Answer / sumit salunke

Theory
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

Answer / vinothbabu.k

genraly cable slection was based on Rated load amps,voltage
drop,& fault current.
cable which can carry the 1.5 times of the rated amps,cable
shall be consider..

Answer / arun

Based on the load criteria we need to use specific formulae.

For example if it a motor then voltage drop calculation is
required, samething for transformers, generators, MCC and
other loads

can you any one give exact mathematical relation between AC current and DC current?

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