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how to choose the cable size for 100A(is there any formula 
please update)
 Question Submitted By :: Electrical-Engineering
I also faced this Question!!     Answer Posted By  
# 1
According to NEC , i want to reduced grounding cable from
4mm2 to 2.5 mm2 ,can u give me technical sheet to depend it
from NEC
Is This Answer Correct ?    1 Yes 2 No
# 2
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.
Is This Answer Correct ?    2 Yes 3 No
# 3
Cable size should be 25

For Ex: 1000A MCCB
For Al - We have to Consider 0.8/
For Cu - We have to Consider 1.6/

For 1000A = 1000*0.8
= 800A
We have to go for (100X10)mm Bus Bar
Is This Answer Correct ?    3 Yes 5 No
# 4
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 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
Is This Answer Correct ?    1 Yes 3 No
# 5
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
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.
Is This Answer Correct ?    0 Yes 2 No
# 6
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 ?    4 Yes 16 No
Amrish Sharma

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