My New Hot Water Heater requires a 25 amp breaker rating at the panel. I currently have a 30 amp at the breaker at the panel for the hot water heater. Should I change it to a 25 amp?
Read the instructions carefully. 30A is an industry-standard convention in North America for tanked water heater circuits. The NEC derating rules require you to provision service for 125% of the actual water heater draw; water heater manufactureres know this, and size their water heaters to take about 23 amps. (Which derates to just under 30 amps, voilà).
Underwriter's Laboratories (who pretty much writes the book on what is acceptable, and harmonizes that book with NEC), goes in lock step with this. They certify the labeling and instructions as part of the UL listing (which is why instructions are often so bizarre, it's because they are copy-pasting instructions UL has approved in the past). So I would expect approved instructions to state 23-ish and require a 30A breaker.
Because 30A circuits are so standard, and 25A breakes such an odd duck (and difficult to find, encouraging misuse of alien breakers), I think UL would expect poor compliance with any "change the breaker to 25A" instructions, and would push back on the water heater manufacturer to make the heater safe with a 30A breaker.
The only advantage I can see to a 25A breaker is if you are using 75C rated wires (not NM/Romex!!!), you could wire it with 12 AWG instead of 10 AWG. But that would encourage another badness: up-breakering 12 AWG to 30A when the owner gets any other water heater on the market that of course requires 30A.
Any existing 30A circuit should be 10 AWG wire, and 25A is well within its limits. So changing the breaker will suffice.
Yes you can downsize a Breaker. You need to be sure that the Breaker is larger than or equal to the Appliance. You need to be sure that the wire that is used is for the Breaker size or Larger.
Typically a 30A Breaker uses #10 wire (here in California)
If it was a properly installed 30A breaker there should be a 10/2 or a 10/3 Wire coming from it. You can use a #10 wire for a 25Amp Breaker.
So Check your Wire size and you are good to go. As the Breaker will pop before the line melts.
You could leave it as is and it would still function, though if its only max Consumption is 25A, then it would take more than 30A to trip the breaker, which might be too late for the device at the time.
Breakers can go down in size relative to the Existing Wire. Wire can Upsize relative to the Breaker being used.
Yes if the max over current protective device is 25 amp and you have a 30 it is ok to down size. it is always ok to reduce the breaker size if the wiring is legal or compliant with the larger breaker. In some cases a very large wire size is used to reduce voltage drop, if the big wires are two large for the breaker they can be pigtailed to a smaller size that will fit the breaker. In your case you probably have #10 wire possibly #8 and these sizes are within the range of your 25 amp breaker. Some stores don’t cary 25’s but they can be found for major brands, or you may have to look on line.