Cable Selection

1. Lecture Notes by Dr.R.M.Larik
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2. ▪ The selection of cable size is based upon the following factors:
▪ Current carrying capacity
▪ Voltage drop
▪ Short circuit rating
▪ These factors should be evaluated before selecting a cable size! In many instances
voltage regulation and short circuit rating factors are overlooked.This oversight
can result in danger to property and personnel, as well as destruction of the cable
itself.
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3. ▪ The current carrying capacity of a cable is based upon its thermal heating.
▪ Thermal dissipation
▪ Safe CCC- maximum permissible temperature rise.
▪ Cause of temperature rise
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4. ▪ Thermal dissipation is affected by:
▪ Ambient temperature
▪ Proximity effect : A horizontal spacing is desirable in order to reduce the effect of mutual
heating. In case they close or touch one another the effect of mutual heating increases and
accordingly, the current carrying capacity of cable decreases sufficiently.
▪ Thermal resistivity of soil : When the cables are buried in the ground the heat transmitted
through the cables passes into the surrounding soil.Therefore, soil thermal resistivity
should be an essential preliminary to the determination of the correct current for a given
cable.
▪ Method of laying
▪ Directly buried
▪ In a duct
▪ Cable tray in air
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5. ▪ Electrical load of 80KW, distance between source and load is 200
meters, system voltage 1000 V three phase, power factor
is 0.8, permissible voltage drop is 5% Cable is directed buried in
ground in duct at the depth of 1 meter. Ground temperature is
approximate 35 Deg. Number of cable per trench is 1.
▪ A 3 core XLPE armoured cable with PVC sheath is used, Copper
conductor.
▪ Thermal resistivity of the soil is 2 K.m/Watt
▪ I= P/(1.732*V*pf)=57.73 A
▪ Iccc =I/ derating factor
▪ Derating factor= product of various correction factors
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6. ▪ Grouping factor= 1
▪ Ambient temperature factor= 0.89
▪ Soil thermal resistivity factor=1.05
▪ Depth of laying factor= 1
▪ Correction= 0.89*1.05=0.93
▪ Table 40
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7. ▪ Iccc= 57.73/0.93 = 62 A
▪ From the table we select 16 sq mm cable which has a capacity
of 75 A.
▪ Next is we check for voltage drop.
▪ 2.5 mv/A/m
▪ Voltage drop= 2.5*57.73*200/1000= 28.865 V
▪ % voltage drop is 2.8865% and is permissible.
▪ Cable short circuit capacity should be higher than system short
circuit capacity at that point
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