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I'm renovating a small house and want to install an electric tankless water heater to save space. A local store offers one that requires 2 50-amp breakers; the house has 200-amp service. The house also has:

  • an oven (40 amp breaker)
  • maybe a stackable washer/dryer unit (30 amp)
  • heat pump (2 ton - not yet installed, I'm guessing needs 25 amp breaker)

Will the 200-amp service be enough to handle all this?

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    What about freezer or refrig, washer, dryer, toaster, toaster oven, other kitchen appliances? – Yehuda_NYC Jun 26 '20 at 2:23
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    How many square feet is this house, and how many breakers do you have to turn off to turn all the kitchen counter receptacles off? Also, what is the actual kW rating on the tankless heater you want, and who's your electric utility? Finally, can you post a photo of your breaker panel please? (Door open, but cover still on) – ThreePhaseEel Jun 26 '20 at 3:03
  • There will be a dishwasher, refrigerator, microwave. The homeowner will add their own things to the house for sure. The heater is a Marey ECO180, drawing 18kW. We are on Dominion Power in VA. – GregH Jun 26 '20 at 10:27
  • @GregH can you post photos of the breaker panel then, please? – ThreePhaseEel Jun 26 '20 at 11:46
  • With the heat pump, how is emergency heat going to be handled? (When the weather is too cold for the heat pump to function). Typical midwestern house that is two 70A circuits of electric strip heaters. – Harper - Reinstate Monica Jun 27 '20 at 19:07
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The decision to ask for a "load calclation" in order to get a permit is a local authorities prerogative.

A load calc includes various loads like heating, laundries, kitchen, bathrooms, square footage and others and it is hard to guess if a 200A service would be adequate.

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You're getting very close to the limit. You're up to 170 Amps without the kitchen appliances and recreational devices. The tankless water heater is a large load, takes about 4 times the load of a regular tank heater.

  • what about doing the laundry and baking a cake on a hot day or on a very cold day? – jsotola Jun 26 '20 at 2:20
  • Seems like 195 to me. 100+40+30+25? – Joe Jun 26 '20 at 3:47
  • @Joe That's a 240 volt circuit Two single 50 Amp breakers or 1 double pole 50 Amp. 50 amps either way. His oven is a double 40 for 240 Volts, he could use two single 40's. The current alternates between the two breakers so each one has to handle the max. – JACK Jun 26 '20 at 12:27
  • @jsotola The burners aren't being used and all these items will be cycling on and off.. save me a piece of the cake. – JACK Jun 26 '20 at 12:29
  • Ah, gotcha - thanks for explaining ! – Joe Jun 26 '20 at 14:30

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