I'm swapping out a traditional hot water heater for a tankless. My house is old and has a double pole 20amp breaker (outside electrical panel). The tankless calls for 3 x 40DP im guessing that's 3 double pole 40 amp breakers. Space is the issue i have no slots left on the panel any tricks to keep from have to replace the whole panel enter image description hereenter image description hereenter image description hereenter image description hereenter image description hereenter image description hereenter image description here

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    Maybe, but it'll depend on your panel. We'll need to see some pictures of it, at the very least. Try to include pics of the breakers, the labeling, and the wiring with the deadfront removed if you can.
    – Nate S.
    Feb 4, 2021 at 22:58
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    This is opinion based, but I really don't like electric, on demand water heaters, they have limited flow rates, require incredible amounts of electricity when many areas have a stressed electrical grid. Like Harp said in another post, new tank type water heaters are very well insulated and efficient. You could replace it with a similar type with no panel upgrades. As others have said, you'll need to do load calculations to be sure you can support that. IE: Electric range, clothes dryer, furnace? Adding a potential 120 amp draw is not trivial. We are here to help. get us more details. Feb 4, 2021 at 23:15
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    @WalterMoose, the one picture I'd like that you didn't add is a picture of the panel's labeling -- it's usually on a sticker inside the door. The reason I ask is because I can't tell for sure, but this very much looks like a 100A panel, in which case you're definitely not going to be able to add a 120A water heater to it at all, regardless of breaker spaces.
    – Nate S.
    Feb 4, 2021 at 23:45
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    This is an extremely trendy, extremely bad idea. And extremely expensive by the time you upgrade not just the panel, but the service to the panel. Return it and get a tank type heater....
    – Ecnerwal
    Feb 4, 2021 at 23:52
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    Why are you wanting to install a tankless heater to begin with? And yes, can we have a photo of the labeling on the inside of the panel's door please? Feb 5, 2021 at 0:38

1 Answer 1


With that panel the answer is no way, I can’t read the main breaker value but I would bet your main 150 at max you have a total of 240 amps of breakers ok 200 because we use the larger of heat / ac so 200 and if the existing water heater is 20 it is two small for a full sized tanked water heater. Even if a 200 and I seriously doubt it. Your service is two small. Are you feeding an inside sub with the 60 for lights and receptacles?

  • And maybe additional load provisioned for the feeder tap? Feb 5, 2021 at 0:50
  • Yes a sub was ran inside. Feb 5, 2021 at 2:22
  • Yes a sub was ran inside. I haven't changed anything electrical the wiring was like this when we purchased the house. Feb 5, 2021 at 2:29
  • that's a 200A panel (by the label) and a 200A breaker so far as I can tell (by looking up TQD and M322)
    – Jasen
    Feb 5, 2021 at 4:22
  • Walter, the inside label states 200 amp I am surprised that is a large buss for that small of a panel but I would stick with my first sentence NO way you can add 27kw to that panel. If you seriously want this I would suggest going to a 30/40 panel 300 amp continuous and 400 amp peak, or a 320a panel called different things in different areas. This will allow for the addition of the 50 amp car charger that will be needed in the future if we go to electric cars and that size upgrade will be needed for both to be added.
    – Ed Beal
    Feb 5, 2021 at 14:37

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