Principles of electric heating

When a current  flows through a conductor of resistance , heat is produced and there is temperature rise of conductors in electrical circuits. Temperature rise is undesirable and must be limited by design to avoid overheating of the conductor or its electrical insulation. This is often termed as permissible surface loading of a conductor. The conductor could be in the form of wire, strips or any other shape.

Permissible surface loading is often a prime consideration in choosing the best size of the conductor or heating element. The relationship is

(1)

In which  is current,  is resistance,  is overall heat transfer coefficient,  is surface area of the conductor and  is temperature difference resulting due to flow of current.

Consider a wire of circular cross section. The resistance is

(2)

In which K is resistivity in  is length of conductor in and  is diameter in . The surface area of the wire

(3)

Combining 1, 2 and 3 we get,

(4)

This relationship gives minimum diameter in  of a conductor of specific resistivity which can be used to carry current without increasing the temperature of the conductor to a value more the above the surrounding.