WIRES AND CABLES
- Wires are conducting materials which are made of copper, aluminum, silver, iron, manganin, and alloyed metals like nichrome wire, constantan, and German silver wire. - It is used to conduct power from the point where it is generated to the point where it is used. - Wires are all electric conductors but not all conductors are wires. These wires are either made of solid or stranded. Most wires are round although square and rectangular forms are also used in specific applications. These wires are manufactured with or without insulation depending upon the application. - Copper or aluminum wires without insulation are usually used for grounding connection and also for high tension or high voltage transmission lines. - Nichrome wire, constantan, manganin, and German silver wires are used as resistance wires in electrical equipment and appliances. - All wires have resistances that oppose the unlimited flow of current and causes voltage drop resulting in the eventual heat developed in the wire. The heat developed varies according to the square of the current in amperes. - There is a limit to the degree of heat that various types of wire insulation and sizes can safely withstand. They should not be allowed to reach a temperature that might cause a fire. - The Electrical Code specifies the maximum current-carrying capacity in amperes that is safe for wires of different with different insulations and under different circumstances and conditions. (see Table I – Types of Conductors, Applications, and Maximum Operating Temperature of wires and cables.)
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Wire Sizes - Sizes of wires are specified in circular mil (cmil) which is the cross-sectional area of any round wire. - They are also specified in gauge number which is the American Wire Gauge (A.W.G.) or the Brown and Sharpe Gauge (B&S). These gauges are different from those used for steel wires or for non-electrical purposes like fence wire. - In keeping with the metric system, however, electrical wire sizes are now specified in Metric System. See Table II – for sizes of electrical wires, their current-carrying capacities, and number of wires in conduit.
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- Number 14 A.W.G. has a diameter of 0.064 inch or 64 mils or 2.0 mm . - Sizes larger than number 14 are nos. 12, 10, 8, 6, etc. The bigger the number, the smaller the diameter of the wire. 2 - Wires larger than number 4/0 or 000/19 are designated in mm and in cross-sectional area in circular mil. 2 - So 250/37 would have the same size as 127.0 mm or 250,000 mils (250 kcmil, sometimes abbreviated MCM). - The largest recognized size of wire is 2,000,000 mils (2,000 kcmil or 2,000 MCM) wire. - Figure below are the actual diameters of typical sizes of electrical wires without insulation.
- Wires are either stranded or solid depending upon their sizes. 2 - Number 8 AWG wire or 8.0 mm and above are stranded. 2 - Number 10 AWG or 5.5 mm and below are solid except flexible cords and fixture wires where flexibility is needed. 2 2 - Number 8 mm up to number 33.6 mm wires have 7 strands of conductors while number 2 2 42.4 mm wires up to number 107.0 mm have 19 strands of conductors. - The more number of strands it contains, the bigger the size of the stranded wires. (See figure below)
- In a raceway, it is easier to install stranded wire than solid wire because of its flexibility to handle. If a wire is stranded, it is larger in over-all diameter than a solid wire of the same size, but the number of the circular mils in any one side is the same whether the wire is stranded or solid. - The cross-sectional area of each strand in cmil added together is the total cross-sectional area of the stranded wire in circular mil. - Wire sizes of number 20 and below are used mostly in manufacturing electrical equipment of all kinds, like for coils of portable transformer. 5
- Number 18 and 16 are used chiefly for flexible cords, signal systems, and similar purposes where small current are involved. - Numbers 14 to 0000/19 or 4/0 are used in residential and farm wiring and industrial and commercial work. - Numbers 12 and 14 are commonly used for general lighting and general outlet installation respectively. Wire Insulation - Most wires used in wiring buildings have thermoplastic insulation. - The insulation consists of a layer of plastic insulating compound, the thickness of which depends on the size of wire. - The different types of plastic wires are type T, which is the most ordinary type of plastic insulated wire, type TW, which is used only in dry locations; and type THW which is similar to type TW but the former's insulation withstands a greater degree of heat, and consequently has higher capacity ratings and is therefore frequency used. - Both types "THHN" and “THWN" have thermoplastic insulations with an outer nylon jacket. - Type "THWN” is moisture resistant. - Type "THHN” is suitable for higher temperatures than types “THW” and "THWN”, and consequently has higher capacity than the other types. - Nylon has better insulation qualities and great mechanical strength, which is much smaller in diameter than the ordinary types "T”, “TW", and “THW" of corresponding size. It permits greater number of small diameter wires (such as types "THHN" or "THWN”) in given size of conduit than it does for ordinary wires such as types "T”, "TW” or "THW”. - Although type "THHN" or “THWN" is more expensive than the ordinary wires, it permits the use of smaller sizes of conduit and consequently cheaper in actual cost. - Type "XHHW" wire resembles the appearance of types “T", "TW”, or “THW". It has a thinner layer of insulation and the over-all diameter is smaller. The insulation is crosslinked synthetic polymer which has excellent insulating heat resisting, and moistureristing properties. It may be used in dry or wet locations. - Rubber-covered wire is made of neoprene, natural or synthetic rubber, and other inflatable materials. It consists of copper conductor thinned to make it easier to remove the insulation, and an outer-moisture, and flame resistant covering. Such wire is type "RHW" which may be used in dry and wet locations. - Type "RH" and "RHH" have insulations that will stand more heat and higher capacity rating than "RHW". Type "RH" and "RHH” are only in dry locations. Figure below shows the types of wire insulations.
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Cables - For several purposes, the cable has two or more wires grouped together to form a cable. - A cable that contains two no. 14 wires is known as "14-2" (fourteen-two), if it contains three no. 12 wires, it is called "12-3" (twelve-three), etc. - If a cable has two insulated no. 14 wires a bare uninsulated grounding wire, it is called "14-2" with ground. - Sometimes the cable contains two or three conductors with insulator of different colors. - A few cables are single conductor type, such as type USE for direct burial in the earth, and type UF which may also be buried but subject to restrictions. Sometimes, ordinary wire in sizes larger than no. 4/0 are called cables but that is an improper use of the term. Figure below – Types of Cable Insulations.
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Description and Uses of Wires and Cables: 1. Copper or aluminum conductor – usually bare wire, solid or stranded, hard or soft, sizes up to 1,000 MCM used for overhead transmission lines and ground wires.
2. Thermoplastic wire (type TW) – flame retardant, moisture-resistant wire, sizes no. 14 AWG up to 1,000 MCM used for building wiring in dry or wet locations .
3. Heat resistant, flame retardant, moisture resistant cable (type THW) – sizes no. 14 AWG up 10 1,000 MCM, usually seven strands conductor in a table used for building wiring in dry or wet locations.
4. Thermoplastic-covered fixture wire (type TF) – sizes no. 18 to no. 16 AWG solid or stranded used for wiring lighting fixtures and as terminal wires.
5. Non-metallic sheathed cable (type NM) – sizes no. 14 to no. 10 AWG with 2 or 3 conductors inside the cable used for exposed or concealed installation in buildings.
6. Copper or aluminum PE insulated line wire – sizes no. 10 AWG up to l,000 MCM, used for line wires which is weather resistant, used also for primary distribution.
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7. Flat cord (type SPT) – sizes no. 18 AWG up to no. 14 AWG 2 conductors all plastic insulated conductors which are stranded used for building wiring or cord of portable electrical equipment and for extension wiring of single bulb.
8. Air conditioning unit safety cord (type SPT) – sizes no. 14 AWG to no. 12 AWG with 2 conductors and ground used for wiring air-conditioning unit.
9. Thermoplastic portable cords (type SJT and type ST) – sizes no. 18 AWG to no. 16 AWG for SJT junior hard service and sizes no. 18 AWG up to no. 6 AWG for ST extra hard service both may have either 2, 3 or 4 conductors stranded used for wiring construction of portable motors and appliances.
10. Underground feeder and branch circuit cable (type UF) – unipass or jacketed, sizes no. 14 AWG up to no. 4/0 AWG used for wiring installation direct burial.
11. Intercom cable – (D-wire) shielded or unshielded sizes no. 22 AWG, 3 conductors to 100 PRS, used for wiring interior communications and sound distribution system jacketed.
12. Hypertemp 2000 wire (Westinghouse insulation polyester amide-imide class H ) – sizes no. 40 AWG to no. 6 AWG magnet wires used for wiring or winding hermetic systems of dry type transformers, high speed winding, wet winding operations, and oil filled transformers.
13. Interlock armored thermoplastic cable or armored cable (type ACT), (BX) – sizes no. 14 AWG to no. 2 AWG with 2 or 3 conductors, used for wiring installation in buildings.
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14. Welding cable – a flexible portable cable with sizes no. 16 AWG to no. 4/0 AWG, used for wiring cable of electric arc welding.
15. Control cable – sizes no. 16 AWG to no. 12 AWG, PVC insulated, PVC jacketed multiconductor control cable used for wiring installation o f machinery controls.
16. Power cable – HMWPE insulated cable with sizes no. 8 AWG to 1,000 MCM, used for underground, aerial, duct and direct burial wiring from 6,000 volts to 35 kV.
17. Temperature wire (type HI) – sizes no. 18 AWG to gauge no. 8 AWG, commonly used for wiring automotive engines and appliances with high temperature, and for switchboard wiring.
18. TV lead-in wire – sizes no. 22 AWG and no. 20 AWG, used for TV antenna lead-in wire, 300 ohms impedance.
19. Telephone exterior service drop wire – twisted or flat constructed vinyl plastic insulated wire, used for wiring telephone.
20. Telephone cable – (Type CCP) sizes no. 26 to no. 19 AWG, installed aerial and duct type for telephone wiring with color coded conductor insulators.
21. Submersible pump cable – sizes no. 14 AWG and no. 4/0 AWG insulated thermoplastic, used for wiring submersible pumps of deep well water systems.
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22. Flexible conduits – sizes with 1/2" to 4" inside diameter, used as armored cable conductors.
23. Service drop cable – sizes from no. 6 AWG to no. 4/0 AWG, aluminum conductor with neutral messenger, used for service drop cable.
24. Asbestos covered nickel cord (Rockbestos) – used for wiring electric ranges, hot water heaters, stoves and hair dryers where both heat and moisture resistance is desirable. Constructed of 16 strands of no. 30 AWG nickel wire, insulated with pure asbestos yarn.
25. Royal cord – sizes no. 22 AWG to no. 4 AWG with 2, 3 or 4 conductors, stranded, used for portable cords in electrical equipment.
26. Telephone cable (Type ADT) – the cable consists of 6 pairs thru 909 pairs of strands; used for aerial and duct installation.
27. Appliance or electric stove heater cable – sizes no. 22 AWG to no. 12 AWG, used for wiring cord of heating equipment in dry locations.
28. Sparkplug and battery cable – used for wiring automobile, truck-motor, and starter cable.
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29. Nonmetallic sheathed cable (type NMC) – is a multiconductor cable used for interior wiring in dry or wet locations.
30. Self-supporting aerial telephone cable (REA-38) – used for aerial telephone wiring with color code insulation of conductors.
31. Coaxial cable – used for radio frequency antennae installation and radar transceiver installation.
32. Duralene (type WP) – polyethylene insulated line wire used for wiring installation of residential buildings.
33. Machine tool wire – used for wiring machine tools, appliance and control circuit.
34. Direct Burial telephone cable – used for wiring installation of telephone line. Conductors have different colors of insulation.
35. Durex wire – used for building wiring specifically for interior wiring.
36. Steel tape armored power cable with ground wires (copper) – used for general wiring installation including underground.
37. Switchboard semi-rigid plastic insulation – stranded cable used for wiring switchboards inside power plant.
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38. Service entrance cable (type SE) – it consists of two or more insulated conductors with one or more bare conductors for grounding. It is time-retardant, moisture-resistant cable used for service entrance and interior wiring.
39. All plastic parallel cord (type SPT) – used for wiring pendant or portable equipment or device.
40. Auto cable PVC and braided – used for wiring automotive light and for hook-up wire.
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Review Questions 1. Explain the difference between a wire and a cable. 2. What is the maximum size of a wire or conductor that is considered solid? 3. What is the size of a cable that is to be made stranded? 4. How many strands of wire does a size no. 6 conductor has? 5. How many strands of wire does size no. 0 conductor has? 6. What is the difference between TW or THW conductor and THWN conductor? 7. What are the most common uses for type RHW? 8. What are the most common use for RHH? 9. Bare copper or aluminum conductors are for overhead transmission line. And if so why and why not? 10. What type of wire is used for cords of appliances? 11. What type of cable is used for direct burial in the earth? 12. Type NM cable is used in what locations? 13. Describe the insulation of type NMC cable and explain where it is commonly used. 14. What kind and size of conductor is commonly used for general lighting? 15. What kind and size of conductor is commonly used for wiring general outlets of appliances? 16. What is the other name for armored cable wire? 17. What type of internal wiring is suitable for 300 volts only? 18. What is the size of gage no. 6 wire in millimeter? 19. Name the different kinds of wires and cables commonly used in dry and wet locations. 20. Name the different kinds of conductors suitably used in corrosive locations.
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