How to Select Cable for Electrical Installation
Table of Contents
Selection of Cable
How to Select Cable for Electrical Installation- Before carrying out electrical installation or wiring at any place, the selection of the most suitable cable is very necessary, because until the size of the selected cable is suitable, electrical appliances going to be connected with the cable will not be able to perform efficiently or correctly. For example, if an under-size cable is used, its resistance will be high. As a consequence of high resistance, its voltage drops will also be high, owing to which reduced voltages will be received by lamps, fans, etc. As a result, lamps will provide less light compared to their actual capacity. Fans’ speed will also be reduced. Moreover, wires will also get warmer, due to which wires remain at risk of catching fire due to the damage or weakening of their insulation. Moreover, as insulation weakens over time, issues like electric shocks as well as the melting of fuses frequently will emerge. However, if a proper size cable is selected before the installation, the voltage drops will be low, and lamps, etc. will provide full light according to their rated power. Further, wires will also heat up less, as a result of which wires insulation will not damage quickly, and the risk of wires catching fires, will also be reduced.
If an over-size wire is used, it will be considered much better. Because, in such a situation, voltage drops in the wiring will be extremely low, the wire will be heated less, moreover, the risk of fire will also be eliminated. However, electrical installation or wiring may prove more costly, in a situation of using an over-size cable, and it becomes hard for a common man to bear such extra expense. The selection of an appropriate cable before initiating wiring or an electrical installation is inevitably important.
Factors on Which Conductor Size Depends
The following factors or points from an electrical and mechanical perspective are kept in mind for determining the correct size of a cable or for finding out an accurate size of the cable to be used.
Conductor Material
At the time of selecting a cable, first of all, it is seen that what type of metal is the cable conductor being used for wiring, be made from? That’s whether it is a copper conductor or aluminum conductor? As the electrical characteristics of copper and aluminum are slightly different, therefore current ratings of both the metals are also somewhat different. Generally, copper is used as a conductor for residential wiring purposes.
Type of Insulation
The current carrying capacity of a cable also depends on insulation. For example, a butyl rubber insulation cable can be used at excessive temperatures compared to a commonly used PVC insulation cable, therefore, under the same or similar conditions, the rating of a butyl rubber insulated cable tends to be higher than PVC insulated cable. Thus, at the time of selection of a cable, the type of insulation of a cable is also taken into consideration, whether it is PVC, VRI, butyl rubber, or varnished cambric insulation cable, or else it consists of some other insulation type. Cables with inferior insulation, have an inferior efficiency, moreover, any sort of accident can also take place as a result of their application.
Load Current
The size of a cable also depends on the load current. As there are different parts of the wiring, and the load current of every part tends to be different (i.e., every part of wiring has a capacity to carry a different load current), therefore, the place wherever wiring or installation is desired to be done, load current of that place is determined through evaluating the power of all loads in watts (i.e., summing up the power of all loads) and then dividing it by supply voltage. (i.e., I = W/V). Then, keeping in mind the future addition, a minimum 10%, and maximum 20% allowance is also added to the actual load current. For example, wiring is desired to be carried out in a residential building, wherein the following loads are to be mounted.
5 lamps, with every lamp having a power of 100 watts, then total watts = 5 x 100 = 500 watts
4 lamps, with every lamp having a power of 75 watts, then total watts = 4 x 75 = 300 watts
2 lamps, with every lamp having a power of 100 watts, then total watts = 2 100 = 200 watts
Therefore, total watts = 1000 watts
Now, if total power is divided by 230 volt, we get load current;
Load current = watt / volt = 1000 / 230 = 4,34 amperes
20 % future current allowance = 4.34 x 0.2 = 0.868 amperes
Therefore, total current = 4.34 x 0.868 = 5.208 amperes.
Finally, the size of the nearest high rating cable according to this load current is selected with the help of a table for sizes of cables. It must be remembered that a cable having a rating higher than a load current should always be selected.
Method of Installation or Wiring System
The selection of a cable is always done based on the wiring system (i.e., what type of cable should be used for what type of wiring), because as a result of a change in the wiring system, most often the cable size also changes. For example, emission of heat occurs easily in an open type of wiring (which is commonly done cleat or batten system), because outer air always touches bare wires, and keeps the heat being produced in wires, exuding into the atmosphere. Inversely, fresh air does not touch wires in underground wiring, or conduit pipe wiring, therefore an adequate emission of heat cannot take place in such type of wiring. Similarly, one cable which is suitable for one type of wiring becomes unfeasible for another type of wiring. As such, the selection of a cable is always done according to the wiring system in such a way that no substantial heat be produced in the selected cables, and even if heat is produced, it can be emitted easily. In batten wiring, often a PVC wire, whereas in casing capping wiring a vulcanized rubber-type insulation wire is generally used.
Bunching
If more than two single-core cables or more than one twin-core cable pass nearby each other or are mounted close together, a magnetic field will be formed around each cable upon the passing of current through it, as a result, the value of the rated current passing through each cable will be reduced (i.e., the flow of current passing through the cables spread together will be reduced). As the flux of each cable affects other nearby cables by increasing their resistance, therefore rated current passing through the cables turns out to be low. It means that without power loss and overheating, a normal rated current cannot transmit through the bunching affected cables. Thus, bunching is also taken into consideration while selecting any cable.
Assume that we have to do conduit wiring in the presence of a 230-volt A.C supply. If we insert supply and load wires separately into the conduit pipe, a flux will produce around wires due to the conduction of the conduit pipe. As such, a power loss will occur as a result of the conduit pipe, and wires will also heat up. Therefore, during the choice of a cable, this point should also be taken into consideration the wires should be inserted into a conduit separately, rather as far as possible, wires should be put into one conduit so that the least damage to wires occurs and the effect of heat on wires also is least. In figure 6.1, both correct and incorrect methods have been illustrated.
Figure – Bunching conductors in conduit
Voltage Drop
Every cable has some sort of resistance, if a cable that has extreme resistance, is used, in such a situation more voltage drops will be high, as a result, a reduction will occur in the supply voltage, and appliances will not be able to operate correctly as a result of receiving low voltages. Moreover, as a result of an increase in a cable’s length, its voltage drops will also increase, because, with an increase in cable length, its resistance also increases.
According to regulation B-23 of the Institute of Electrical Engineering, total voltage drops at any point of the wiring (from the energy meter to the last point of the final sub-circuit), should not exceed 2.5 % of the supply voltage. As we have 230 volts as single-phase supply voltage, therefore total voltage drop should not be more than 5.75 volts (i.e., 2.5 x 230 / 100 = 5.751). Thus, while making a selection of any cable, a cable consisting of conductors having a low resistance, should always be selected. As voltage drop also occurs in circuit wiring from the distribution board to the different sub-circuit points, therefore the value of voltage drop on a sub-circuit should always be 1.25 % compared to the aforementioned value (i.e., 2.5 %).
Temperature
As the temperature of different regions tends to be different, therefore at the time of selection of cables, local temperature is also taken into account to assess how much temperature can they withstand both internally as well as externally. Moreover, heat produces as a result of the conductor’s internal resistance, and cable temperature also increases through passing current above its allowed rating.
Heat is closely related to the type of insulation. During summer, cable turns out to be warm due to heat, and its resistance also increases, as a result of which voltage drops occur, and appliances cannot work properly. Therefore, an insulation-type cable must be used for open wiring, so that it is not much affected by heat during the summer. PVC insulation generally comes up to this standard due to the feature that it can be used continuously at 70°C, and at 130°C at intervals without any kind of loss or damage. Whereas other ordinary insulation is greatly affected by temperature. Therefore, different cable manufacturing companies prepare cable tables according to the variations in the temperature. Therefore, whenever a cable is desired to be selected, a cable should be selected according to one’s requirements with the help of these tables. However, if a cable has to be selected with the help of a table prepared for a certain temperature, by determining the current at some other temperature, then in such a situation, it must necessarily be multiplied with a temperature factor.
An increase in temperature also depends on the load factor. If a full load current passes through a cable, a voltage according to the product of its resistance and current will produce, i.e., (V = IR), however, if the rate of current passing through it is low, the voltage drop will also become low with respect to that ratio. And, if the value of current passing through it exceeds its full load current rating, the voltage drop of the cable will increase, as a result cable’s temperature will also increase.
Cables are generally affected by the following two types of temperatures;
External Temperature; Temperature produced due to the working conditions around a cable, or atmospheric temperature around cables.
Internal Temperature; Temperature produced as a result of current passing through a conductor
Both external, as well as internal temperatures, should be given due consideration while making a selection of cable.
Mechanical Injury or Damage
Mechanical risks should also be considered while selecting a cable. In all those places, where risks of mechanical breakdown, mechanical shocks, shuddering, or mechanical injuries exist, such a cable should be selected which can tolerate all these risks, so that they can function efficiently without any kind of damage in the presence of these risks.
Tough rubber sheath (TRF) is one such cable, which can sustain mechanical breakage and shocks. Wherever there is a risk of any sort of damage to cables, a TRF cable is used in such places by enclosing them in conduits for more protection. For example, residential workshops, etc. An armored cable can also be used in places where quaking or jolts etc. are feared so that both conductors, as well as insulation, remain protected.
Nature of Work
In the place where wiring is desired to be carried out, a cable is selected according to the nature of work going to be done in that place. If the place is greasy, oily, or has chemical effects, a cable that cannot compete with such things, must never be used. Usually, a PVC cable is used in such places. In places having moisture, steam, and water, generally such type of a cable is selected which has an outer sheath around it, in order to do away with the effects of these things. Whereas, for household wiring, ordinary insulation cable without bearing any type of sheath, can be suitable. Wherever work has to be done nearby a fire, or high temperatures, a vulcanized rubber cable should never be used for open wiring, because a rubber insulation cable has a risk of catching fire.
Selection of Different Sizes Cables for Different Circuits
Another important point in the selection of a cable turns out to be the selection of different types of cables for a different types of circuits. For example, the load situation on different sections of wiring from an energy meter to a final sub-circuit tends to be different. A current equivalent to the total load of all sub-circuits from service box to energy meter, from meter to the main switch, and from the main switch to the distribution board, transmits, whereas the total load current of the final sub-circuits passes through the sub-circuits. Therefore, the selection of the most appropriate wiring cable can be done according to the current on every section or part of the wiring. For example, the current-carrying capacity of a main and sub-main cable tends to be higher than an ordinary wiring cable. Therefore, a current-carrying capacity of 66 to 100 percent is taken into account when selecting a cable despite not using the total load during any one time. At the time of selecting a cable for the final sub-circuit, it must be inculcated that the current passing capacity of a cable to the corresponding appliance, should be equal to the maximum capacity of that appliance (i.e., the capacity of a cable providing electricity to a 15-ampere socket, must essentially be 15 amperes).
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