I have a 48A 240VAC instantaneous water heater. What size circuit breaker do I need? I am thinking a 50A would do because it is not continuous.

Read the mfg. data on the heater. It will tell you circuit size and protection. If it's not there, track down the information on the internet. How do you know it's 48 amp?

The tag on the heater says 47.8 Amps and the installation instructions recommend a #6 AWG wire. Its say to use appropriate size breaker.

The reasoning in your first post is good. A 50 amp breaker is what you want.

Slightly off topic, if this were a storage type water heater it would be considered a continuous load. In this case it is instantaneous so it is not considered continuous.

I hate to ask, but why are you using an instantaneous water heater? They are real energy hogs and usually cost more to operate than tank type. Specifically the electric ones.

It's going in a cabin and won't be used much. I won't have to heat the water all week when we are not there.

OK. I guess that is the one circumstance where it may be cost effective. Thanks for your response. An alternative would be a timer on a tank type, if you knew when you were going to be there. Good luck.