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Good Afternoon Everyone,

I have a few question about amperage sizing. I am looking to install a tankless water heater in our home because its a small home and the current tank water heater is placed right next to the kitchen and upgrading to a tankless water heater would allow the addition of a very much needed pantry to the already small kitchen. Well in preparation to the project I wanted to make sure I had the amperage availability to do so and thus the research started and I have a few questions I couldn't find answers to.

One post a read (I cannot find the link) said that modern houses can have a "dual pole" incoming electrical service, so the home actually can have double total available amperage than what is listed. I think the poster was either misleading in his explanation or I am not using the right search terms to find more information on this. Is this a possibility?

I did information gathering and found the rated amperage of all of the major devices in the home and they totaled 176.8 AMPs which is about 88% of the total 200 AMPS my home is currently rated for. Is the AMPs listed in the the maximum amount the devices can pull (such as when the device is first turned on) or is this the average running amperage?

The tankless water heater I want to get is the ECOSMART ECO 24, which has an amperage draw of 100 AMPS.

ArchonOSX
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Nick W.
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2 Answers2

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That is very likely the "average running" current rating, not the peak inrush current when you first turn something on. However, you figure of 176A seems very high for an average home. If that is really what you are currently using then your power charges will be quite high every month, AND you have NO EXTRA CAPACITY to add a very-high current device like a tankless water heater.

To really know how much current is being drawn from any branch circuit (breaker) or even from the total load (via the incoming service lines), you would use a "clamp on" AC ammeter. You don't need great precision, so an inexpensive meter like https://www.amazon.com/dp/B00NWGZ4XC would give you ACTUAL numbers to work with.

You did not reveal how much current your tankless water heater needs. But they are typically VERY HIGH current devices and demand heavy wiring and separate branch circuits.

Richard Crowley
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Using the total value of all loads connected is not an accurate way to calculate the system demand. For example, how often do you run the heat and AC at the same time? Code allows for the larger of the 2 to be used. Depending on the number of appliances connected there are derating adjustments that are used. A double pole main at 200 amps has 200 amps of 240v available, There is 200 amps on each leg at 120v, but any 240 devices reduces the total on both legs. Electric on demand water heaters are extremely high current devices usually at 240v a under sink model I just looked up 13000w/240v =54 amp draw, the larger units I have installed draw close to 150 amps this requires multiple 60 amp feeders ( 60 is the largest size allowed 3 were required) .I have only installed a few electric whole house models due to the cost of the service upgrade required. You may be able to place several smaller units close to the locations they are needed without requiring a 300 amp service like the large single units require. Tankless electric water heaters only draw power when the hot water tap is turned on then they draw close to the listed value ( in KW). To calculate the current draw divide kw by the voltage as in the example above. If you get two small of a heater it will not be able to raise the water temperature with a full flow the 13000W unit was only 4 gallons per minute but at that flow it will only raise the temperature 20 degrees. With a 1.5 gallon per minute flow it can raise the temperature 55 deg. Make sure to do your homework on this or you may find yourself less than happy due to the cost of the service upgrade or having cold showers.

Ed Beal
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