A new method of planning the capacity ratio between feeders and main transformers in a distribution network is proposed. Firstly

two indices including the average load rates of feeder and main transformer under the total supply capability(TSC)are defined. Then

the law of variation of the two indices with the capacity ratio between feeders and main transformers is analyzed. It is revealed that the two indices cannot reach the maximum value at the same time. Secondly

according to the construction cost differences between feeders and main transformers

the optimal capacity ratio between feeders and main transformers is calculated. At this ratio

the total construction investment in the unit power supply capability is minimum. Finally

the optimal capacity ratio of multiple cases in different situations is obtained

which declines with an increase in the cost ratio between feeders and substations and rises with the increase in tie-line balance degrees. The optimal ratio is suggested to be 1. 0-1. 6 in current cost level of an urban network

which is obviously lower than the range of 1. 5-2. 0 for the conventional distribution network and the planning empirical value. This work is supported by National Natural Science Foundation of China(No. 51277129)and Program for New Century Excellent Talents in University(No. NCET-10-0624).