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:


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.

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.