Conductor Size Related To Current

http://cr4.globalspec.com/search/sitesearch?do=show&sort=textmatchrank&srch=busbar%20sizing&order=asc

Chandru

Let me know what you find here. I want to know the answer.

Are you aware of Skin Effect & Proximity Effect?

yes sir

An important limitation for bus bars is the thermal limit. The allowable current/cross sectional area is based on the electrical power dissipated and the convection cooling of the bus. At low frequency, the cross sectional area of the bus will allow you to calculate the power dissipated, but the exposed area of the bus is part of the cooling calculation. This means, a large piece of sheet metal will dissipate more heat than a round or square cross section of the same material. There is more to it, but that's the fundamental reason. Obviously, anything that inhibits cooling (like insulation) changes the equation.

But the doubt is that for a bare aluminium flat, factor usually considered for capacity is 0.8 to 1 ampere per square metre.

In the above statement the words "square meter" should be read as "square mm".Coming to the answer to your question,it is the skin effect .it may be noted that current flows in the periphery of the cross section of the conductor ie.the surface .Hence major part of the cross section of the Al. flat remains unused,where as in case of cable which comprises of a number of strands of smaller diameter,optimum utilisation of the cross section is achieved,thereby resulting in higher current flow in cables of equivalent cross section.

thanx sir, i can understand. Can i get a table relating ampere rating and cable size (al / cu)

You can see the links given by TonyS & Wareagle in response to your question.