True Hot Water Heater Wattage

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Grouch1980

Grouch1980

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New York, NY
I went through a thread from years ago, and I had this follow up question:

A 240 volt, 4500 watt hot heater heater connected to a 208 volt panel will operate at 75% of its nameplate wattage. so 4500 watts x .75 = 3375 watts.

When performing feeder / service calculations from article 220, whether standard or optional it is stated you have to use nameplate ratings. So we 4500 watts. Is this correct?

And when do you use the 3375 watts? Is it when you calculate the branch circuit ratings?... the breaker and wire size?

Thanks.
 
HEYDOG said:
220.82(B)(3) for Optional calculations!
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Interesting that the find function in the PDF reader did not come up with this. This would appear to apply only to the optional calculations though.

I wonder if I misspelled it when I did the search. So I tried it again, and it found a bunch of uses of the word. Then I tried again, and it found none. Very odd quirk.

Since it uses the word nameplate, unless the nameplate has a different Amp rating for the lower voltage, you would have to use the nameplate rating for 240 Volts even though it was operating on 208 V if the code requires nameplate rating be used.
 

True Hot Water Heater Wattage​

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Ok, I was unable to resist commenting on there being no reason to heat water that is already hot, but then…
A 240 volt, 4500 watt hot heater heater…
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Ok, so in the first place why would one heat a heater, and especially a heater being used to heat a hot heater?
😂
 
Jraef said:
Ok, I was unable to resist commenting on there being no reason to heat water that is already hot, but then…

Ok, so in the first place why would one heat a heater, and especially a heater being used to heat a hot heater?
😂
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woops! ... I meant hot water heater.
 
petersonra said:
I was unable to find the word "nameplate" anywhere in article 220. Where are you finding it?
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Standard Calculation: Section 220.53, for appliance loads... it says 75% demand factor to the NAMEPLATE rating load of 4 or more appliances. This includes water heaters.

Optional Calculation: Section 220.82, it says the NAMEPLATE rating of the following, which includes water heaters.
 
Grouch1980 said:
I went through a thread from years ago, and I had this follow up question:

A 240 volt, 4500 watt hot heater heater connected to a 208 volt panel will operate at 75% of its nameplate wattage. so 4500 watts x .75 = 3375 watts.

When performing feeder / service calculations from article 220, whether standard or optional it is stated you have to use nameplate ratings. So we 4500 watts. Is this correct?

And when do you use the 3375 watts? Is it when you calculate the branch circuit ratings?... the breaker and wire size?

Thanks.
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Just curious where does the 75% come from. It comes out pretty close to what you get using ohms law. 240 * 240 / 4500 =12.8 ohms. 208 * 208 / 12.8 = 3,380 watts.
 
Jraef said:
Ok, I was unable to resist commenting on there being no reason to heat water that is already hot, but then…

Ok, so in the first place why would one heat a heater, and especially a heater being used to heat a hot heater?
😂
Click to expand...
Hot water heaters:

www.yahoo.com

Ex-employee charged after stolen water heaters appear for sale on Facebook

A San Angelo man has been indicted after a company's storage unit was burglarized, followed by the stolen items appearing for sale online.
www.yahoo.com www.yahoo.com
 
Grouch1980 said:
I went through a thread from years ago, and I had this follow up question:

A 240 volt, 4500 watt hot heater heater connected to a 208 volt panel will operate at 75% of its nameplate wattage. so 4500 watts x .75 = 3375 watts.

When performing feeder / service calculations from article 220, whether standard or optional it is stated you have to use nameplate ratings. So we 4500 watts. Is this correct?

And when do you use the 3375 watts? Is it when you calculate the branch circuit ratings?... the breaker and wire size?

Thanks.
Click to expand...
It's the same as we just recently discussed with dryers.

If they only give the nameplate in watts then that's what you have to use regardless of the voltages. If they give you different watts or amps for the different voltage, you can use that for the voltage you have.

Note also that heating element resistance actually changes depending on the temp of the element which means the 75% calc is not reliable. The resistance increases with temp which means the element likely reaches equilibrium at higher amps on 208V than you're thinking. I've got a pic of a dryer label that says 24A at 240V and 23A at 208V. That's not a 75% difference even after accounting for the motor.
 
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