CONTENTS AT A GLANCE Overview
Insulated Wire Conductors Wire Insulation and Cable Coding Service Entrance Cables
Nonmetallic Cables Metal-Clad Cables Communications Cables Metal and Nonmetallic Conduit Conduit Bodies and Fittings Electrical Boxes
Outlet Box Wiring
Wall-Mounted AC Switches Lamp Dimmers and Fan Controls Motion/Occupancy Sensors
Wall-Mounted Receptacles and Plugs Ground-Fault Circuit Interrupters Isolated-Ground Receptacles Locking Receptacles and Plugs Pin-and-Sleeve Devices Wire Mesh Cable Grips
Glossary of Wiring Device Terms
6
Overview
This chapter focuses on the standard wire, cable, and electrical devices used in homes and offices, with emphasis on branch circuits. Many different kinds of standard wire and cable are available for use in 120/240-V AC branch-circuit wiring. Wire is defined as a single solid or twisted copper or aluminum conductor with or without insulation, and cables for power and lighting typically consist of two or more insulated wires and a bare ground wire with a protective jacket. There are also untwisted and twisted wire pairs for telephone service and coaxial cables for TV and Internet reception.
Conduit is tubing or pipe that encloses and protects wires and cables. It can be metal or nonmetallic, and rigid or flexible. In outdoor locations, conduit protects the enclosed wires or cable above ground from rain, snow, condensed moisture, and abrasion as well as accidental severence during excavation or trench digging. It is also used to protect buried cable. Short lengths of conduit are used under concrete sidewalks to make it eas-ier to pull wires or cable under those obstructions. In addition, short lengths are used to protect wire or cable as it emerges from the ground to a meter base or other enclosure.
Conduit bodies are fittings that permit electrical metallic tubing (EMT) conduit to be joined in straight lengths or in 90° turns. Many different kinds of conduit connectors and couplings are available for coupling or connecting similar or dissimilar conduit.
These products are made from materials such as zinc-plated steel, malleable iron, alu-minum, die-cast zinc, and nylon.
Junction and pull boxes provide access points for pulling and feeding conductors into a raceway system. Their use is mandatory in conduit runs where the number of bends between outlets exceeds the maximum permitted by the National Electrical Code(NEC). Conduit is also used inside homes or buildings to protect wiring or cable from moisture, dust buildup, or abrasion.
Wall switches are designed for mounting inside wall-mounted outlet boxes and before being covered with decorative wallplates. Standard switches are available in all specification grades with ratings of 120/240 V AC, and some industrial and commer-cial grades are rated for 347 and 600 V AC. Switches can be single-pole, three-way, or four-way, and they can include pilot lights, lighted handles, and locks.
Conventional dimmers are wall-mounted devices for reducing the illumination lev-els of incandescent lamps. Other versions with different internal circuits are designed for dimming fluorescent lamps or controlling the speed of fans. Dimmers can change the illumination levels in a room and can help to conserve energy.
Motion or occupancy sensors are switches that can turn on lights or other appliances automatically when a person enters a room, and turn them off when the person leaves.
The most popular motion-sensing technologies are passive infrared (PIR) and ultra-sonic. These sensors are also used in security systems to warn of intruders inside or outside homes or buildings.
General-purpose duplex straight-blade receptacles are the familiar electrical outlets most often seen in homes and offices. They are designed to accept two- or three-prong plugs on line cords. Ground-fault circuit-interrupter (GFCI) receptacles, now required by the NECin many areas of a home, protect persons from electric shock should a bare or abraded conductor come in contact with an exposed metal object.
Locking-type plugs and receptacles are widely used in industry to clamp line cords from heavy-duty power tools securely in their receptacles. They can preventing possible injury to operators or damage to property if the plugs are accidentally pulled from their receptacles. Pin-and-sleeve devices are heavy-duty connectors for distributing electric power in industrial plants. They include plugs, receptacles, inlets, and connectors.
Insulated Wire Conductors
The American Wire Gauge (AWG) is used in the United States to identify specific wire diameters. The AWG values for wires commonly used to conduct 120/240-V AC and communications signals in homes and offices are illustrated in Fig. 6-1.
Ampacity,a word coined by combining the terms ampereand capacity,expresses the current-carrying capacity of wires and cables in units of amperes. It is important in the selection of wire and cables for various applications because it signifies maximum safe current-carrying capacity for conductors.The numbers used to identify wire diameter and ampacity are inversely related to AWG sizes. For example, 10 AWG wire has an ampacity rating of 30 A, No. 12 AWG wire has an ampacity rating of 20 A, while No.
14 AWG wire has a lower ampacity rating of 15 A, all for use in 120/240-V circuits.
On the other hand, relatively fine No. 16 and No. 18 AWG wires are not given ampacity rating because, as low-voltage or signal-level conductors, they are incapable of carrying useful power.
INSULATED WIRE CONDUCTORS 175
Figure 6-1 Cross sections and ratings of common copper wire gauges.
The ampacity of insulated wire, solid or stranded, depends on its gauge, metal com-position, and the thermal properties of its insulation. Copper is preferred for electrical conductors because of its high conductivity, ductility, and resistance to corrosion, but aluminum and copper-clad aluminum wire are approved for some applications.
The classification of insulation, either thermoplastic or thermoset, generally deter-mines its suitability for use under various ambient conditions. Thermoplastic materi-als, typically plastic resins, soften and flow when they are heated and subjected to pressure, but they become rigid when cooled. Thermoset materials, by contrast, are initially set or cured when heated, but after curing they will not soften, flow, or distort.
Some kinds of insulation are approved for wire only if it is to remain in a dry envi-ronment, preferably within a building, but other insulating materials have been for-mulated to stand up to environments that are dry and damp, dry and wet, or wet, and still other materials can withstand extreme temperatures. For example, some insulated wire can perform effectively only up to 60°C (140°F), while others can perform effec-tively at temperatures as high as 90°C (194°F).
All modern electrical wire insulation is rated as flame-retardant, but only a few of these insulation materials are capable of remaining effective insulators following long-term exposure to sunlight (ultraviolet radiation), ozone, or nuclear radiation.
A letter code has been established to simplify the selection of the optimum insulat-ed wire for specific tasks while considering operating temperatures and application environments. A knowledge of this code will permit the user suffucient background information to make the best selection. Examples of these code designations are RHH, THHN, TW, and XHHW.
Copper and aluminum wire are identifiable on sight by their color—reddish brown for copper and silver for aluminum. However, it is difficult to identify copper-clad alu-minum wire on sight without making a clean cut through the wire to examine the core.
On the other hand, even experts have trouble identifying wire insulation. For this rea-son, manufacturers print an identification code directly on the wire insulation, giving:
■ The trade name of the insulation
■ The wire’s maximum operating temperature
■ Environments for safe application (dry, damp, wet, or combinations of these)
■ Insulation properties (e.g., flame-retardant and moisture- and/or heat-resistant) Table 6-1, excerpted from Table 310.13 of NEC 2002, translates the insulation codes printed on the most commonly used insulated wires for electrical power power conduction. (To keep the table short and easier to read, codes identifying special-pur-pose insulated wire were omitted.)
Table 6-2, excerpted from Table 310.16 of NEC2002, gives the ampacity values of copper, aluminum, and copper-clad aluminum wires from 14 to 4/0 AWG, with a wide range of insulations indicated by their manufacturing code types such as RHW, THHW, or XHHW. The maximum temperatures at which these insulated wires can function effectively are classified into three groups: 60°C (140°F), 75°C (167°F), and 90°C (194°F).
TABLE 6-1 IDENTIFICATION OF INSULATION CODES ON COMMON ELECTRICAL WIRES
MAX.
TRADE TYPE OPERATING APPLICABLE
NAME LETTER TEMP. LOCATIONS INSULATION
Thermoset RHH 90°C Dry and damp
194°F
Moisture-resistant RHW 75°C Dry and wet; ozone- Flame-retardant,
thermoset 167°F resistant when2000 V moisture-resistant
applied thermoset
Moisture-resistant RHW-2 90°C Dry and wet Flame-retardant,
thermoset 194°F moisture-resistant
thermoset Heat-resistant THHN 90°C Dry and damp (for use Flame-retardant,
thermoplastic 194°F in conduits) heat-resistant
retardant thermoplastic
Moisture- and THHW 75°C Wet Flame-retardant,
heat-resistant 167°F moisture- and
thermoplastic 90°C Dry heat-resistant
194°F thermoplastic
Moisture- and THWN 75°C Dry and wet (for use in Flame-retardant,
heat-resistant 67°F conduits) moisture- and
thermoplastic heat-resistant
thermoplastic
Moisture-resistant TW 60°C Dry and wet Flame-retardant,
thermoplastic 140°F moisture-resistant
thermoplastic
Thermoset XHH 90°C Dry and damp Flame-retardant
194°C thermoset
Moisture-resistant XHHW 90°C Dry and damp Flame-retardant,
thermoset 194°F moisture-resistant
75°C Wet thermoset
167°F
Moisture-resistant XHHW-2 90°C Dry and wet Flame-retardant,
thermoset 194°F moisture-resistant
thermoset The letter translations are:
Hindicates a maximum allowable temperature of 75°C; if not present, the maximum is 60°C.
HHindicates a maximum allowable temperature of 90°C.
HHWindicates a maximum allowable temperature of 90°C for dry locations and 75°C for wet locations.
Nindicates a nylon sheath around the thermoplastic insulation.
Rindicates thermoset insulation.
Tindicates thermoplastic insulation.
Windicates suitability for wet and dry locations.
Xindicates cross-linked polyethylene thermoset insulation.
2indicates allowable temperature of 90°C, wet or dry.
Source: NEC2002, Article 310, Table 310.13, pp. 70-138 to 70-141.
TABLE 6-2ALLOWABLE AMPACITIES OF INSULATED CONDUCTORS TEMPERATURE RATING OF CONDUCTOR 60°C75°C90°C60°C75°C90°C (140°F)(167°F)(194°F)(140°F)(167°F)(194°F) Types TW, UFTypes RHW,Types RHH,Types TW, UFTypes RHW,Types THHN, THHW, THW,RHW-2, THHNTHHW, THW,THHW, THW-2, THWN, XHHW,THHW, THW-2,THWN, XHHW,THWN-2, RHH, USE, ZWTHWN-2, USERHW-2, USE-2, USE-2, XHH,XHH, XHHW, XHHW, ZW-2,XHHW-2, XHHW-2ZW-2 SIZE (AWGSIZE (AWG or kcmil)COPPERALUMINUM OR COPPER-CLAD ALUMINUMor kcmil) 18——14———— 16——18———— 14202025———— 1225253020202512 1030354025303510 84050553040458 65565754050606 47085955565754 38510011065751153 29511513075901002 1110130150851001151
1/01251501701001201351/0 2/01451751951151351502/0 3/01652002251301551753/0 4/01952302601501802054/0 CORRECTION FACTORS For ambient temperatures other than 30°C, AMBIENT TEMPS.multiply the allowable ampacities shown shown ˚C˚Fabove by the appropriate factor shown below 21–2570–771.081.051.04 31–3587–950.910.940.96 41–45105–1130.710.820.87 51–55123–1310.410.670.76 61–70141–1580.330.58 Conditions: No more than three current-carrying conductors in raceways, cables, or buried directly in earth; based on ambient temperature of 30°C (86°F). Source: NEC2002, Table 310.16, p. 70-144.
The first half of Table 6-2 gives the values for copper wire separately from those for aluminum and copper-clad aluminum in the second half. The ampacity ratings for alu-minum and copper-clad alualu-minum are identical for all guages and temperatures. Many of the same insulations are used on both copper and aluminum wires. Examination of the table reveals that all insulated copper wire gauges have higher ampacity ratings than comparable aluminum wire gauges for all insulations.
Table 6-2 assumes that not more than three of the current-carrying conductors list-ed are locatlist-ed in one raceway or cable or burilist-ed together directly in the earth. A selec-tion of correcselec-tion factors for five ambient temperature ranges, from 21–25°C (70–77°F) to 61–70°C (141–158°F), was taken from NEC2002, Table 310.16. It will permit reasonable estimates to be made of the ampacity values of individual insulated wires at temperatures above and below the base ambient temperature of 30°C (86°F).
The complete NEC table includes 10 ambient temperature ranges, from 21–25°C (70–77°F) to 71–80°C (159–176°F), for more precise ampacity estimates.
NEC2002, Table 310.16, also includes wire gauges from 250 to 2000 AWG. A foot-note reference to NEC2002 Article 240.4(D) limits the maximum current permitted in selected wires as follows:
■ 14 AWG copper wire shall not exceed 15 A.
■ 12 AWG copper wire shall not exceed 20 A.
■ 10 AWG copper wire shall not exceed 30 A.
■ 12 AWG aluminum and copper-clad aluminum wire shall not exceed 15 A.
■ 10 AWG aluminum/copper-clad aluminum wire shall not exceed 25A.
Wire Insulation and Cable Coding
INSULATED WIRE CODING The wire manufacturers’ wire markings give:
■ Wire gauge
■ Maximum voltage rating
■ Insulation code (R for thermoset, T for thermoplastic, X for cross-linked polyeth-ylene thermoset)
■ Maximum temperature code (H for 75°C, HH for 90°C, nothing for 60°C)
■ Abbreviation for sheath if present (e.g., N for nylon)
The identification codes for wires with thermoplastic insulation approved for use in conduit are given in Fig. 6-2afor THHN and in Fig. 6-2bfor THWN.
CABLE IDENTIFICATION AND CODING
Cables are identified by their wire gauge and the number of insulated wires they con-tain. In addition, all cables have a grounding wire. The codes printed on the jackets of
the major classes of cable for lighting and power applications, service entrance (SE), nonmetallic (NM or Romex), underground feeder (UF), flexible armored (BX or AC), and metal-clad (MC) are given in Fig 6-3.
The code for SE cable shown in Fig. 6-3aprovides the following information:
■ Cable type (e.g., service entrance)
■ Insulation code
■ Maximum voltage rating
■ Number and gauge of “hot” conductors
■ Number and gauge of ground conductors
■ UL listing
■ Year of manufacture
The codes on nonmetallic (NM) and underground feeder (UF) cable, as shown in Fig. 6-3band c,provide the following information:
■ Wire gauge
■ Number of conductors
■ Presence of ground wire (W/G)
■ Function (e.g., nonmetallic sheath or underground feeder)
■ Maximum operating temperature (H for 75°C, HH for 90°C, none for 60°C)
■ Identification of sheath, if present (e.g., N for nylon)
Service Entrance Cables
Service entrance cable, type SE, as shown in Fig. 6-3a, conducts power into the building from the splice with the utility’s service cables. Standard SE cable has two
SERVICE ENTRANCE CABLES 181
Figure 6-2 Codes printed on the thermoplastic insulation identify the wire as suitable for use in conduit: (a) THHN;
(b) THWN.
insulated conductors and a bare ground wire. It is approved for powering 240-V AC heat pumps and electric furnaces, but it cannot be used where an insulated neutral is required. Two of the insulated cables are for the 240-V AC load and a return neutral is for the 120-V AC load. SE cable is available in three different styles, U, R, and USE, all rated for 600 V AC. The letter X on the insulation indicates that it is cross-linked polyethylene.
NEC2002, Article 338, “Service-Entrance Cable: Types SE and USE,” covers the use, installation, and construction specifications for SE and USE cable.
Service entrance cable, type SE, style U,is a flat cable approved only for above-ground use. It has two insulated conductors. Both are black, but one might have a red stripe for
Figure 6-3 Codes printed on the jackets of cables identify them as:
(a) service entrance (SE), (b) nonmetallic (NM), and (c) underground feeder (UF).
phase identification. The neutral is multistranded braid enclosing the two insulated con-ductors. Conductors and braid are wrapped in glass-reinforced tape and jacketed in polyvinyl chloride (PVC) pipe. The jacket is flame-retardant, moisture-resistant, and has ultraviolet-resistant properties. It is available in AWG 6 and larger. U-style SE cable can-not be used to power 120/240-V AC appliances such as electric dryers and stoves, because these appliances require three insulated conductors and ground.
Service entrance cable, type SE, style R,is round cable that contains three insulated conductors and a grounding wire. As with style USE cable, it is approved only for above-ground use. The conductors are wrapped with glass-reinforced tape and enclosed in a PVC jacket. It is available in AWG 6 and larger. R-style SE cable is approved to feed 120/240-V AC loads such as electric clothes dryers and ranges, because it has three insulated conductors with an additional bare grounding conductor.
Service entrance cable, type USE,is intended for underground service or for use within conduit. It contains individual conductors that can be buried or placed in a con-duit. The conductors are insulated with cross-linked polyethylene. This cable is avail-able in AWG 6 and larger.
Nonmetallic Cables
Cables labeled 12/2 WG contain two insulated AWG 12 wires and a grounding wire.
Some commonly used cables suitable for use in 120/240-V circuits are described below.
Nonmetallic sheathed cable, type NM,shown in Fig. 6-3b,is widely used for indoor wiring in dry locations such as bedrooms, family rooms, or kitchens. NM cable, also called Romex, is available in a wide range of wires sizes with either two or three insu-lated wires and a bare ground wire. Insuinsu-lated and bare wires in NM cable are protected with paper insulation and a thermoplastic jacket. This cable is sold as rolls in boxes of 25 to 250 ft of cable.
NEC 2002, Article 334, “Nonmetallic Sheathed Cable: Types NM, NMC, and NMS,” covers the use, installation, and construction specifications of nonmetallic sheathed cable.
Underground feeder cable, type UF,shown in Fig. 6-3c,is used for wiring in damp or wet locations including outdoor lighting circuits. It has two insulated wires and a bare ground wire that are embedded in a white or gray solid-core vinyl sheathing to protect against dampness or water seepage. Most local electrical codes allow UF cable to be buried directly in the ground, and it is also suitable for use indoors in such loca-tions as laundry rooms or built-in basements where the humidity can be high.
NEC2002, Article 340, “Underground Feeder and Branch-Circuit Cable: Type UF,”
covers the use, installation, and construction specifications for UF cable.
Large appliance cablehas either two or three insulated wires and one bare ground wire embedded in a white or gray solid-core vinyl sheath. This cable is intended for powering kitchen ranges, clothes dryers, and water or other electric heaters rated for 30 to 50 A that require No. 8 or 6 AWG wire. Its construction is similar to NM cable, but its stranded copper wires make the cable easier to bend.
NONMETALLIC CABLES 183
Metal-Clad Cables
Flexible armored cable, type AC,shown in Fig. 6-4a, also called BX or Greenfield cable, has either a steel or aluminum armor sheath that serves as a grounding pathway.
Its conductors are insulated with thermoplastic and individually wrapped with a waxed paper jute. Available with from one to four conductors, it has a No. 16 AWG aluminum bonding wire inside its jacket that runs its entire length and contacts the metal armor to reduce its grounding resistance. Any short-circuited current flows through the metal sheath back to the loadcenter.
NEC2002, Article 320, “Armored Cable: Type AC,” covers the use, installation, and construction specifications for AC cable.
Metal-clad cable, type MC,shown in Fig. 6-4b,has either interlocking armor like AC cable or a smooth corrugated-tube armor. Standard MC cable has either three or four insulated No. 6 AWG or larger conductors. The bundle of conductors is wrapped in paper or plastic. However, it is also available with more smaller-gauge conductors and one bare grounding wire.
NEC2002, Article 330, “Metal-Clad Cable: Type MC,” covers the application and construction specifications for MC cable, and NEC 2002, Article 332, “Mineral Insulated, Metal-Sheathed Cable: Type MI,” covers the use, installation, and construc-tion specificaconstruc-tions for type MI cable.
Figure 6-4 Metal-clad cables: (a) flexible armored (AC);
(b) metal-clad (MC).
Communications Cables
Communications cables are made to carry audio, video, and data signals for tele-phones, TVs, and computers. These specialized cables are generally installed the same way as power cables.
Coaxial cablecan carry TV, video, and data signals. Its central conductor is insu-lated by a dielectric material which is covered by conductive wire braid or metal foil and then an insulating jacket. Coaxial cable is available in lengths up to 25 ft.
Telephone cable(also called station wire) can carry audio or voice signals between telephones and answering machines, and data for fax machines and computer modems, over the wired dial-up public telephone system. It is available in the following cables:
■ Quad or four-wire, four-color (green, red, black, and yellow)
■ Six-wire, six-color (green, red, black, yellow, white, and blue) wires configured as three twisted-wire pairs
■ Eight-wire, six-color (blue, red, green, and brown) banded wire, configured as four twisted-wire pairs
These cables are discussed in greater detail in Chap. 9.
Metal and Nonmetallic Conduit
Indoor metal and nonmetallic conduit can keep dust and moisture out of wiring and protect it from mechanical abrasion. Short sections can protect wiring close to the floor used to power washing machines and clothes dryers in damp basements. These sections must be anchored to masonry walls where there are no wood studs for sup-porting them. Conduit is also used as vertical ducting between floors to ease the task of pulling power and communications cables through floors. It will also protect all power and communications cables in warehouses and other unoccupied storage build-ings where there are no dry internal walls or ceilbuild-ings.
THHN and THWN wire with thermoplastic insulation are approved for use in con-duit. THHN wire, rated for a maximum operating temperature of 90°C (194°F), can be used in both damp and dry locations, while THWN wire, rated for 75°C (167°F), can be used in both wet and dry locations. Service entrance (SE) cable, underground feeder (UF) cable, and nonmetallic (NM) cable can also be installed in conduit.
METALLIC CONDUIT
Metallic tubing, type EMT,is light, thin-wall metal conduit used primarily to protect indoor wiring from physical abuse. It is easy to install, but its thin walls provide only marginal protection from abuse. EMT is available in 10-ft lengths with diameters of
12, 34, 1.0, 112, and 2.0 in. EMT 12-in.-diameter conduit can hold up to six No. 14 AWG wires, five No. 10 AWG wires, or two No. 8 AWG wires.
METAL AND NONMETALLIC CONDUIT 185