# 240V HWT on 208V



## telsa (May 22, 2015)

The effect is NIL.

The thermostat merely keeps the 208V coils running longer.

The user never notices the difference.

You'd have to be dealing at Industrial power levels before the voltage difference kicked in.

Which you'll never see in your entire career. 

Serious industry uses natural gas, coal, oil ANYTHING but electricity to create hot water.


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## wcord (Jan 23, 2011)

Funksparky said:


> Does anyone know how much the performance will be affected of a hot water tank with 240V elements running on 208V? As in recovery time or amount of hot water available before it can’t keep up? Will it be much different? Are 240 volt tanks used all the time on 208? They seem far more common. Thanks y’all.
> 
> 
> Sent from my iPhone using Tapatalk


The elements will run roughly 25% less. A 3000 watt element will only produce 2250 watts.
There is a noticable delay in heating up an entire tank.
Here's the data from a Rheem 4500 @240 volt
Watt Density Classification: LWD, Input Voltage (V): 208, Input Voltage (V): 240, Wattage at 208V (W): 3380
Watt Density at 208V (W/sq. in.): 70, Wattage at 240V (W): 4500, Watt Density at 240V (W/sq. in. ): 93


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## splatz (May 23, 2015)

AC voltages are based on resistance heating so it's straightforward...

208 / 240 = .86666666666666666

∴ If it takes 10 minutes to recover at 208V, it would take 8:40 at 240V


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## CoolWill (Jan 5, 2019)

As a matter of watts, a 4500 watt, 240 V element will be only 3380 watts at 208 V.


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## 3DDesign (Oct 25, 2014)

CoolWill said:


> As a matter of watts, a 4500 watt, 240 V element will be only 3380 watts at 208 V.


Please clarify your math. I'm coming up with 3900 watts.
208/240*4500=3900


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## CoolWill (Jan 5, 2019)

3DDesign said:


> Please clarify your math. I'm coming up with 3900 watts.
> 208/240*4500=3900


Wattage varies by the square of the change in voltage, not in direct proportion to it. Or you can use Ohm's law to work it out. The resistance of a 4500 Watt element is 12.8 ohms. At 208 volts, 12.8 ohms will draw 16.25 amps. 208 x 16.25 = 3380


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## 3DDesign (Oct 25, 2014)

CoolWill said:


> Wattage varies by the square of the change in voltage, not in direct proportion to it. Or you can use Ohm's law to work it out. The resistance of a 4500 Watt element is 12.8 ohms. At 208 volts, 12.8 ohms will draw 16.25 amps. 208 x 16.25 = 3380


I've obviously been out of the classroom too long, you're correct, thanks.


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## wcord (Jan 23, 2011)

3DDesign said:


> I've obviously been out of the classroom too long, you're correct, thanks.


I've long forgotten the theory, but experience taught me that resistive heating elements produce way less heat than a simple voltage calculation would indicate


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## MechanicalDVR (Dec 29, 2007)

wcord said:


> I've long forgotten the theory, but experience taught me that resistive heating elements produce way less heat than a simple voltage calculation would indicate


While all that maybe true I've never heard of anyone saying they noticed the difference in the hot water the elements in their tank put out.

When a bad bottom element was replaced I have had them say they had hot water longer in the shower but they obviously didn't know when the element had died in the first place.

Recovery on an electric tank is crap compared to a fossil fuel, oil having the best recovery rate..


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## CoolWill (Jan 5, 2019)

MechanicalDVR said:


> While all that maybe true I've never heard of anyone saying they noticed the difference in the hot water the elements in their tank put out.



The only difference is the time it takes to reach temperature. And since most people aren't timing it, no one will care.





> Recovery on an electric tank is crap compared to a fossil fuel, oil having the best recovery rate..



I have a tankless on propane. And while its true that I have yet another bill to keep up with, it is superior. I also cook with propane because electric stoves suck.


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## MechanicalDVR (Dec 29, 2007)

CoolWill said:


> The only difference is the time it takes to reach temperature. And since most people aren't timing it, no one will care.
> 
> 
> 
> ...


*I don't have propane like that yet but it's coming soon!*


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## glen1971 (Oct 10, 2012)

wcord said:


> I've long forgotten the theory, but experience taught me that resistive heating elements produce way less heat than a simple voltage calculation would indicate


I vaguely remember it so I had to look it up.
"A Watt is a unit of power, defined as the rate of energy transferred per second. The energy is usually defined as Joules; therefore, one Joule per second is one Watt. When measuring heating effects, a unit of thermal energy known as a calorie is used. One calorie is 4.184 Joules. Materials have a property called thermal capacity or specific heat. This is a measure of how many calories are needed to raise 1.0 gram of the material one degree Centigrade."

Less watts=less joules=less calories, which means less heat available to the surface of the material trying to heat. If the thermal capacity stays the same, and the energy to it drops it will take longer to heat that material up one degree.


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## wcord (Jan 23, 2011)

Ask a family with teenage kids how long to takes to heat a tank , after said kids had showers.
Electric recovery sucks at the best of times.


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## 99cents (Aug 20, 2012)

Essentially you have a dual wattage unit. If you want the unit to operate at 208 the same as 240, you need to up your watts which will probably mean upping your volts, if that makes sense.


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## 99cents (Aug 20, 2012)

CoolWill said:


> Wattage varies by the square of the change in voltage, not in direct proportion to it. Or you can use Ohm's law to work it out. The resistance of a 4500 Watt element is 12.8 ohms. At 208 volts, 12.8 ohms will draw 16.25 amps. 208 x 16.25 = 3380


You only square it if it’s 3 phase. Am I wrong?


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## CoolWill (Jan 5, 2019)

99cents said:


> You only square it if it’s 3 phase. Am I wrong?



You're thinking of the square root, not the square.


Consider how current is proportional to voltage in a resistive circuit. And power is the product of current and voltage. So, if you double the voltage, you also double the current. Since each quantity doubled, the power is 4 times greater. That is, the change in voltage is squared. 2 squared is 4. If the voltage were increased 3 times, the power would be 9 times.


It works the other way too. If you cut the voltage in half, the power is 1/4, since 1/2 squared is 1/4. In the case of 240 vs. 208, the change in voltage is 0.866 times... which squared is 0.7511.... 4500 x 0.7511 = 3380.


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## MikeFL (Apr 16, 2016)

If the facility has 208 power I doubt it has teenagers taking showers (although anything's possible).


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## CoolWill (Jan 5, 2019)

MikeFL said:


> If the facility has 208 power I doubt it has teenagers taking showers (although anything's possible).



Lots of apartments are 208. Just sayin'


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## eddy current (Feb 28, 2009)

This is actually a common question on our red seal license exam. 

How many amps will a 4500 w / 240 volt hot water tank draw if it is connected to 208 volts?

The common error is using the full wattage (4500) and ohms law. Many don’t realize that there will not be the full wattage if you don’t give it it’s rated voltage. You need to find the resistance to get an accurate amperage if using ohms law to solve.


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## MikeFL (Apr 16, 2016)

CoolWill said:


> Lots of apartments are 208. Just sayin'


That did come to mind after I posted. We don't do that around here but up north seems to speak often of it.


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## micromind (Aug 11, 2007)

Interesting note with 4500W 240V water heaters connected to a 208 system........

We're all used to using a 30 amp 2 pole breaker and #10s to these devices but if the system is 208, using a 30 is a violation of 422.11(E). 

This states that the overcorrect protection shall not exceed 150% of the rating of the device. 

A 4500W resistive load connected to 208 will draw 16.25 amps. 16.25 X 1.5 = 24.375 Therefore, a 25 amp breaker is the maximum. 

Further, using a 20 amp breaker and #12s is compliant. The reason being that 220.5(B) allows you to drop any decimal that's less than 0.5 So the 16.25 amp load becomes 16.


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## Southeast Power (Jan 18, 2009)

I remember punching out a condo and having a complaint about an electric stove under performing on 208 volts. 
We didn't specify or supply the appliances so, have a nice day.


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## eddy current (Feb 28, 2009)

I did a renovation in an apartment complex where the GC supplied all the baseboard heaters. Original ones were actually rated 208 volt and the new ones were dual rated for 240 and 208. The new ones were physically the same size but the Problem was the new ones at 208 did not get as warm as the old ones did. Many complaints from the tenants.


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