My question concerns the installation of a 40 gallon electric hot water heater. It will have 2 elements (upper and lower), each of which carries a load of 4500 watts. I believe it requires 240 hookup. I plan on hardwiring it (no plug), but I need to know what wire size and breaker size to use. Right now, my tentative plan is to use #10/2 with ground, with a 30 amp double pole breaker. Can someone confirm if that is appropriate and, more importantly, safe? Based on previous posts, I do not believe that the upper and lower heating elements of a hot water heater run simultaneously (thus requiring a total load of 9000 watts), but I'm not sure. Obviously, 9000 watts will require a larger conductor and breaker.

Here is an excellent resource on water heaters: Both electric and gas

http://www.hotwater.com/lit/training/tc049r2.pdf

Aparently some water heaters can use both elements at the same time and it's not common.

Also this resource states that the breaker should be 125% of the nameplate rating.

The link provided in the answer is from one of the oldest water heating companies in the business. It is an excellent reference.

That would be correct and would meet NEC requirements. I would, if I were you and have no experience doing this kind of worh (anything electric is hazardous to work on) , hire an electrician or qualified plumber to do this job for you.Remember, electricity and water do not mix!!
PS: not to be a nit-picker but,, it is not a "hot" water heater... it is a water heater. Why would you need a hot water heater?? The water would already be hot!!