Splitting 60 amp 2 pole to 3 -40 amp 2 pole subpanel

Question

Can you split a 200 amp main circuit breaker box with a 60 amp 2 pole circuit breaker into 3 - 40 amp 2 pole circuit breakers in a subpanel for a tankless water heater?

Answer 1

NO!

If I understand correctly, you want to use a 60A double-breaker to feed a subpanel with 3 x 40A double-breakers. There are certain situations where you could do that, but not for tankless water heating. Why? Because with tankless water heating those 3 x 40A double-breakers will all be running at full capacity at the same time. Which means the feed to a subpanel of 3 x 40A breakers in this situation (other applications may be different - e.g., 3 x 40A EVSE with load sharing) requires a feed of 120A.

So then you might say, I'll just put the 3 x 40A breakers directly in the main panel. Well, whether you can do that or not depends on whether you have 120A of spare capacity or not. Whether you should do that or not? No, you should not do that.

Tankless electric water heating is a great concept. Instead of having a big tank that takes up space and wastes energy just sitting there, you have a tiny box that heats the water when you need it. Except:

• In order to provide the level required in a typical house (several gallons per minute for bathing, dishwashing, laundry, etc.) you need an insane amount of power. 120A is typical, and that is much power as many entire houses, including lighting, HVAC, cooking, tank water heater, etc. use. And at least double what a typical electric car charger (EVSE) needs, and actually more along the lines of what 4 EVSE need. That's a LOT of power.
• Tank water heaters are actually very efficient because they are very well insulated. Put your hand on the outside of a traditional tank water heater (anywhere except the hot water output pipe and (if gas) the exhaust ductwork). It is cool to the touch - basically room temperature. Yes, if you heat up a tank of water and let it sit unused for a couple of days, the heating element will turn on every so often for a little while to reheat to compensate for heat lost to the surrounding room. But that is really very little.
• Except in a small apartment, it is usually not hard to find space for a tank water heater. 3 feet x 3 feet is generally more than enough.

If you really want to go tankless, before looking at breaker locations, subpanels, etc. you need to do a Load Calculation. This is a formula that incorporates all your existing large loads (e.g., HVAC), cooking appliances, the size of the house and other factors to come up with an estimated load for the entire house. You then compare that to your utility service size. If you have 200A service (typical) and 80A or more of existing Load Calculation (excluding your water heater if you currently have a tank water heater) then you can't add 120A of tankless hot water heating because that 120A could all be turned on all at once at any time while everything else (based on Load Calculation) is also running and exceed the 200A supply. It is possible to upgrade to 400A/320A service, but I wouldn't recommend that just to get tankless water heating.