Firstly, according to the transformer capacity and connection method, normally 0.4kv is Y connection. According to the capacity of 630 and 400, the high voltage input extra current is 909A and 577A.This current can be used as a temporary task current to select;Or, of course, Ig= sigma Igt+K* sigma Idt, which means stop the selection based on temporary load and short-term load;Now you can start calculating the cable section.Ixu is the allowable cable load capacity, Ig is the temporary allowable task current, which can be selected based on capacity or load.To be on the safe side, according to the additional capacity to choose only large;K is the correction coefficient under different cable laying conditions, which can be divided into single cable in air, multiple cable in air, pipe penetration in air, single cable in soil and multiple cable in soil.See the attached table of chapter 17 of the design manual for the coefficient, which has a lot of content.According to Ixu=Ig/K, the load capacity can be calculated, and the cable can be selected according to the load capacity.For example, 909A is used for laying more than one wire in the air. The environment temperature is 35 degrees uniformly, and the cable task temperature is 65 degrees. The spacing is the outer diameter of a single wire.Refer to manual K=0.865*0.9=0.7785, Ixu=909/0.7785=1168A;1168/2 = 584 a;Check the cable load capacity. The load capacity laid in the air of section 300 is 636A, with 2 pieces, greater than 584;So I can use both of them!In addition, the uniform ambient temperature of 400kVA is 35 degrees, and the allowable cable task temperature is 65 degrees. The single 577/0.865=667A, and the section of 300 is not satisfied.So want to see the environment that oneself lay, the environment that assumes here does not agree with practice demand necessarily!Overhead wire!