It depends upon multiple things such as radius of electric wire 'r1', critical radius of insulation '(r)cr' for a given insulating material and heat transfer coefficient 'h' of given conditions.
In the case of electric wires, what we want is higher heat loss from the wires. (The heat is generated because of the electric resistance of wire).
The addition of more and more insulation material should increase heat transfer. But as shown in the fig below, the addition of insulating material increases the heat transfer up to a 'critical radius' and then heat transfer starts reducing (which we don't want).
Hence, for choosing the thickness of insulation in the electric wires the critical radius of insulation is a very important factor.