Wires: Types, Grades and Size | Wiring | Electrical Engineering

In this article we will discuss about:- 1. Classification of Wires 2. Types of Wires 3. Grade 4. Size.

Classification of Wires:

Wires may be classified into two groups:

(i) Ordinary wires and

(ii) Cables.

For different electric work conductors used are usually made of copper, although aluminium conductors are also used extensively now-a-days. In India availability of copper is extremely meagre in com­parison to its demand. A large quantity has to be imported from other countries. For this reason the use of aluminium conductor is being increased day by day.

At present even the armature and field coils of different machines and instruments are made of aluminium wire. For resistance coils alloys like eureka, nickel-chrome, german silver, platinoid etc. are used. For fuse wires, use of lead-tin alloy or copper wire is the normal practise.

Wires are used to serve mainly two different purposes:

(i) For the manufacture of armature and field windings of generators, motors, electric bells, electrical instruments etc.,

(ii) For house wiring and for drawing overhead transmission and distribution lines.

For armature and field coils and for underground cables annealed copper wire or soft aluminium wire is used; for house wiring and for overhead lines, hard-drawn copper wire or hard aluminium alloy wire is suitable.

While bare conductors are used for overhead lines, for all other work insulated wires are used. For the use in generators, motors and other apparatus, wires may be either cotton insulated or silk covered or enamelled. Again, cotton or silk covering may be applied on the conductor in single layer or two layers or three layers according to requirements of the system.

When single layer is applied, it is called single cotton covering (S.C.C.) or single silk covering (S.S.C.), when two layers are applied, it is called double cotton covering (D.C.C.) or double silk covering (D.S.C.) and when three layers are applied, it is called triple cotton covering (T.C.C.) or triple silk covering (T.S.C.). Wires used for house wiring and underground cables have various other types of insulation.

Types of Wires:

Different types of insulated wires used for house wiring and service connections are discussed below:

i. Vulcanised Indian Rubber (V.I.R.) Insulated Wires:

In this wire tinned copper or aluminium is used as conductor. On this conductor one or more layers of vulcanised Indian Rubber, i.e. rubber treated with sulphur at high temperature, is applied. In order to remove the bad effect of sulphur on copper or aluminium, the conductor is thoroughly tinned or a layer of pure rubber is applied on the conductor at first.

Finally the rubber insulation is covered either with cotton tape impregnated with moisture resisting compound such as bitumen or wax, or covered with cotton tape. This tape is again covered with cotton braiding impregnated with moisture resisting compound. Sometimes rubber insulation is covered only with cotton braiding impregnated with moisture resisting compound.

V.I.R. wires are single-core type. These wires are usually used in conduit wiring, wood casing & capping wiring, cleat wiring etc. But now-a-days single-core P.V.C. wires have largely replaced V.I.R. wires from different fields of application.

ii. Tough Rubber Sheathed (T.R.S.) or Cab Tyre Sheathed (C.T.S.) Wires:

In these cables also tinned copper or aluminium conductor is used. Over the conductor a layer of pure rubber or rubber treated with sulphur is applied. As outer protective layer tough rubber sheath in T.R.S. wires and cab tyre sheath in C.T.S. wires are provided. These sheaths are not readily affected by sun or water. Hence, they may be used in damp places or in open atmosphere.

T.R.S. or C.T.S. wires are cheaper in cost and light in weight. Such wires are available in single-core, twin-core, three-core and twin-core with an earth continuity conductor. Single-core and twin-core wires are widely used in house wiring, and twin-core and three-core wires are used for giving supply to cranes, hoists etc. and for drawing service connections from overhead distribution lines or from one building to another. The rubber insulation over each conductor has usually different colour for different core.

For service connections and house wiring at present P.V.C. wires are preferred over T.R.S. or C.T.S. wires.

iii. Metal Sheathed or Led Sheathed Wires:

In these cables V.I.R. wires are provided with a continuous lead sheath covering in place of cotton tape or cotton braid sheath. Lead covering is mechanically strong and moisture proof. These wires are used in lead sheath wiring system. Owing to its high cost as compared to C.T.S. wires, lead sheathed wires are not usually preferred for internal wiring. These cables are used for service connections and for wiring under abnormal climatic conditions having lot of moisture.

Lead sheathed cables are available in single-core, twin-core flat, three-core flat and twin-core flat with an earth continuity conductor. Here also rubber insulations over the conductor have different colours for different cores.

iv. Weather Proof Wires:

Weather proof wire is normally used in outdoor work where the wire remains exposed to open atmosphere. These cables are made of V.I.R. insulated wires suitably taped, braided and compounded with weather resisting material. The conductor is made of tinned copper or aluminium over which a layer of rubber treated with sulphur is applied.

The rubber insulation is covered with cotton tape soaked in insulating varnish. Finally the cotton tape is covered with cotton braiding soaked in an insulating compound which is not affected by the sun, water or humidity.

Weather proof cables are used for giving supply to industries and for outdoor wiring at low and medium-voltages.

v. Wires with Thermo-Plastic Insulation (P.V.C. Wires):

In these cables conductor is insulated usually with polyvinyl chloride (P.V.C.) which is a thermoplastic material. This type of insulation is not affected by acid, alkali, ozone, humidity or the sum rays. Rubber insulation is very quickly deteriorated when comes in contract with oil, but P.V.C. remains unaffected. P.V.C. is much harder than rubber.

Hence no further protection like cotton tapping or cotton braiding is required against mechanical injury. It is inflammable and it burns so long it is in contact with a flame. But as soon as the source of flame is removed, it stops burning. Thus it proves that P.V.C. insulation does not help combustion.

P.V.C. becomes semi-melted when excessively heated and becomes brittle under extreme cold condition. Therefore P.V.C. wires cannot be used for giving connections to the heating appliances and other such applications. Similarly, it cannot be used for wiring in a place exposed to weather where there is frequent snow fall.

P.V.C. wires have replaced V.I.R. wires and C.T.S. wires in conduit wiring and in C.T.S. wiring where wire is run on wooden batten. P.V.C. cables are also used as power cables, control cables, signal and communication cables etc.

Besides P.V.C. other thermoplastic insulations like poly-chloroprene, polythene are also available in the market.

Grade of Wires:

For house and industrial wiring at present 250/440 volts grade and 650/1100 volts grade wires are usually used.

(i) 250/440 volts grade cables may be used where d.c. supply voltage does not exceed 250 volt, or the line voltage of a.c. supply does not exceed 415 volts.

(ii) 650/1100 volts grade cable may be used where the circuit voltage exceeds 415 volts, but the pressure between line conductor and earth does not exceed 650 volts.

In India cable manufacturing concerns manufacture cables according to Indian Standard Specifications (I.S.S.).

The numbers of specifications laid down by Indian Standard Institution are given below:

Size of Wires:

The current-carrying capacity of each conductor is limited. When current flows through a conductor, the conductor gets heated. This heat is proportional to the square of the current. Again, there is a limit to the degree of heat which a particular insulation can withstand safely.

According to specification laid down by the Indian Standard Institution the maximum currents which can safely flow through the wires of different sizes with different types of insulations are fixed. The specification is given in table no. 5. If this specification is not strictly adhered to, there is possibility of damage of the insulation due to excessive heating of the conductor.

One method of expressing-the size of electric wire is in terms of Standard Wire Gauge (S.W.G.). The gauge commonly used is the British Standard Wire Gauge. This is given in table no. 6.

In order to determine the wire, gauge number, a circular steel disc with different sizes of holes all along its periphery (fig. 73) is used, each hole representing a particular size. It can be seen in fig. 73 that each hole has a slit attached to its side, and different slits have different widths. In front of each hole there is a number.

For determining the gauge number, a wire is drawn through different slits. The number mentioned against a slit in which the wire is neither slack nor too tight is the gauge number of that wire. For example, if the number against a slit in which the wire is neither slack nor top tight 18, the size of the wire is 18 S.W.G.

If the wire consists of a single conductor, it can be expressed as 1/18. Similarly, if the size of the wire is expressed as 37/16, it means the wire has a stranded conductor with 37 strands and the size of each strand is 16 S.W.G. The largest size of wire available in the market has a number of “0000000” (seven zero) S.W.G., while the smallest size has a number of 48 S.W.G.

At present the size of a wire is not expressed in terms of S.W.G. number, but in terms of diameter of each strand. In column 1 of table no. 5 the sizes of wires are expressed in terms of number of strands and the diameter of each strand in inch, and the cross-sections of the wires in inch are given in column 2.

In column 3 the size of a wire is given in terms of number of strands and the diameter of each strand in mm, while the cross-section of the wire in mm² is given in column 4. Thus the size of the wire previously expressed as 37/16 is now expressed as 37/.064″. It means the stranded conductor of the wire has 37 strands and diameter of each strand is 0.064 inch.

The wires available in the market are usually denoted by cross-sectional area in sq. inch or in sq. mm. Copper wires are manufactured with diameters measured either in inch or in mm, but aluminium wires are manufactured with diameters measured in terms of mm only. In table no. 5 the current-carrying capacity of both copper and aluminium conductors has been given. The conductors have the capacity given in the table at an ambient temperature of 30°c.

At temperatures exceeding 30°c the conductors must not carry the currents given in the table. In that case the conductor should carry a current which is given by the current shown in table no. 5 multiplied by a factor shown in table no. 7. Otherwise the insulation of the conductor will be damaged due to excessive heating.

For example, let the ambient temperature of a place be 35°C. Let a 2-core, copper conductor, P.V.C. insulated wire is to be used. From table no. 5-it can be seen that the maximum current-carrying capacity of this wire at 30°C is 62 amperes. Now, since the ambient temperature of this place is 35°C, the maximum current which should be allowed to flow through the wire must not exceed 62 x 0.87 = 54 amperes.

From the following discussions it can be understood how one can use table no. 5:

(a) If the circuit current is known, the size of the cable can be selected from the table. Let a single-core cable is to carry 15 amperes. In the fourth line of column 5 of the table, current rating is mentioned as 15 amperes. Following this fourth line it can be found in column 1 that the size of the copper conductor for 15 amperes current is 3/.036″. Also in the fifth line of column 5 the current rating is 15 amperes. In the fifth line of column 3 the size of the aluminium conductor for 15 amperes is mentioned to be 1/1.80 mm.

(b) If the cross-sectional area of a conductor is known, its current-carrying capacity can be determined from the table. Let the cross-sectional area of a conductor be 0.003 sq. inch. This cross-section can be found in the fourth line of column 2. In the fourth line of column 1 the size of the cable with copper conductor is mentioned as 3/.036″. Depending on whether the cable is 2-core or 3-core the current-carrying capacity of the cable may be determined either from column 5 of from column 6.

In this connection it is to be noted that the current-carrying capacity of a conductor is not fixed.

It depends on the following factors:

(i) Permissible voltage drop at the far end of the feeder.

(ii) Permissible temperature rise of the conductor.

(iii) Whether the cable is single-core or multi-core.

In a d.c. circuit or in a single-phase a.c. circuit either a twin-core cable or two single-core cables may be used. Similarly, either a 3-core cable or three single-core cables may be used in a 3-phase a.c. circuit. The surface available for heat radiation in single-core cable is larger than that available in a twin-core or a three-core cable.

Hence, for the same size of conductor and at the same ambient temperature, a single-core cable can carry more current than a twin-core or a three-core cable. This can be clearly understood from the current ratings given in column 5 and in column 6 of table no. 5.

Thus, the cross-section of conductors in a cable is determined either considering the permissible voltage drop along the length of the conductor or taking into account the permissible temperature rise in the conductor. In lighting circuits usually the voltage drop up to the farthest point of the load is considered.