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

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)

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

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

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.

can I connect step down transformer(25KvA,240V Phase to
phase)to output of online UPS(15KvA,440V PHASE TO PHASE)
Load connect to TRANSFORMER is 13KvA,240V PHASE TO PHASE.
please brief if it is wrong