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.

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.

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
starting)

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
IEC.

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

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
(Volts):
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
ΦST)

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.

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

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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How to find Cable size?
Caterpillar generator produce (Two generators) 127*2=254 amp
at 11 kv .
What would be the cable size (XLPE) to supply 256 amp at 11
kv? Air or surrounding Temperature is 32 degree C. Cable
will go under ground at 2.5 feet depth. Group Rating Factors
is 3 cables. Clearance between cables is 7 cm. Soil
Temperature (Ambient) 28o C Soil Thermal Resistivity is 1.2
K.m/W
I would like to use 3 separate single core instead of using
3 core cable.
Some says I should use three separate single core 120RM cables.
Some says 70RM .some says 95.
I m totally confused.
Is there ant specific rule to determine cable size?
What would be the LT cable size to flow 500 Amp current?
Ifte
My e mail address is ifte13@yahoo.com
Anyone please send me the answer to my e mail address,
i will be very glad.