My home has two water lines and therefore two hot water tanks. I just replaced one with the ECO27 unit and can barely make the space for the three 40 amp double pole breakers needed. I am trying to think ahead as I will likely want to replace our main tank as well. Our house is 100% electric with separate radiant systems in each room. My 200 amp main panel has a couple circuits which could be moved into my basement panel. I currently have a 100 amp sub panel in the basement and another one on the second floor. The second unit would be near the basement sub panel. How should I run the next 3 40 amp double pole breakers to the second unit and do I run the risk of my entire house shutting down?

A 200 amp service cannot handle the added load of a tankless water heater.

I believe you as you have great knowledge, but why do the instructions say 200 amp service is required and what options do I have?

I believe the water heater needs a 200 amp service alone, not including the existing load.

Have an electrician do a service rating calculation to confirm.

You may need to go to a 400 amp service.

Do a load calculation in a MS exel work sheet with load(amp)in Y-axis and time(clock) in X-axis. Calculate the maximum load by adding loads in vertical column,which gives the main breaker size. If it exceed 200Amps contact the utility.

Assuming the question is regarding a home in the USA or Canada, service rating calculations is not done in the fashion:

Quote:

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Originally Posted by sinnadurai

Do a load calculation in a MS exel work sheet with load(amp)in Y-axis and time(clock) in X-axis. Calculate the maximum load by adding loads in vertical column,which gives the main breaker size. If it exceed 200Amps contact the utility.

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There are derating factors used by the National Electric Code and the Canadian Electric Code that will arrive at a service rating less than the total sum of actual loads.

Davenmoni, the original poster, needs to have a local electrician do a service rating calculation to confirm the size service needed.

tkrussell,When I mentioned load it include factors like diversity/simultaneous/derating etc. Anyone in electrical engineering field should know it.

That may be, but the poster is a homeowner, not an engineer.