There is no need to calculate ampacity of the cable it would
be provided by the manufacturer its self or else if you want
to calculate it by your self you should also know its
specific resistivity,working temp,core dia & insulation
thickness.

Current carrying capacity = FLC / DF
To refer cable catelouge choose the cable sizes
-Derating factor (DF) =0.72
-Full load current (I)= P / (1.732 x V x P.F)

For calculate the current carrying capacity.
First we should calculate the cross section area of the
conductor (sq.mm) and conductor material used in cable.

Each and every matals are having its own current carring
capacity. This table is avail at IS.

Current carring capacity may reduce by some other factors
like the cable length and insulation's heat withstandable
capacity.

The current carrying capacity of a cable is called
Ampacity. For the porpose of the above calculation there
are some concepts that must be taken in consideration .
like Dia,Insulation material and thickness,type of conductor
(Copper/Aluminium),Resistivity and the working conditions.
For BETTER CONCEPT I advice the readors to please refer
the "Recommended Cable Sizes and their load capacity as per
IS:13947 & IEC 60947". Below given table can help you.
Rating Cu Al
10A 1.5MM2 1.5MM2
15A 2.5MM2 2.5MM2
20A 2.5MM2 4.0MM2
25A 4MM2 6MM2

Actually current carrying capacity of the cable we can from
the manufacturer, But for the referance purpose I can give
some idea about current capacity of the cable.
So generally we are using two types of cable i.e,. copper &
Aluminium.
copper is the best conductor than the aluminium
At 1Sqmm copper conductor will Approx 2 amp in ground
At 1Sqmm copper conductor will Approx 2.4 amp in air
At 1Sqmm Aluminium conductor will Approx 1.2 amp in ground
At 1Sqmm Aluminium conductor will Approx 1.6 amp in air.

For example:
p=50kw
load current=87amps
So
for aluminum cable size(air)= load current/1.6
=87/1.2
=72.5
But In the market there is no availability for 72.5 Sqmm so
we can propose 70Sqmm or 95Sqmm.
This is the one of the best procedure for for cable
selection with using catelogues

Well, First try to calculate the total load of the system.
For example total load is 1000KW. The current can be
calculated as P=1.732VICOS(PHI). Hence total current will
be 1732A by considering V=415volt, power factor=0.8.

Step2: To calculate the cable size

Vd = mV/L/A

where Vd= Allowable voltage drop which is normally 2.5
percentage to 4 percent.
L = Lenght of the cable from the source to feeder
A = Ampere rating

Let us consider L = 1000Meters
A = 1732A
Vd = 2.5 percentage of 415V as in U.A.E.
= 2.5*415/100 = 10.375

Use these value in the above formula

mV= Vd/L*A
= 10.375/1000*1732
= 0.005 volt

Refer to the cable catalogue in your region and find out
the current carrying capacity of the cable which will be
totally depend upon the atmospheric condition, in cold
place a smaller diameter of cable can carry larger current
and in hot areas the same size will carry lesser current.

In U.A.E. we are normally using 11x1c 630sqmm cable with
2500A incomer breaker. But cross check the voltage drop as
calculated above is within the limit of voltage drop of
these cables else you have to go to higher size of cable
untill the voltage drop met the design milli volt drop of
the cable.

while doing the excitation current test on power
transformer what is the value of voltage to be applied?
Nomonal rated voltage or 10% of rated voltage? Is it
necessary to apply on high voltage side?