The water pressure required for a private home, according to the International Plumbing Code, is based on the pressure required by the plumbing fixture with the highest pressure needed for proper operation. Two other things that affect the pressure requirement are the pressure drop from the main house shutoff valve to the plumbing fixture and the pressure drop from the water main to the main house shutoff valve. The minimum pressure supplied at the water main tap must be higher than the sum of these three pressures for the house to meet the plumbing code.
The highest plumbing fixture pressure requirement is typically 20 pounds per square inch (psi) for a temperature-controlled shower. Most other plumbing fixtures, such as bathtubs, single-handle faucets, dishwashers, sinks and hose outlets, require a pressure of 8 psi. Certain special types of toilets may require from 15 to 25 psi.
Pressure loss occurs in the interior piping through each valve, fitting and section of pipe, because of friction and turbulence. The loss takes place between the house main shutoff valve and the plumbing fixture with the highest pressure requirement. For a temperature-controlled shower fixture both the cold and hot water piping must be considered. There is also a pressure loss of 0.43 psi per foot of elevation rise that must be added to the total interior loss. Tables and graphs in the International Plumbing Code allow proper pipe sizes to be selected and the pressure drops calculated.
Pressure losses occur from where the water main is tapped for the house water supply through the "curb" shutoff valve, the water meter and the exterior piping. Elevation change between the water main and the house main shutoff valve will create an additional pressure drop. Information to calculate these pressure drops is also in the International Plumbing Code.
The minimum water pressure at any tap location can be obtained from the water utility or from the water supply itself. Water utilities commonly provide a minimum pressure of about 55 psi. This is sufficient for typical residences.