The selection of transformer core is related to voltage, while the selection of transformer conductor is related to current, that is, the thickness of conductor is directly related to heat generation. In other words, the capacity of the transformer is only related to the heat generation. For a designed transformer, if it works in an environment with poor heat dissipation, if it is 1000KVA, it may work in 1250KVA if the heat dissipation capacity is enhanced. In addition, the nominal capacity of the transformer is also related to the allowable temperature rise. For example, if a 1000KVA transformer is allowed to have a temperature rise of 100K, and if it is allowed to work to 120K under special circumstances, its capacity is more than 1000KVA. It can also be seen that if the heat dissipation condition of the transformer is improved, its nominal capacity can be increased. Conversely, for frequency converters with the same capacity, the volume of the transformer cabinet can be reduced.