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05292007, 10:40 AM  #1 
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Amps vs wattage
I've been looking to put in an electric tankless water heater which requires quite a bit of power. Looking at the charts it is reasonable to expect one that requires at least 80 amps, and I only have 100 through the meter right now.
So I had an electrician come out to give me a quote on boosting power and we got into a discussion about why I needed more. He started into a conversation of wattage vs amps, saying that even though they might require 80 amps, the wattage will say it draws much less power and I might not really need more than 100amps. I was confused and he wasn't very good at explaining the difference to me. I wasn't going to bring voltage into the equation. So I went back to the charts and it confuses me further, I'll give you an example: The one I'm looking at says 208 volts, 14.4kw, 70 amps; OR 240 volts, 19.2kw, 80 amps.... and the 80 amps requires 2 50amp breakers. I hope somebody has an easy way to explain this! Thanks! 
05292007, 03:22 PM  #2 
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Amps vs wattage
You need all three thing to figure load, and ALL are important. Wattage, voltage and amperage.
Wattage is what you draw, NOT amperage. The wattage is constant, the amperage goes up and down according to voltage. Household voltage is 240v, more accurately 120/240v. 19.2Kw?!?!????? Watts divided by volts equals amps. So.... 19,200 divided by 240v = 80 amps. There is NO WAY you will run this unit efficiently with only a 100 amp service to the house. Upgrading to 150 amps is almost worthless. The difference to 200 amps cost wise is negligible, so I would not even consider 150 amps.
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05292007, 05:22 PM  #3 
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Amps vs wattage
You should also know that two 50 breakers means two 50 amp double pole breakers. That is four breaker spaces in your panel.
The reason the two voltages are listed is because you could use this heater in an industrial location where the voltage would be 208 volts. In you home the voltage is 120/240 volts. 
05302007, 08:29 AM  #4 
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Amps vs wattage
This is extreemly rare, but I do have to disagree with my friend Speedy on one point. Wattage is not the constant, resistance is. If you take an x watt 240 volt heater and install it on a 208 volt system it will draw less wattage, and the deta t rise will be less across the coil.
If a home has a 240 volt 100 amp service it would have a total of 24000 watts available. If only 120 volt loads were connected, you could have up to 200 amps of such loads. 24000/120. Since most residential panels have a mixture of 120 and 240 volt loads the only practicle way to add them together is to convert them all to wattage, add them, then divide by the system voltage 240. As to the OP original question. KW / Volts = Amps ..... Volts / Amps = Ohms If you do the math on your appliance you will get 3 ohms at either voltage. I agree with Speedy that upgrading to 150 amps is a waste of time and money. I also think you will be disappointed with your higher electric bill. I have no direct experience, but have heard of alot of people who were disappointed at the savings after installing tankless water heaters. Either the number of years to pay off were longer than the customer expected to live, or the electric usage actually went up.
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05302007, 08:33 AM  #5  
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Amps vs wattageQuote:
And Petey, can you elaborate on "19.2Kw?!?!?!?!" I'm assuming you thing that's a sick amount of power to be drawing? I do know that if I increased I'd likely go to 200 vs 150amps. The other idea that I kicked around was adding another meter and shutoff panel just for the water heater..... that would be even cheaper than upgrading the entire service. Thanks all! 

05302007, 09:12 AM  #6 
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Amps vs wattage
Not to speak for Speedy, but 80 amps is a sick ammount of power for a 100 amp service. The service should only be loaded to 80 percent to start. 80 amps on a 100 amp service would max it out with just the water heater.
The second service may not be an option. Check with your utility company. They may not allow this for a single family home. In addition find out the monthly cost for service and minimum kwh. Then figure out how many years it would take for the extra utility bill to equal the cost of a 200 amp service install.
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05302007, 12:39 PM  #7  
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Amps vs wattageQuote:
As far as the separate service, billing wouldn't be an issue, there is no minimum on electricity (just gas). He did tell me to call the power company first and ask if this was an option on my home. 

05302007, 02:50 PM  #8  
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Amps vs wattageQuote:
Thanks Jeff for the clarification.
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Sometimes I feel like if I answer any more questions it is like someone trying to climb over a fence to jump off a bridge and me giving them a boost. Answers based on the 2008 & 2011 NEC. 

05302007, 07:13 PM  #9  
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Amps vs wattageQuote:
wattage = voltage x amperage (ohms law) 

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