The optimization in reliability cost/benefit analysis for distribution investment planning has been more attended on energy delivery services in Thailand. In this research, the reliability costs for the customer point of views represented as the outage costs are presented. The data for analysis is provided from customer survey-based method during 2008-2009. Due to the direct and indirect impacts for electric customer in each category are different, the concept and methodology for data collecting based on customer survey is described. According to the investigation of outage costs in different sector, the results from customer survey are classified into four customer sectors. In each sector, the individual customer damage function (ICDF) derived from survey are aggregated to develop sector customer damage function (SCDF). The composite customer damage function (CCDF) for the large area is obtained by using SCDFs and electrical energy utilization ratio in each customer sector under interested area. The CCDFs of four service areas under the rural distribution utility is also evaluated in this article. © 2011 IEEE.