Mississippi Code Title 75. Regulation of Trade, Commerce and Investments § 75-51-3

Temperature and pressure relief valves, or combinations thereof shall have thermosetting of not more than two hundred ten (210) degrees fahrenheit and pressure setting not to exceed the tank or heater manufacturer's rated working pressure. The relieving capacity of these two (2) devices shall each equal or exceed the heat input to the water heater or storage tank.

Temperature type or pressure-temperature type valves shall be installed in the shell of the water heater tank or may be installed in the hot water outlet, provided the thermo-bulb extends into the shell of the tank, and in all cases installed at the highest practical point. For installations with separate storage tank, said valves shall all be installed on the tank and there shall not be any type of valve installed between the water heater and the storage tank. Temperature relief valves shall be so located in the tank as to be actuated by the water in the top one-eighth ( 1/8 ) of the tank served.

Pressure relief valves may be located adjacent to the equipment they serve. There shall be no check valve or shut-off valve between a relief valve and the heater or tank for which it is installed.

The outlet of a pressure, temperature, or other relief valve shall not be connected to the drainage system as a direct waste.