CABLES USED IN LOW VOLTAGE SYSTEMS

Mainly three types of cables used PVC/CU ; PVC/PVC/CU ; XLPE/PVC/CU . There are various conductor sizes for cables 1,1.5, 2.5,4,6,10,16,25,35,50,70,95,120,150,1`85, 240,300,400,500 ,630,800,1000 mm2 . When the cross section size of the cable is high , the current carrying capacity of the cable also go high . as well as the width of the insulation increased ,voltage drop propotionally decreased.For every cable there is a recommended voltage drop for per amp/ per meter (mv) .

Cables are usually equipped with cable ties and cable glands . Armoured cables use Cu/ Brass glands & non armoured cables use plastic glands for low cross section areas and Cu /Brass glands for relatively high cross section areas .

Cable laying methods -: 1) Perforated tray 2) Cable Trunking 3) Laying in trenches 4) Laying in conduits 5) Ladder support 6) Cable Ducts
Supporting metal work of the ladder support ,under the cable occupies less than 10% of the plain area. The perforated tray having not less than 30% of it's surface area removed by the perforation .Cable ties are used to lay cable in cable trays .

Cable Selection methods -:
Voltage Drop (Vd) calculation ; Vd =( L*I* Vt)/ 1000 :Vp>Vd
Vd - Actual voltage drop ; L - length of circuit ; I - The current to be carried out by the cable
Vp - Max permissible volt drop 4% of supply voltage in volts .

Procedure of selected cable :
1) Determine the design current of the circuit (lb)
2) Select the type of overload protective device (ln) : (ln > lb)
3) Select the type of cable & method of installation ; whether it is pvc/cu or pvc/pvc/cu or XLPE/pvc/cu or lay in duct ,tray ,ladder ,trench
4) Determine the rating factors ; ca,cg,ci,cf
5) Calculate the current for the circuit taking into account all correction factors
Ic=(In)/ (ca*cg*ci*cf)
6)Select the cable having current carrying capacity more than that It > Ic
7)Effective current carrying capacity of the cable ; Iz = It* ca*cg*ci*cf

cg - correction factor for grouping ca- correction factor for ambient temperature
ci- correction factor for thermal insulation cf- correction factor for protection device
Ic - Calculated current for circuit It - Tabulated single circuit carrying capacity of cable
Iz- Effective current capacity of a cable
Finally we consider about the It value and permissible voltage drop for select a cable .If the cable is non armoured , up to 16mm2 earth cable size is same as the size of main cable ,above 16mm2 earth cable size is half of nain cable.