Water system size The fastest way to determine the size of a residential drinking water system using the 'Water System Size' application is without having to search or use Notepad on the table. How it works: After making the required measurements, simply enter the data and select the fixture to be used. The water system size main line will immediately calculate the main line or branch line size 1/2" to 1-1 / 2". The water system size will not replace your U.P.C. code book. Each plumber must refer to a code book next to the water system size main line to understand and comply with your state's pipeline “law”. The water system size is an ongoing, independent, private work that is not affiliated with iapmo in any way. Your input helps the code develop quickly, so we can make positive changes to the future.