I'm upgrading residential water heater to tankless.

No extra spaces on 200 A service panel. Existing heater uses two spaces (30 amp, 2-pole).

I know I'll need new wiring and breaker, but I'd like to keep existing service panel.

I'm intrigued by this one, because it says it can do 29 kW on two spaces (i.e. one 2-pole breaker) with a 125 A breaker:


By comparison this comparable 27kW unit wants three 40 A breakers: table

My question:

Why do the breaker configurations differ so wildly when it seems like it would always be preferable to use as few spaces as possible?

  • 4
    It might be preferable for you, but not the maker. The maker might have designed the system so that more smaller elements work better than one or two very big elements. 40s probably more common than a 125 to fit on a panel(instead of a main breaker type).
    – crip659
    Jan 12, 2022 at 22:26
  • 6
    DON'T DO IT! I hate electric tankless WHs. They draw a LOT OF POWER while utilities can barely manage to keep the lights on. As they shut down base load generation facilities (nuclear, coal and gas) in favor of unreliable green power, it will get worse. Your plan isn't an "upgrade" it's a downgrade. When your power goes out on a tankless, your stuck, but with a traditional tank style WH, at least you'll have (typically) 50 gallons of hot water avail. I know this is a rant, but a traditional electric WH has virtually no heat loss and it evens out the load on the grid. Jan 13, 2022 at 8:49
  • 1
    I'm always surprised why so many people want to use tankless too. Gas is okay but tankless electric is silly.
    – izzy
    Jan 13, 2022 at 19:14
  • 1
    @GeorgeAnderson so yep, I eventually settled on a hybrid HPWH instead. I didn't even know they existed when I posted this question, and I wish I had because it would be saved a lot of needless research into electric tankless!
    – davetapley
    Jan 17, 2022 at 18:47

3 Answers 3


Because tankless heater manufacturers aren't even slightly concerned with breaker spaces, and you are sweating what's actually not a big problem.

With tankless, the heater should be as near as practicable to the point of use. Because they are compact and don't need a vent, and being closer cuts down on the wait for hot water. Better off crossing the distance with wire than pipe. Wire in instantaneous. As such, the heater is probably not near the panel.

The answer to “not enough spaces" is a subpanel. Always, not just in this case. So you run one fat feeder to the sub, and then your three 40A circuits in the sub to the heater hopefully nearby.

So we need to run wire some distance. Do we want to run three #8 copper cables? Or one 1/0 aluminum cable? The 1/0 is cheaper by the foot than even one of the #8, so that's a no-brainer. Aluminum feeder is perfectly reliable at these large sizes.

The money savings using 1/0Al for part of the distance instead of #8Cu pays for the subpanel, and then some.

I wish I could say the water heater sub will solve your "out of spaces" problem, but no. It's using nearly 120A of the 120A capacity of 1/0Al feeder, and there's no room for anything else.

However a second subpanel for general use might be a good plan.


It depends solely on how the water heater is manufactured and who it is manufactured by. Some use three breakers so the components can be a smaller size and the elements are protected by the customer's breakers in the main panel. The smaller wire that required is easier to work with and it used in other areas of the house when wiring it.

This particular water heater uses one large feed and one large breaker. It has it's own breakers protecting everything on the inside of the unit. (See photo below) enter image description here

The wiring is large and hard to work with and would be unique to just the water heater. With the one breaker, which would be expensive, if it fails, you lose your hot water. With the three breaker system you would still have hot water if something went wrong.


Preferable for the customer, sometimes. But not always, and not for the manufacturer.

125A breakers are not as readily available as 40A breakers for some panels. Even when they are, they often cost a lot more. 15, 20, 30, 40 and 50 amp breakers are all typically around the same price. Going for 3x40 works on almost all panels. Unless you're totally full, in which case you likely can't spare 120A of current anyway.

In addition, the design may be easier to use 3 heating elements instead of 1 large heating element. Allows for partial heating, smaller controls, etc. Once you do that, splitting the supply makes sense.

Finally, wrestling 125A capable cable around in a basement is a LOT harder than 40A cable. Doesn't matter in a new house but often goes in a retrofit.

To answer the unasked question: Do I have enough capacity in my panel?

Having enough spaces - whether for 3 x 40A or 1 x 125A - is only one of the parts of the answer. You also need to have large enough electrical service to support 125A of water heating. You may have 200A service but already be using on the order of 150A, in which case a net gain of (optimistically) 80A (i.e., adding 120A but subtracting an existing 40A from the tank heater that is being replaced with tankless) just won't work. You'll trip your main breaker occasionally and your utility will complain too. Or you may have 200A service but actually only use 80A because it is a small house with some gas appliances and no bitcoin mining or grow lights, in which case a net gain of 80A to a total of 160A will, just barely, work. This is a truly major change in your electricity usage and, unlike the other common change, electric vehicle charging, harder to manage. Hmmm, I'll charge the EV at night only (easy) and use the hot water during the day. Then someone gets stuck in traffic for hours (happened to quite a few people in Virginia recently) and come home in the middle of the night and take a nice hot 120A shower...and suddenly everyone is in the dark. Load calculation, possibly followed by an increase in your electric service, is necessary. And hope you never get peak demand charges.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.