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Electrical resistance is the opposition of a given material to the flow of electricity. The resistance of a an electric conductor is given by the formula:

R = eL/A
Where R = Resistance in ohms
            L = Length in meters
            A = Cross sectional area of conductor (c.s.a)
            e =  Resistivity of the conductor in ohms meter

Resistivity e = RA/L
If A = 1msq, then: resistivity is defined as the resistance per meter for a unit c.s.a
The two most popular conductors encountered in the electrical engineering field are Copper and Aluminium

The electrical resistance of a conductor is dependent on the following factors:
(a) temperature
(b) length of conductor material
(c) cross sectional area (c.s.a) of conductor

Ohm's law states that the voltage applied across a conductor is directly proportional to the current passing through the conductor provided the temperature and physical condition of the conductor remains constant.

This implies that:
V = IR
Where V = Voltage applied across conductor
            I  = Current passing through the conductor
           R  = Resistance of conductor

For direct current systems, resistance is the appropriate term to use. For alternating current systems, impedance is the right term to use. Impedance is the opposition to the flow of alternating current through a conductor. Impedance is as a result of resistive, capacitive and inductive effect of alternating current on the conductor. For small sizes of conductor, the capacitive and inductive effects due to alternating current is usually negligible and the term resistance could be used

Often in electrical works, there is the need to determine approximately the resistance of a conductor.
Resistance in ohms per km is given by:
R = (22.5 Ohms mmsq)/S(c.s.a) per km for Copper

R = (36 Ohms mmsq)/S(c.s.a) for Aluminium
Inductive reactance is negligible for conductors of c.s.a less than 50mmsq

This formula works pretty well for most design calculations of resistance less than 50mmsq

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