Water heater 4.5kw at 240 circuit breaker compared to 208V circuit breaker size

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Cartoon1

Cartoon1

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Florida
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Electrical Engineer
Hello All. I noticed at one of my surveys that a rated 240V 4.5KW water heater single phase has a 2-pole 30AMP breaker, which is fine. But the panel voltage is 208V. Shouldn't it technically be a 25A breaker for 208V since the power of the water heater now 3.3KW? I don't see it as being a problem since the cable is a #10 copper capable for 30Amp. But is this considered a violation? Thanks!

The water heater cutsheet model does not give a FLA or circuit breaker size, just that is rated for 4.5KW and is capable of doing 208V also.
 
On a load of 3.3 kw on 208, you are correct the breaker should probably be a 25 amp based on the wording of 422.11. I agree , no big deal.
 
Do the math. A 4.5 KW heater @ 240 volts draws 18.75 amps and has 12.8 ohms resistance . A heater with 12.8 ohms @ 208 volts only draws 16.25 amps and would only consume 3.38 KW. Hard for some peop!e to grasp that if you halve the voltage on any heater you only receive a quarter of the wattage. Ran into trouble where somebody looked at a 10' baseboard heater and sized the BTU'S they would produce while running on 240 volts but were connected to 208 volts. Once it got down below around 25 degrees they were a!ways cold.
 
With these types of calculations, it is good to keep in mind that the upper limit line voltage can be as high as ~223v, with power companies creeping their domestic phase voltage toward 125, 127v etc. My last apartment in my home town would always see 128 - 129v peak throughout the day. This may also continue to change over the next few decades, and it will be more than feasible thanks to the advent of LED drivers, switch mode power supplies and VFD appliances having wide voltage ranges.

On the other hand, like garbo mentioned, if you need a certain amount of heat output or capacity, it is good to account for voltage on the lower extreme as well.
 
Garbo's number:
3380/208=16.25*150%=24.375 amps. Maximum OCPD is 25 amps.

422.11(E)(3) Not exceed 150 percent of the appliance rated current if the overcurrent protection rating is not marked and the appliance is rated over 13.3 amperes. Where 150 percent of the appliance rating does not correspond to a standard overcurrent device ampere rating, the next higher standard rating shall be permitted.
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david luchini said:
I don't see any violation. The appliance is rated 4.5kW, 240V. The appliance rated current is 18.75A. A 30A c/b meets 422.11(E)(3).
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Yep, my take too. 422.11 does not say watts, it says current. Now IF the appliance showed the lower current rating at 208V on the nameplate, that would be different.

Think of it this way; 422.11 is there to help protect the appliance against what happens if it draws too much current. In this case the appliance remains RATED for 18.75A. It will never DRAW that much because of the lower voltage, but it is still RATED do do so, even if the voltage was 120V, 12V, 600V etc. Current is current is current. 422.11 doesn’t require you to recalculate the current RATING of the device based on the change in voltage.
 
If you really want to dig into it you should check the voltage / wattage stamped on the actual element.
If the plumber really needs 4500W of water heating for the water demand they sometimes swap the elements with 208V 4500W elements at their shop.
The other thing is 110.3(B) if the manufacturers instructions say to use a 25A breaker.
Here is a chart with the various elements and OCPD sizes that comes with a common water-heater
waterheater-smaller.jpg
 
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