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From the 2015 UPC Illustrated Training Manual

608.2 Excessive Water Pressure.
Where static water pressure in the water supply piping is exceeding 80 psi (552 kPa), an approved-type pressure regulator preceded by an adequate strainer shall be installed and the static pressure reduced to 80 psi (552 kPa) or less. Pressure regulator(s) equal to or exceeding 1 1/2 inches (40mm) shall not require a strainer. Such regulator(s) shall control the pressure to water outlets in the building unless otherwise approved by the Authority Having Jurisdiction. Each such regulator and strainer shall be accessibly located aboveground or in a vault equipped with a properly sized and sloped boresighted drain to daylight, shall be protected from freezing, and shall have the strainer readily accessible for cleaning without removing the regulator or strainer body or disconnecting the supply piping. Pipe size determinations shall be based on 80 percent of the reduced pressure where using Table 610.4. An approved expansion tank shall be installed in the cold water distribution piping downstream of each such regulator to prevent excessive pressure from developing due to thermal expansion and to maintain the pressure setting of the regulator. Expansion tanks used in potable water systems intended to supply drinking water shall be in accordance with NSF 61. The expansion tank shall be properly sized and installed in accordance with the manufacturer's installation instructions and listing. Systems designed by registered design professionals shall be permitted to use approved pressure relief valves in lieu of expansion tanks provided such relief valves have a maximum pressure relief setting of 100 psi (689 kPa) or less.

A limit of 80 psi (551.6 kPa) is the maximum static pressure of any water supply system. The reason for this is to reduce water hammer, unnecessary use of water, splashing, excessive discharge of pressure relief valves and to protect appliance and fixture valves and mechanisms from pressure that exceeds their design limits. Any installation with pressures above 80 psi will require a pressure regulating valve to limit the pressure to 80 psi or below (see Figure 608.2).

By design, pressure regulating or reducing valves are modulating valves, which have a high level of flow resistance and consequent pressure drop through them even when fully open. Therefore, pipe sizing downstream of the pressure regulator must be based on "worst-case" pressure loss during a maximum demand water flow. Worst-case pressure loss through a listed pressure regulator is presumed to be no greater than 20 percent; therefore, the water system is sized based upon 80 percent of the pressure regulator’s "set" pressure, this being a selected static pressure that is presumed not to exceed 80 psi. Therefore, all pipe size determinations downstream of the regulator must be based on 80 percent of this reduced pressure when using Table 610.4.

For example, a water system has a pressure of 100 psi. A pressure regulator will be installed and set at 80 psi. For sizing purposes using Table 610.4, the maximum pressure would be 64 psi, which is 80 percent of 80 psi.

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