Answer / vinod sharma

In a three phase system p=1.732x3xVxIxCOS@
When load is given 50KW
Then I=50000/1.732x3x415x.8
=86A near about
taking some future expectancy you can take a cable of
having 100A capacity.SO by this u can take a cable of 70mm
square in aluminium and 50 mm square in copper

Answer / engineer

Right cable size Sellection depends on 3 factors:
1- Load Current.current(I) = 50kW /(1.732*415*0.85
(p.f)) =139A

2- Distance(From feeder to your load)
3- Allawable Voltage droop in your system (Should be less
than 5% )

As experience you can use 10sqrmm for 40A,for 120meter.

Answer / mohammad farman khan

Fisrt calcultate Load in Ampere as basic formula below

I (Amp ) = P ( KW) / 1.732*Voltage (400 or 415 or 380 )*
p.f

Voltage depend on frequency rating (50 Hz or 60 Hz)
then check where you have to laid in AIR or Ground
you have to consider so many factor.
suppose you laid in Ground then factor to be consider as
below
(1) Soil Thermal Rseistivity
(2) Group factor
(3)Rating factor of ground temprature
(4)Rating Factor depth of laying

then calculte Voltage drop on behalf of amper rating by
load.
once you calculate then check length

V.D = mv x I X L/1000 OR mv =V.D X 1000/IXL

VD= Maximum acceptable volt drop (in volt)
mv =appropriate volt drop (in mv /amp /metre
I = Current per phase (in ampere)
L= Length ( in meter)

Answer / sunil patil

Given- KW-50
Now from formula I= KW X1000/(V X I X 1.732 X .8)
=86.95 A
Now by refering fenolex cable rating table
For 25mm2 =80A Which is less than our o/p current so we
can select 35mm2 having current capacity is 102A

Answer / m.saad

Calclate the load Current
I = p/((root3)*V*P.F )
where I line current, v line voltage
then go to cable tabe
and check the Drateing factors.such as
if this cable underground,free,soil,depth of laying,
ambient temperture, armoed or unarmoed etc.

then calcute the voltage drop
if more than 4%
you should to select the next bigger cross section at table.

by experince for copper PVC cables
1mm take 2.5 AMP

Answer / shabeeb

50kw load will take 87 amps (approx.). There is table of
different types of cables available in the manufactures
site. More over there is so many factors are there to
select a cable. Such as the type of material (copper /
aluminium), single core or multicore, how and where the
cable is being laid, the length of cable(for voltage drop
calculation), type of load and its starting current (for
motors), future load etc. Normally if the load is 50KW,
70sq.mm Aluminium cable or 50sq.mm copper cable can be used.

Answer / ajith

380V/50HZ,50KW=4core50mm

Answer / mathhar

LOAD : 50kW , 415V , 0.8p.f
1- I = 28.75A
2- 28.75 * 1.2 = 34.5A
3- temp. factor * group factor = 0.75 "ex."
4- 34.5/.75 = 46A
5- choose 16mmsq
6- cal. Vd = (I * L(m) * delta(v)/ (V * 1000))*100%
its should less than 5%
7- short circuit temp. rise

Answer / imtiaz ahmed

i m an engineer and i want to formula to calculate the size of cable in the form of load killowatt
kindly send me this formula wi appropriate example even then i understand easly.
this will be your greatness.


thanks and regards
imtiaz ahmed
from kashmir
cell no +92 345 9748199

Answer / electrical engineer kaleem 055

As my 5 years of experiance. I am still working in construction field as Electrical Engineer. Let me tell you 1 easy and good method to find the cable size for load , Earth cable size for load and to find the exact circuit breaker size which is require for load.
Always keep it in your mind that if load will 30kw or 20kw or 50kw etc always select the cable size 1 step down from the given load.Example for 30kw load cable size will be 25sqmm. and for 20kw it will be 16sqmm.etc
for earthing cable always load divided by 2. Example for 30kw load earth size will be 15 but in market 15 is not available so use 16sqmm yellow green cable and for 20kw load use 10sqmm cable etc.
Now for circuit breaker.
Always load multiple by 2 so that will be your mcb size.
Example if you are using 30kw load
30 x 2 = 60 so you will use 60 ampear mcb for 30kw load.And for 20kw use 40ampear breaker.

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