The ratio of current transformer should be selected according to the calculation of primary load current IC. 

Current transformer primary side rated current ratio (such as 20,30,40,50,75,100,150,2 A / C) and other specifications, the secondary side rated current is usually 1A or 5A. 2 A / C means that there are two current ratios of the same product, which is realized by changing the connection mode of the connecting piece of the product. In general, the selection of the converter ratio of the metering current transformer should make its primary rated current I1n not less than the load current in the line (that is, the calculated IC) . If the load calculation current in the line is 350A, then the current transformer conversion ratio should choose 400 / 5. In order to ensure the accuracy of the current transformer used for protection, the transformer ratio can be selected to be larger.