In usual practice, the carrying capacity of copper is taken
as 2.5 A/sq mm and 1.5 for Al.If the load is steady and
continuous 60 % of these values are adopted.

There is no formula for calculating the cable size. Usually
the cables are sizing based on standards like NEC,BEC.,
etc. if you want to size the cable you first choose the
std, which is using in ur location.as per NEC std just
multiply the 100*1.25=125A, choose the cable ( table-
310.16) for that size. after you choose the cable you have
to derate the cable for your laying location & way of
laying.

Theory for Cable Sizing.
From Ohms law, the voltage drop in an electrical conductor is
V = I R
where
I = Current, amps
R = Resistance, Ohms

The electrical resistance can be derived from the properties of the conductor material.
R = ρ L / A
where
ρ = Resistivity, Ω mm2 /m
L = Length, m
A = Cross sectional area, mm2
Watch the units in the above equation.

The resistance can also be expressed in terms of the material conductivity (ψ) which is just the reciprocal of the resistivity.
R = L / (ψ A)

Table 1. Typical material electrical conductivity
Material Conductivity, ψ
Copper 58
Aluminium 36
Mild Steel 7.7

Combining Ohms law with the resistance expression, we get.
V = (I L) / (ψ A)

Now, we can define an acceptable voltage drop in a conductor. I have calculated the voltage drop that was used in several published tables and get around 5.5 Volts.
We could therefore re-arrange this equation to give the cable size. But cables come in standard sizes measured in mm2. In addition, you also need to consider the maximum current for each cable size.

Table 2. Conductor sizes and maximum currents
Cable Size, mm2 1.5 2.5 4 6 10 16 25 35 50 70 95 120 150 185 240 300
Maximum current, A 13 21 28 36 46 61 81 99 125 160 195 220 250 285 340 395
So, the theory is relatively simple, let us now see how to use the program.

there is no funda or formula to get size of conductor wrt
to load. just calculate the ampere of the load, and then as
per ampere ,,, as per the cross sectional area of the
conducter like 1.5mmsq can bear the 4-6amp load,,,so on..

My switchgear busbar is rated at 2000A and just imagine the actual load on busbar is 1000A. From that busbar there are 10 outgoing feeders. Now the question is will the load be 1000A on each outgoing feeder or 100A per outgoing feeder. 2nd thing how do i calculate the load on individual feeder. Thanks for your time. Please provide some clarification on this issue.

low voltage means how much voltage?
extra low voltage means how much voltage?
medium voltage means hos much voltage?
high voltage means how much voltage?
Ex: low voltage - 1volt to 240volt like this

generally on the roadside v hav 11kv line which is being
step down to 440v line to line....so any1 can tell me which
type of transformer they r using whether it is star to
delta,delta to star etc...???