I am looking at adding a tankless water heater but I don't have enough spaces in my breaker box for the breaker requirements of the heater. It is asking for 3 double pole 40 amp breakers and I am thinking about adding in a sub panel for the water heater. I was wondering if this is ok and what size breaker do I need to install for the sub panel. If this helps anyone answer my questions or give me advice I am looking at a eco smart 27. it requires 3 40 amp double breakers and im new to electrical work and don't know much I am having a electrian come out and do the work so it is done right and so I don't kill myself. I do know that the eco smart 27 has 3 heating elements in it and thasts the reason for the 3 breakers.
This heater alone required 113 Amps with really heavy hot water usage. So first you need an electrician to look at your house and your electrical panel/s to see if you have that much power available. The sub-panel idea is good if you have the available power. The sub-panel would need a rating of at least 125-Amps. You need to have newer and quality service equipment. Otherwise this type of a load could really raise havoc.