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
Theory for Cable Sizing.
From Ohms law, the voltage drop in an electrical conductor is
V = I R
I = Current, amps
R = Resistance, Ohms
The electrical resistance can be derived from the properties of the conductor material.
R = ρ L / A
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
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..
Is This Answer Correct ?
Other Electrical Engineering Interview Questions
Why we transmit the power at the voltage in the multiple of
11 or 1.1 like 11kv, 22kv, 66kv, 132kv...... some one told
me that it is due to fomr factor 1.1 of sine wave. its fine
but while transforming we can choose any voltage, so why
only multiple of 1.1 there also.......? plz tell me.
what is the problem in manufacturing transformer with 0
degree phase displacement.???why alwasys go for DYn11 or
DY5?Whys not DY0 i.e zero degree displacement between
primary and secondary side vectors?