guess I'm still confused. with the 100 amp branch circuit breaker, will this work
This will help:
Think of it this way:
Your main panel is like a library; it is a 100 Amp size library, so it has 100 Amps of "books" it can loan out at any one time.
Most of the time, your home "borrows" / uses maybe 30 to 50 amps (30 to 50 books) at one time. If your house is using 50 of the amps (50 books), there are only 50 left (50 books left).
Your new water heater wants to borrow 100 Amps (all 100 books) all on it's own. If you only have 50 Amps (books) left in your library, where are the other 50 Amps (books) going to come from????
The answer is they are NOT going to come from that main 100A panel, regardless of sub panels or other fancy setups, it defies basic electrical principals.
If you connect it anyway, what is going to happen when the water heater starts running is you will open (trip) your main breaker due to drawing too much current at one time. This will disable power to your entire home leaving you in the dark until you reset the breaker, and the process will be repeated until the water heater is removed or the main service panel is upgraded so it has more amps "books" to loan out at any given time.
I hope this was useful and didn't seem condescending, it was only intended to be helpful
Jamie