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Answer Posted / 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
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