Ours was installed from the factory. I have used it with 30 amp hook up with no problems. I have not experiences low volume/pressure problems to make it work. It is set at a temp that is right at scalding temps, so with a slight adjustment of the handle, I get perfect water temps for as long as I want to stand there.
My only complaint with it is that I waste a lot of water waiting for the hot water to travel the distance from the heater to the manifold and then back to the shower. I usually turn on the water in the shower and then take off my clothes. By the time I get in the shower, the water is hot.
Highly recommend it.
Ron