Rheem Hot Water Cylider Thermostat keeps tripping

It could be element shorted to ground through steel tank, so don't touch tank when breaker is ON.
Or you have undersized 12 gauge wire and 20 amp breaker and the heat has loosened breaker on the busbar. Breaker trips when heat exceeds breaker limit. Jiggle breaker and listen for crackling sound.

To restore hot water now:
Turn off circuit breaker, and open top cover then Push in reset button on top of heater to restore functionality. Put back insulation and factory cover.
To fix water heater: test and replace bad element.
http://waterheatertimer.org/How-to-test-water-heater-element.html
If problems continue
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html#trip-breaker
http://waterheatertimer.org/Troubleshoot-household-electricity.html

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Thermostat could be bad causing the breaker to trip due to it being shorted. It's either your breaker or the thermostat, more than likely the upper thermostat is bad.

You have electric water heater.

For tankless electric, more information is needed about water flow, water pressure, water filter, water hardness, water softener, maintenance done so far, etc.,

For tank-type, open following link for troubleshoot walk-through.
With 240Volt water heater, elements test for 120V live power at all times.
No hot water, then suspects include: Upper element is bad, 240Volt circuit breaker tripped on one leg, bad upper thermostat, loose wire, water leak causing.
http://waterheatertimer.org/How-to-troubleshoot-electric-water-heater.html

Check 240 circuit breaker.
Open top cover on water heater and push in ECO red reset button.
If reset button will not reset, then replace thermostat.
If reset is not tripped out, then either upper thermostat is bad, upper element is bad, or water heater might be getting 120V instead of 240V.
If you have some hot water, then best suspect is lower element.
http://waterheatertimer.org/How-to-troubleshoot-electric-water-heater.html

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And take advantage of fixya expert assistance live.
For a price, expert works with you while you work on water heater or any do-it-yourself project.
Fixya is always less expensive than a service call.

You posted under Rheem 81VP which is 6 gallon point of use electric water heater.
3 things can cause problems: Power going to water heater, thermostat with reset button, and 1500watt element.
Check that 120Volf circuit breaker is not tripped.
Open cover on side of tank and push in red reset button.

If reset button is not tripped out or will not reset, and water heater has power, then replace thermostat is best start point.
http://waterheatertimer.org/How-to-replace-thermostat-on-electric-water-heater.html

Also test element, since elements burn out:
http://waterheatertimer.org/How-to-test-water-heater-element.html

Upvote the help.
And take advantage of fixya expert assistance live.
For a price, expert works with you while you work on water heater or any do-it-yourself project.
Fixya is always less expensive than a service call.

It's probably bad upper or lower thermostat.
BUT... new element could also be shorted.
OR ... original tank is rated 1500 watt elements look at label on side of tank.
I always replace both thermostats to save time.
Lower thermostat costs $10 at Lowes.
http://waterheatertimer.org/How-to-replace-thermostat-on-electric-water-heater.html

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Also take advantage of fixya phone service.
For a price, expert speaks with you over phone while you work on heater or any do-it-yourself project.
Fixya is always less expensive than a service call.

You posted under Rheem 40 gallon electric water heater, which is two-element 240Volt heater.
If this is not your heater, add a comment.

If you suddenly have no hot water, that narrows the suspect list.

Step1:
Check for tripped circuit breaker.
Open top cover on water heater and push in reset button on upper thermostat.
If reset button is tripped out, and will not push back in, then replace upper thermostat.
http://waterheatertimer.org/How-to-replace-thermostat-on-electric-water-heater.html
Put ear against tank and listen for bubbly-fizzing sound that says heater is ON.
If reset is not tripped, then move to step 2

Step 2: use tester or multi-meter and test across top two screws on upper thermostat for 240Volt.
If heater is not getting 240V, then test circuit breaker.
Move water heater wires from 240V breaker to another same-amp 240V breaker and see if that restores power.
You may need to replace breaker:
http://waterheatertimer.org/How-to-replace-circuit-breaker.html
If problem persists, then you might have a line problem between breaker and water heater.
Open cover on top of heater, and check connections on top of heater.

Step 3:
If heater is getting 240V, then upper thermostat or upper element is bad.
I suggest 30 minute troubleshooting which could save you a service call, and will be valuable information in the future:
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html

Add a comment for more free help.
Also take advantage of fixya phone service.
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Hello Check the red overtemp reset button at the temp control.Check for contintuity at the elements with a volt meter (pull one wire off first) they should have a reading otherwise they are open & shot.Last is overdrawing (busted pipe) shut off the cold water supply for a 1/2 hour and open it back up and feel the hot water supply pipe.Is it hot?The tank can't keep up with the demand.

I am assuming that everything was fine and then all of the sudden available hot water usage time dropped.

On a dual heating element hot water heater it can be

1. Internal tank loaded with sediment (drain and purge from lower garden hose fitting)

2. Thermostat or element problem with ONE of the 2 heaters. (Trouble shoot element thermostat)

3. Check Circuit breaker for partial trip of the breaker.

If you don't have current, 0v from terminal to terminal, then you don't have voltage to the elements. Replace the thermostat but check that you have 220v to the thermostat. It is possible that your breakers to the unit are either bad or tripped.