Practical Advice on Choice of Electric Cables

Practical Advice on Choice of Electric Cables Points : Practical Advice on Choice of Electric Cables, Example of Choice of a electric Cable Advice on Choice of Electric Cables Fig sets out in a convenient manner, recommended cables for most conditions of installation. To find the correct range of cables for a particular method of installation, look up in the first column the method that will be used, and opposite that will be found’ a range recommended V.R.I. and P.VC., insulated cables manufacture. All these cables are illustrated in Figs. Together with their respective reference numbers. Having previously determined the size of conductor, number of cores, voltage grade and type of insulation, the cable best suited for- the job can then easily be selected and ordered.

Fig. also serves to remind engineers of the voltage limits of various methods of installation e.g. heavy gauge conduit, trunking or casing can carry medium volts supplies (up to 650 volts), cleated wiring can carry a supply of up to 440 volts, but only low voltage supplies (up to 250 volts) may be carried by cables run in light gauge conduit, wood casing or battens.

The Pakistan Cables Limited reference number consists of five consecutive indices, sometimes followed by a Icier. The first two indices give the voltage rating the cable i.e., 04 indicates a cable the rating of which is 250/440 volts, and 05 indicates cable the rating of which is 660/1100 volts. The third and fourth indices a single core 2 for a twin core etc. The suffix letters are used where the construction of the cable flexible cord is similar to a standard V.R.I. type, except for say, insulation. For example X means P.V.C. insulated instead of V.R.I and Y means P.V.C. insulated and P.V.C sheathed, instead of V.R.I. and tough rubber sheathed. If the reference number is not followed by a letter, then the cable is vulcanized rubber insulated.

Example of Choice of a Cable An example is given below of a typical problem faced by an electrical engineer in an industrial concern. E.g. A new distribution fuse board is to be installed in an existing part of a chemical factory. This will supply some additional machinery that has been ordered. The problem confronting the factory electrical engineer is to select the correct sub-main cable to run from the main distribution fuse board to the proposed new distribution board. The details of the installation are listed below:-
Three phase supply stated to be 440 volts a.c.
Air temperature in factory regularly reaches 113°F in summer.
A duct has already been made in the concrete floor to the desired position of new fuse board. The duct is at present open along all its length so that the cable can laid straight in. When measured distance in 95 yards.

The three phase load to be supplied
Three —5 h.p. Induction motors.
Twelve — h.p. Induction motors.
4.5 kW heating and drying plants.
The fuse of this loading is estimated as follows-
Two of the 5 h.p. motors will run contentiously.
The other will be used occasionally.
At an/one time there will be approximately nine of the 1 h.p. motors running.
The 4.5 kW heating plant will be used all the time.