Selection of Electric Cable

Selection of Electric Cable Points : Selection of Electric Cable, Electrical Considerations, Bunching, Selection of Electric Cable At first sight, the selection of one particular type and size of cable seems very easy. All that needs to be done, so it appears, is to find out the current the load will take, look up the nearest cable rating to this in the I.E.E. tables, buy the appropriate roll of cable and install it.

Unfortunately, as with many things, complications arise which are not at first obvious, and the purpose of this section is to outline most of the factors affecting the choice of a cable for a particular job. Details are given of these factors grouped under two headings, Electrical Considerations and Mechanical Considerations. Electrical Considerations The following is an example to illustrate that there is no simple answer to the question “How many amps will a 19/.083 cable carry?”

There are in fact, four main items which effect the current carrying capacity of a cable. These are:-

Bunching
Method of installation
Volt Drop, Temperature Bunching If more than two single core cable, or more than one twin core cable are run close together, then the rated current that they can carry must be reduced. One of the reasons for this is that a current flowing in a cable sets up a magnetic field or flux around it. If 3 or over cables are run together the flux from one cable interferes with the adjacent cables and creates an apparent increase in resistance. Method of Installation If more than two cables are bunched together in conduit, their current rating is different from the same size cables run on battens. The current rating again is different for cables run in cleats. These differences are due to the flux interference mentioned previously and also to the cooling of the cables available.

Obviously, for cables enclosed in conduit, there is no free flow of air past the cables to cool them; the air inside the conduit is static and so not much cooling is available. However, if cables are freely around each cable and give maximum cooling. With cables run on battens, the cooling is not quite so effective if more than two cables are run together since air cannot pass around the surface of each cable but it is still better than the equivalent if more than two each cable but it is still better than the equivalent number of cables in conduit. If P.V.C sheathed cables are buried directly in plaster the cable will conduct more heat away from the cable than if it was in conduit.

Since the temperature effects the current carrying capacity of the cables, then the better the cooling the more current the cable Table I to 20 for cables run in conduit, cleats, battens or buried direct in plaster. Thus a cable has a different rating for each method of installation and so the table corresponding to the method employed should always be used. Volt Drop Since every cable has a definite resistance, then there is always a volt drop along a cable supply current. With short runs of cable, the volt drop is generally negligible, but with, long runs of cable of small cross sectional area, the volt drop can become an important factor.

The voltage drop from the incoming supply terminals to any point in an installation should not exceed 1 +2% of the declared supply voltage (this does not apply to motors). This means that for a declared supply voltage of say, 250 V, the maximum allowable volt drop is:

1+ 2/100 x 250 = 250 = 1 + 5 = 6 volts.

If the incoming Voltage stated by the supply company is 250 volts, then the volt drop calculation must always be done using this figure, even if the voltage as measured by a voltmeter is only, say, 220 volts).

As there is also a volt drop m the circuit wiring from the distribution board to various sub circuit points, the volt drop at the load being supplied should half the above Value i.e 3 volts.