In order to calculate transformer price changes after one large transformer is substituted by several small transformers with the same total power or after one three-phase transformer is replaced by three single-phase transformers it is necessary to use transformer change ratios which are functions of transformer rated power decrease or increase. For three-phase transformer, greater amount of active materials is needed than for a single-phase two-coil one with the same rated power, but less amount of them - for three single-phase transformers of the same total rated power. Applying design size power formula one can quickly adjust transformer’s no-load losses on account of its load losses and vice versa.