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How do you select a cable size(Cu&Al) for a particular load
 Question Submitted By :: Electrical-Engineering
I also faced this Question!!     Answer Posted By  
# 1
While Calculating The Proper Size Of Cable You Should Know
The Full load current Also You Should Consider The Derating
Factor Of Cable , Cable Current rating varies according to
different types of Cable distribution ie where it above
ground or underground.
Is This Answer Correct ?    47 Yes 8 No
# 2
Calculate the full load current(10% more).Choose prper
rating of Cu & Al. conductor from the chart.
Is This Answer Correct ?    43 Yes 19 No
Panda N.n
# 3
depending upon full load current of the load, the size of
cable is selected. Also calculate the % voltage drop and
see it is in permissible limit or not if is more select the
next highier size of cable.
Is This Answer Correct ?    33 Yes 9 No
Tauseef Tarique, Engineer(e)
# 4

Is This Answer Correct ?    21 Yes 4 No
# 5
A cable said to adequately sized when all the following
conditions aer satisfied:
1. Current carrying for steady state operation
2. Short circuit withstand capacity
3. Voltage Drop (steady state as well as during motor

As cable data provided by manufacturers/standards (ERA
report 69.30 etc) is for certain test conditions, which may
be very different from actual site conditions where the
cable is being installed.

Hence, to obtain the actual current carrying capacity of
the cable during steady state operation of the instalation
coditions of the cable shall be considered and suitable
derating factors (derating factor for group of cables,
Ambient temperature correction factor, derating factor for
thermal resistivity of soil, derating for depth of
installation) shall be multiplied to the manufacturer
supplied data. tehse values could be supplied by the
manufacturer or u can refer standards such as ERA 69.30 or

The short circuit withstand level of the cable must be
greater than the energy let through value of the protective
i.e. k2S2 >I2t where,
k = cable core insulation constant (143 for
S = Cable cross sectional area in mm2
I = Prospective fault current, rms in A
t = Disconnection time of protective device (in

The voltage drop in A.C. cables shall not be more than 5%
based on the continuous maximum current loading and rated
voltage. During motor starting or re-acceleration the
transient voltage dip at the terminals of any motor will
not exceed 18-20% of the rated equipment voltage.

A cable of length L (km) having a resistance of RC (ohm per
km) and a reactance XC (ohm per km) operating in a three
phase ac system carrying a balanced current I (Amps) with a
power factor of Cos Φ will develop a voltage drop ΔV
1. For three phase ac systems
ΔV = √3 x I x L x (RC.Cos Φ + XC.Sin Φ)
2. During starting of 3 phase motors ( with a starting
current of IST at a power factor of Cos Φ) the voltage drop
will be:
ΔV = √3 x IST x L x (RC.Cos ΦST + XC.Sin

3. For single phase ac systems:
ΔV = 2 x I x L x (RC.Cos Φ + XC.Sin Φ)
4. For DC systems:
ΔV = 2 x I x L x RC
The percentage volt drop relative to the rated voltage is
given by:
Vd = (ΔV x VL) /100 %

Also, one may need to size the cable for earth loop
impedance considerations.
Is This Answer Correct ?    17 Yes 5 No
# 6
Factors like the length of the cable, resistance, current
being carried by the cable , short circuit current, voltage
drop all come into play when selecting cable to be used.

First calculate the load current

I = P / x V x pf
pf =0,9 according to TCO Standard P-ST-2035, point 9.9

Then find apparent power

For three phase: S = x V x I
For single phase: S = V x I

Then I find the resistance by Ohm's law:

Z = V / I
V - is particular Load

Then find the short circuit current

I short circuit = S / Z

Then find cable size

S = I sh.circuit x / k k = 143, t = 1

And then select the cable according to the cable catalogs
Is This Answer Correct ?    3 Yes 0 No
# 7
verify motor capacity or kw 
Is This Answer Correct ?    7 Yes 5 No
# 8
Dear Friends,

cable types - XLPE/PVC /SWA

we have to choose the cable depend upon the applied
area/current rating

first step to find voltage drop for a required length as
per IEC regulation the VolDrop should not exceed 4% of
supply voltage it can be calculated

VD= mv/meter/amps this can be get it from cable chart -
supplier .if its ok then we can use

Is This Answer Correct ?    7 Yes 7 No
# 9
Generally for copper cable sq.mm2 *2.5
for alluminum cable sq.mm2 *1.5
Is This Answer Correct ?    6 Yes 6 No
Muralidhara Kumar
# 10
wire size is depend upon the type of wire.
if we used the CU wire then the size of wire should be the 1/5 part of load current and AL wire wire will take around 1/4th of the load current
Is This Answer Correct ?    2 Yes 2 No
Rahul Singh

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