Water interruption is often caused by a rupture in the branch-like singular pipeline. This will cause critical complaints from household and may decrease public service quality. As an alternative of singular pipeline, additional parallel pipeline could be installed for sustainable water supply. This system is called double pipeline system and able to be utilized for water transmission line between treatment plant and distribution reservoir. Construction of double pipeline was thought to increase capital cost, which can be an issue to waterworks authorities. Reducing capital cost was possible by means of installing connectors between two parallel pipelines because of reduced diameter of each pipe. To obtain optimal design condition for connectors, it was necessary to compare water pressure according to accident location, to investigate flow according to connection pipe spacing, connection pipe diameter, and aging of pipe. Reliable and economical connection layouts were determined based on these results. The cost estimation for each design condition was carried out. Cost was approximately reduced by 20 ~ 30 % compared to the double pipeline without connections. In addition to this, connection between double pipelines could expect extra benefits for maintenance since the pipe could be repaired and rehabilitated without interruption.