Main breaker tripping

For residential water heater, 30 amp is best... but must have 10 gauge wire.
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html#trip-breaker
http://waterheatertimer.org/Color-codewire.html
If you have 12 gauge wire, then use 20 amp breaker and install 3800 watt elements in place of 4500 watt elements
http://waterheatertimer.org/How-to-replace-water-heater-element.html
Add a comment and say which size wire you have

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

Your residential water heater is not 9000 watts. Or it would blow the fuse every day and melt the wire. Instead, you have dual-element non-simultaneous 4500 watt water heater.
http://waterhatertimer.org/How-it-works.html
http://waterheatertimer.org/Figure-Volts-Amps-Watts-for-water-heater.html
Look at label for 'total connected watts'
http://waterheatertimer.org/images/Whirlpool-energy-smart-watt-rating-IMG_2600.jpg
25 amp breaker or fuse and 10 gauge wire is minimum acceptable for 4500 watt water heater.
http://waterheatertimer.org/Color-code-wire.html
Troubleshoot blowing breaker or fuse:
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html#trip-breaker
Why does it blow fuse once each year?? I don't know.
Fuses and breakers trip when overheating occurs on wire.
Each fuse or breaker is rated for heat, and over time, it will get hot and then cool and then get hot again ... this action weakens fuse or breaker.
To solve problem: install 3500 watt elements (and clean sediment out of tank). Wire will run cooler, and fuse will last longer. Whole fuse box will demand less power, and run cooler, and will last longer as a result. So there is less stress on electric service panel.
http://waterheatertimer.org/How-to-replace-water-heater-element.html
3500 watt element will heat water to same temperature, but takes longer. For example: 4500 watt heats 21 gallons per hour, while 3500 watt heats 17 gallons per hour.

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

Guessing that you have 240Volt residential non-simultaneous electric water heater? And that you have 30 amp breaker and 10 gauge wire. And that water heater is located within 100 feet of breaker box. And there are no leaks running hot water out of tank. And that you installed fully brand NEW elements of same wattage as old elements. And that fully brand new thermostats were installed, and wiring exactly duplicates wiring that was present before. And that factory-insulation was re-installed over elements and thermostats.

1) Guessing the 'hot water shut off' means the ECO red reset button located on upper thermostat keeps tripping and cutting off electric power to water heater?
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html
http://waterheatertimer.org/images/WH-w-combined2-1000.jpg
http://waterheatertimer.org/How-to-test-water-heater-wires.html

2) If the ECO is tripping, then water is getting too hot, or there is shorted wire located near the upper thermostat, .
Typically replacing both thermostats and replacing elements will solve this problem .... assuming that new thermostats and elements are good, and wiring correct.
Double-Check your wiring on upper and lower thermostats.
Test wires for short.
Make sure factory-insulation is re-installed over thermostats so cool room temperature will not affect temperature reading.
Check that thermostats sit flat against steel tank.
Many short draws hot water can also cause stacking event that will trip circuit breaker.

3) Add comment and say what you find using checklist above.
Be careful to explain findings fully, using same words found on links above, so illuminated diagnosis can be made.

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

You have dual element 240Volt tank-type electric water heater.
You want to change wiring from non-simultaneous to simultaneous.
Copy following link for exact internal wiring diagram, and external wiring change: Remember the wire from breaker box will not support both elements without tripping breaker, so you will need additional 30 amp breaker and 10 gauge wire, or single 50 amp breaker and 6 gauge wire.
http://waterheatertimer.org/How-to-wire-water-heater-thermostats.html#simultaneous
http://waterheatertimer.org/Color-codewire2.html
http://waterheatertimer.org/Figure-Volts-Amps-Watts-for-water-heater.html

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

Before I spent the money on a new heater I would check reset to make sure it did not trip. I would also check element and make sure you are getting flow thru outlet and that it did not plug up with rust and mineral .
Depending on the water quality where you live I would expect to get at least ten years use out of a electric heater. Thank you.

Breaker could be bad.
Let's do a quick check and save you money.
Open top cover on water heater and push in red reset button that might be tripped.
If reset is tripped, water heater should turn ON.
If reset will not reset, then replace upper thermostat:
http://waterheatertimer.org/How-to-replace-thermostat-on-electric-water-heater.html
If reset is not tripped out, then it is OK, so test two top wires on upper thermostat.
Use cheap multimeter from home center, and set dial to 240-250-277 VAC.
Test across top two screws on upper thermostat to see if you have 240Volts.
To do full troubleshoot on ordinary tank electric water heater takes 30 minutes and multimeter
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html

If water heater is not receiving 240V, then the breaker is suspect, or a wire between breaker and water heater is loose.
See basic water heater circuit
http://waterheatertimer.org/240-v-water-heater-circuit.html

Now let's look at circuit breaker:
Mark suspect 240V breaker.
Identify all your breakers, and also mark another 240 Volt breaker that is same amperage as water heater 240V breaker.
Turn off main breaker.
Take front panel off breaker box.
The wires and lugs on main breaker are HOT, so stay away from that area.
http://waterheatertimer.org/See-inside-main-breaker-box.html
Suspect breaker has 2 wires coming from breaker.
See if one or both wires are loose, and then tighten.

If wires are tight, then move these wires to the other 240V breaker that is rated for same amps as water heater.
Now turn on main breaker.
Put ear against water heater tank to hear bubbly fizzing sound.
This will identify if circuit breaker is bad, or if water heater needs a full troubleshoot.
It is not a good idea to run two 240V appliances on same 240V circuit breaker, so you may have to decide one or the other appliance until you can replace breaker.
Instructions to replace circuit breaker:
http://waterheatertimer.org/How-to-replace-circuit-breaker.html

Add a comment for more free help.
Also take advantage of fixya phone service.
For a price, fixya expert speaks with you over phone while you work on water heater circuit.

The Rheem water heater uses 4500 Watt elements
Check label on side of any appliance to see electrical rating details
2 element tank with two 4500 watt elements is still a 4500 watt appliance since both elements are never ON at same time.

Volts x amps = watts
amps = watts divided by volts

amps = 4500 divided by 240Volts or 220Volts
amps = 18.75
Your water heater uses 18.75 amps to 20.45 amps

Your 25 amp breaker should only carry 80% of rated value
25 amps x 80% = 20 amps.

So your water heater breaker is correctly sized.
You should have a 10 gauge wire going to this water heater.
12 gauge wire is a bit undersized for 4500 Watt water heater
Feel the water heater wire to see if it is warm.

If the main breaker is getting hot when 20 additional amps are being used by the water heater.
1) The house is using more amps than your 100 amp main can carry
Using the formula, 100 amps x 80% = 80 amps for the main breaker
Add up your total amp draw.
For example if your 3500 watt oven is ON, it draws 16 amps.
If your 5000 Watt electric heater is on, it draws 23 amps.
Your 1500 Watt 120V microwave draws 12.5 amps.
Add these numbers up plus your water heater, and it comes to 71 amps.
71 amps is getting close to the max 80 amp, but it's not over.
Now if you have a pool pump and and hot tub and a table saw going, the main breaker could be overdrawn.

2) To solve the problem short-term, you can put a timer on the water heater and use it only when needed.
http://waterheatertimer.org/Compare-13-electric-water-heater-timers.html

3) The main breaker is bad and needs to be replaced.

If you need help figuring circuits or timers, add a comment and I will help

Hello,You have a shorted heater ELEMENT.They can be eaisly replaced.Draining the tank will not slove the problem.

sounds like a loose connection ,or possibly a weak main breaker.