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.

KeepItSimpleStupid

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.

Handyman2007

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!!!!