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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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  • Do you have 4 slots available in your service panel ? This will take a minimum of 2ea double pole breakers. – Ed Beal Oct 04 '16 at 19:23
  • I have 6 available slots, (Tankless water heater requires 3 40 amp dual-pole and another 2 slots are currently being used by my current water heater. – Nick W. Oct 04 '16 at 19:51
  • The standard water heater calculation is 5kw or name plate which ever is larger. This can be deducted from your load – Ed Beal Oct 04 '16 at 20:51
  • Can you post the square footage of your home, the number of receptacle circuits you have for the kitchen countertop receptacles, and the nameplate ratings of all appliances on dedicated circuits please? Without that, any attempt at load calculation is a shot in the dark... – ThreePhaseEel Oct 04 '16 at 22:26

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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  • To get the total of 176.8 I added up the total of every major appliance in the house. I don't want to create a situation in which I would have to be wary of what combination of devices are plugged in. As for the amperage of the water heater, The one I am planning on using has an amperage rating of 100A. – Nick W. Oct 04 '16 at 17:56
  • 100 amp would require 2 ea 60 amp 240v feeders. You may be able to get by with 2 50 amp since it is not a continuous load but that would be pushing it. – Ed Beal Oct 04 '16 at 18:11
  • You can see that adding a 100A device to a 200A service that already has 176A of load is wildly out of the ballpark. It is very unlikely that your ACTUAL average load is 176A (or even 76A) That is why it would be beneficial to use a meter to see what the ACTUAL current load is. – Richard Crowley Oct 04 '16 at 18:12
  • Ok, that makes sense. Is there a way to measure the peak amperage over say a 24 hour period? Or is the best way to just watch the meter while various devices are in use? – Nick W. Oct 04 '16 at 18:19
  • Sure, you can get as sophisticated as you want, providing you are willing to spend the $$$ for it. But that is likely way higher cost than benefit. It is the AVERAGE loads that everything has to handle day-in and day-out. There are lots of other factors not in evidence here. Like whether your HVAC and stove/cooker are electric, etc. etc. The ordinary loads like lighting, toaster, microwave, etc. are pretty incidental and likely have rather low average load. But the higher-current loads (existing water heater, electric cook-top, oven, clothes dryer), are where it can add up to overload. – Richard Crowley Oct 04 '16 at 18:27
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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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  • Because I want to be clear, so essentially I have 200 AMPS, if the device is 120v it only uses half the listed amps since it only uses one pole? – Nick W. Oct 04 '16 at 18:13
  • No it uses 100 amps at 120V what is the KW rating? I have not seen a 100 amp 120V water heater that draws anything close to that value. – Ed Beal Oct 04 '16 at 18:42
  • I now see you edited your post after I read it. The echo smart 24 is a 240V 24KW water heater as I posted above 24000 watts /240 Volts = 100 amps. – Ed Beal Oct 04 '16 at 18:47
  • I was talking about a different device (My Microwave specifically) is rated at 15 AMP 120v. Does that mean that device essentially only use 7.5A because its using 15 AMP from one of the two poles? If I am correct in my idea of how the 120v devices work it may mean I can definitely use the tankless water heater. – Nick W. Oct 04 '16 at 18:56
  • If the microwave is listed as 15A (they are usually listed in watts) at 120V it draws ~15 amps when used on high power. There is a difference in the branch circuit required to power a device and the power consumption. There should be a number like 1800W if it was 15 amps. A more common value 1400W or 1200W if it required a 15 amp branch circuit. 1200 W/ 120V = 10 amps. – Ed Beal Oct 04 '16 at 19:06