John: The sequence of operation for your Rheem water heater is as follows when initiated on a cold tank of water.
The upper thermostat will supply energy by way of the blue and yellow wires to the upper element.
(If the power supply is 220 volts and the heating element 4500 watts,
a current of 20 amps(approx.) should be indicated)
Once the upper part of the tank has been heated to the upper thermostatic setting, the power will be then terminated to upper element and the energy will be directed through the red and black wires to the lower thermostat and element.
The lower element will continue heating the water at the bottom of the tank untill the lower thermostat is satisfied.
The lower element then will maintain the heat of the water in the tank at that setting, unless sufficient water is drawn from the tank to energize the upper thermostat again.
Only one element is energized at a time with 220 volts..one leg of the 220 supply voltage is always present(110 volts) at the element terminals, the other leg is controlled by the thermostat when heating is called for.