than six movements of the hand are required, a separate disconnecting main circuit breaker is required ahead of the branch circuit breaker. Each 120-volt circuit requires a single-pole breaker that has its own handle. Each 240-volt circuit requires a double-pole breaker to protect both ungrounded conductors. However, you can place two single-pole breakers side by side and tie the two handles together mechanically to give double-pole protection. Both handles could then be moved by a single movement of the hand. A two-pole breaker may have one handle or two handles that are mechanically tied together, but either one requires only one movement of the hand to break the circuit.


Lamp sockets (Figure A-22) are generally screw-base units placed in circuits as holders for incandescent lamps. A special type of lamp holder has contacts, rather than a screw base, which engage and hold the prongs of fluorescent lamps when they are rotated in the holder. The sockets can generally be attached to a hanging cord or mounted directly on a wall or ceiling in open wiring installations. This requires using screws or nails in the mounting holder that is provided in the nonconducting material, which is molded or formed around the lamp socket. The two mounting holes in a porcelain lamp socket are spaced in such a way that the sockets may also be attached to outlet box ears or a plaster ring with machine screws. The screw threads molded or rolled in the ends of the lamp-holder sockets also facilitate their ready integration in other types of lighting fixtures such as table lamps, floor lamps, or hanging fixtures that have reflectors or decorative shades. In an emergency, a socket may also serve as a receptacle. The socket is converted to a receptacle by screwing in a female plug. One type of ceiling lamp holder has a grounded outlet located on the side.


The most common components in interior wiring signal systems (Figure A-23) normally operate at voltages of 6, 12, 18, or 24 volts, AC or DC. As a general rule, they are connected by open-wiring methods and are used as interoffice or building-to-building signal systems.


Many types of reflectors and shades (Figure A-23) are used to focus the lighting effect of bulbs. Of these, some are used to flood an area with high-intensity light and are called floodlights. Others, called spotlights, -concentrate the useful light on a small area. Both floodlights and spotlights come in two- or three-light clusters with swivel holders. They can be mounted on walls or posts or on spikes pushed into the ground.


The most common light source for general use is the incandescent lamp (Figure A-23). Though it is the least efficient type of light, its use is preferred over the fluorescent type because of its low initial cost, ease of maintenance, convenience, and flexibility. Its flexibility and convenience is readily seen by the wide selection of wattage ratings that can be inserted in one type of socket. Further, since its emitted candlepower is directly proportional to the voltage, a lower-voltage application will dim the light. A higher-voltage application from a power source will increase its intensity. Although an incandescent light is economical, it is also inefficient because a large amount of the energy supplied to it is converted to heat rather than light. Moreover, it does not give a true light because the tungsten filament emits a great deal more red and yellow light than does the light of the sun. Incandescent lights are normally built to last 1,000 hours when operating at their rated voltage.


Bulb for bulb and watt for watt, a fluorescent light provides more light for the money than an incandescent light does. For example, a 40-watt fluorescent tube produces almost six times as much light as a 40-watt incandescent bulb, and a fluorescent tube will last about five times longer than an incandescent bulb.

Unlike the simple principle of an incandescent bulb, which glows when current flows through the filament, an intricate electrical process takes place before a fluorescent tube gives light. Because a fluorescent tube does not have a filament, a ballast (also called a transformer) is necessary to set up voltage within the tube. In addition to ballasts, older-type fluorescent fixtures have starters that assist the ballast in the initial starting process.

The two most common types of fluorescent light fixtures for homes are rapid-start and preheat. It is easy to distinguish between them because the starter mechanism of the rapid-start type is built right into the ballast, whereas each tube on the preheat type has a visible starter unit. The starters, which look like small aluminum cylinders, tend to burn out as often as the bulbs do. A third type, less commonly used in the home, is the instant start. This type has no starter and is distinguished by a tube with a single pin on each end.


Glow lamps are electric-discharge light sources that are used as indicator or pilot lights for various instruments and on control panels. Because these lamps have relatively low light output, they are used to indicate that circuits are energized or that electrical equipment installed in remote locations is in operation.

A glow lamp consists of two closely spaced metallic electrodes sealed in a glass bulb that contains an inert gas. The color of the light emitted by the lamp depends on the gas; for example, neon gas produces a blue light. The lamp must be operated in series with a current-limiting device to stabilize the discharge. This current-limiting device consists of a high resistance that is usually contained in the lamp base.

A glow lamp produces light only when the voltage exceeds a certain striking voltage. As the voltage is decreased somewhat below this value, the glow suddenly vanishes. When the lamp is operated on AC, light is produced only during a portion of each half cycle, and both electrodes are alternately surrounded with a glow. When the lamp is operated on DC, light is produced continuously and only the negative electrode is surrounded with a glow. This characteristic makes it possible to use the glow lamp as an indicator of AC and DC. The lamp also has the advantages of small size, ruggedness, long life, and negligible current consumption; and it can be operated on standard lighting circuits.


The transformer is a device for changing AC voltages into either high voltages for efficient power-line transmission or low -voltages for consumption in lamps, electrical devices, and machines. Transformers vary in size according to their power-handling rating. Their selection is determined by input and output voltage and load-current requirements. For example, the transformer used to furnish power for a doorbell reduces 115-volt AC to about 6 to 10 volts. This is accomplished by two primary wire leads that are permanently connected to the 115-volt circuit and two secondary screw terminals from the low voltage side of the transformer.


Pole-line hardware includes bolts, nuts, washers, braces, eyebolts, anchor rods, lag screws, pole steps, guy clamps, guy clips, guy hooks, guy plates, thimble-eyes, and steel pins. To increase the life of the material, electricians should use hardware galvanized according to specifications of the American Society for Testing Materials (ASTM). Figures A-24 through A-27 illustrate many items of pole-line hardware.

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