208v water heater

I have a tankless water heater that I installed recently. It is 220 volts, 11.8kW, 54 amps and is connected at panel to a 60 Double Pole / 6 AWG Wire (2-1 type, 2 hots + ground) and to the heater via a 60 amp breaker box that sits next to heater. I want to upgrade to a larger heater that is 12kW 55 amps, but 208 volts.

I am confident my wire, panel breaker, and breaker box will work with this set up, but not so sure about how to go from 220v to 208v.

My questions
1. how do I know if my home can provide 208v?

2. what if I connect the appliance to the existing set up, will it burn out or fail in some way?

Thanks for your help. Bob

 

Well residential single family voltage hasn't been 220v for many decades. You have 240 volts. 208 volts is a single phase voltage derived from a three phase commercial power supply but almost all 208 volt equipment is made to work on 240 volts so check with the manufacturer if you aren't sure.

 

One other thing to consider is that if the device is rated a 12k watts, 55A at 208 volts, supplying it with 240 volts is going to drive up the amperage to around 66-67 amps, thus exceeding your circuit capacity.

 

Well, technically, putting something that draws 54 amps on a 60 amp circuit is exceeding capacity, breakers are designed to handle 80% of their rating. So you really need to change that anyway.

 

" Mod note: Above is incorrect for a resistance circuit. Amps equals Watts divided by Volts. 12000÷240=50 amps. "


Mod, please explain why you feel my statement is incorrect. The 12K watts rating for the new proposed hot water heater is at 208 volts. So if you increase the voltage from 208 to 240 the wattage/amperage doesn't remain the same, or I completely misunderstand ohms law calculations?

 

Ray he cant get 208 volts from his panel right? Thats from 3 phase? The OP has single phase and is limited to 240v or 120v equipment period, right?

 

ednu: the wattage is generally the constant in the equation so if the voltage goes up, the amperage goes down.

 

Isn't the only constant the resistance value of the heating element, not the wattage? Putting different voltages on a heating element using ohms law calculations shows an increase in wattage and amperage, if the resistance stays the same?

In this instance the heater is rated at 12k watts with 208 volts, then the resistance using ohms law calculations shows a resistance of 3.605333 ohms.(R = voltage squared / wattage)

If you take that resistance and apply 240 volts(p = voltage squared / resistance) you get just short of 16000 watts and 66.5A?

Am I applying ohms law calculations incorrectly? Thanks for humoring me on this.

 

It's been 50+ years since I took physics so maybe I'm wrong. I have removed my note. Your reply makes sense.

 

With my IQ today I'll let the pros answer.

 

Here's where I was corrected in a similar question....

Read more: http://www.doityourself.com/forum/el...#ixzz2LYoAcZlg

Resistive circuits such as heating elements are different from things like motors and such.

 

I can buy that. I stand corrected for the second day in a row.

 

Resistance changes based on temperature. So the heater that you're measuring cold will have a significantly different resistance when there is 120v put through it. Additionally, the heater will get hotter when you put 208v through it, and even hotter when 240v is put through it. So you can't assume that resistance is constant.

You want to always use Watts = Volts * Amps when dealing with appliances and anything around the home. In this case, 54A at 208v is 11Kw. If you apply 240v to it, it will still be 11Kw, but now only 47A.

That's another reason that this heater needs to be rated to work with 240v. If not, it won't work as expected and could cause issues since the heating coil will get hotter than expected.

For further reading: Fundamentals of HVACR: Electric Heater Resistance - A Moving Target


Ray - don't worry, physics hasn't changed much since you learned it.

 

Thanks, Vic for the conformation. Thanks Ednu for the correction.

 

Well...I guess Furd (who originally posted my quote) is going to have to hold a class for us in another thread....lol.

I'm guessing that a WH might be different from a strip heater as referenced by Zorfdt in the article?

 

But my mind is like a sieve full of oil and the oil is getting thinner.

 

The heaters are not constant power components. Lower voltage will yeald less power. The resistance will change a little but the water temp will keep it very close to a constant resistance.

 

Yes, That's correct. Low-voltage 3-phase power is 120/208V. 120V to neutral and 208V between any two hot phases. Thar's due to the difference in the form of the output from Wye and Delta transformers, but it doesn't matter. The important point for this discussion is the one you stated.

 

Yeah here is a 208/240v tankless heater. Page 9 shows amp draw and Kw between the two voltages.

http://www.stiebel-eltron-usa.com/pd...tempraplus.pdf

 

Zorfdt, I'm not looking to be argumentative here, but I just can't see how what you state to be correct. I've read the article that you reference and also looked at the temperature coefficient of resistance tables for different materials. I don't see in the article where it states there is a significant difference in resistance based on temperature and the article quotes "Fortunately, the relatively small difference we see in electric strip heaters does not create a substantial problem for us, but it does raise questions when trying to demonstrate Ohm’s Law." The temperature coefficient tables show only a miniscule percentage difference in resistance values for most materials per degree Celsius increase.

" You want to always use Watts = Volts * Amps when dealing with appliances and anything around the home. In this case, 54A at 208v is 11Kw. If you apply 240v to it, it will still be 11Kw, but now only 47A."

Unless I read this wrong you appear to state that there is no difference in wattage, but a decrease in amperage if the voltage is increased for the same heater? For the same device to be rated at 47A(240 volt) versus 55A(208 volt) the resistance would need to change from 3.7818 to 5.1063 which is a substantial difference imo and doesn't appear to relate to temperature versus resistance change in the material I see. Granted the difference in voltage from 208 to 240 is only 32 volts, but for that argument to be correct would you also think that putting 120 volts into the same heater would produce 12000 watts? I'm fairly certain that wouldn't be the case.

Lastly isn't the real question here that he is considering using a heater rated 12k watts at 208 volts and 55 amps. Since he obviously doesn't have 208 volts, he's looking to power it with 240 volts and is wondering what that means to his heating capacity and existing circuit.

Edit: I see Ray has posted a document showing differences between voltages on a water heater. Thanks, I didn't mean to beat this to death, just wanted to understand better.

Edit: Oops, that was lawrosa who posted the document, not Ray, sorry.

 

Gee, I take some time off and all heck breaks loose.

Not that it matters to the original topic since the original poster hasn't returned but everyone posting has a bit of the story correct and a bit incorrect.

ALL electrical appliances (meaning everything electrical) are designed to operate at a specific voltage and amperage. Some things are designed to work on multiple voltages and these generally require manual reconnecting or the changing of a switch, the notable exception being a switch mode power supply that will often have a fairly wide voltage input range.

Motors, as one example, need to be made with dual windings and the windings are connected in parallel for the lower voltage and in series for the higher voltage. You CANNOT take a motor designed for 120 volt ONLY operation and connect it to 240 volts and expect it to run at half the amperage of 120 volt operation. In reality, the motor will draw excessive amperage, far more than the 120 volt amperage and it will burn out the windings. By the same reasoning, you cannot take a 240 volt only motor and expect it to run on 120 volts with the only change being double the 240 volt amperage draw. It just plain does not work that way.

Now, looking at a straight resistance heater it IS the resistance of that heater that is the defining specification. Resistance heaters are designed to a specific temperature and heat output with a specific voltage and amperage input. The amperage draw IS, of course, dependent upon the applied voltage AND the resistance of the element. Changing either the applied voltage will most assuredly change the amperage draw and consequently change BOTH the temperature of the element and the quantity of heat released; it also changes the total wattage of the heater. HOWEVER, it does NOT act as most people believe. Increasing the voltage on a resistance heater will NOT cause the amperage to fall, just the opposite, it will have the current rise and the total wattage will increase as well. The operating temperature will increase and the total heat output will increase and if the increased voltage is high enough it WILL cause the element to overheat and burn out. Conversely, lowering the voltage will cause the amperage to decrease, the temperature to decrease and the total heat output to decrease but the life of the element will likely be increased significantly.

If I remember this correctly operating a single phase 240 volt heater on 208 volts will result in a 25% drop in heat output and about a 13% decrease in wattage. Also significantly longer life of the element. So it is often okay to run 240 volt heaters on 208 volts PROVIDED the decreased temperature and heat output is still sufficient for the end results desired. In the case of a large tank-type water heater this is often done with acceptable results. In the case of an instantaneous water heater this might also work but would be far more critical and the result would most likely NOT be acceptable.

Let us now move on to the original question, can an instantaneous water heater designed to operate on 208 volts be operated with a 240 volt supply? The quick answer is NO, it will draw excessive amperage and burn out the heating elements. However, by use of a buck-boost transformer you CAN lower the input voltage from the initial 240 volts to 208 volts. The sizing of the transformer is critical to avoid overloading the transformer but it does NOT need to be anywhere near the full watt rating of the heater. The question of whether or not it is cost effective to use the buck-boost transformer depends on the cost of the heater and the cost of the transformer versus the cost of just getting the proper heater in the beginning.

 

 

Only things I can add to this conversation:

1) You should be able to use the full load rating of a breaker as a tankless water heater is not a continuous load.

2) While 208 is most common in 3 phase Wye systems, it does not require the use of all 3 ungrounded conductors to connect to a single phase load. You still get 208 between any two hots.