clsarmiento
Member
What's the correct amps load for a 4.5KW, 2-element (non-simultaneous operation), 480volt, 3-phase electric water heater?
Thanks.
Thanks.
Water Heater Amperage
- Thread starter clsarmiento
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jeremysterling
Senior Member
- Location
- Austin, TX
I'll be the first to ask:
What do you think it is and how did you come to that conclusion?
What do you think it is and how did you come to that conclusion?
jeremysterling
Senior Member
- Location
- Austin, TX
Last week, I had a similar scenario. Had to hook up three-phase non-simultaneous 240V 12kW water heater to three phase 208V.
At 208V, I figured the heater to actually become a 9kW load. I divided 9k by .36 (constant my boss showed me, you will need a different constant for 480V) to get 25A. By NEC 422.13, I went 125% to get 31.25 amps. I ran #8 to a 3P40A breaker.
The unit pulled 23 amps by the ampclamp on either element.
I used a three phase constant (square root of three) but it is not really a balanced three phase load.
At 208V, I figured the heater to actually become a 9kW load. I divided 9k by .36 (constant my boss showed me, you will need a different constant for 480V) to get 25A. By NEC 422.13, I went 125% to get 31.25 amps. I ran #8 to a 3P40A breaker.
The unit pulled 23 amps by the ampclamp on either element.
I used a three phase constant (square root of three) but it is not really a balanced three phase load.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
Yeah but if the element is 4500 watts then each element is single phase not 3 phase. Never seen a 3 phase element. This leads me to ask why the heater is 480V and 3 phase. That does not sound right.
broadgage
Senior Member
- Location
- London, England
So called 3 phase water heating elements certainly exist, I have one in front of me.
They consist of 3 elements assembled in the same housing housing, with either 3 or 4 terminals according to design.
They consist of 3 elements assembled in the same housing housing, with either 3 or 4 terminals according to design.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
I wired a 3 pahse water heater and it consisted of numerous single phase water heaters split among the phases. I believe it was 100kw unit with individual fuses for each element
broadgage
Senior Member
- Location
- London, England
This sounds a bit like an exam question since it is rather a small water heater to be 3 phase.
Assuming it is really 3 phase, then it would consist of 3 elemements each of 1.5kw, they could be 277 volt elements wired in Y with a neutral connection, or they could be 480 volt elements wired in delta. The total load would be the same in either case.
The current in each phase would be about 5.4 amps
Alternatively it might not be true 3 phase at all, but a single 480 volt, 4.5KW element intended to be connected across any 2 phases of a 3 phase 480 volt supply.
In this case the current would be about 9.4 amps, this sounds more likely for a real world water heater, but if it is an exam question, then if they say "3phase" then calculate on that basis.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
I agree it probbaly is a code question but let's say it is single phase to the elements. How do you get the same answer-- the elements do not come on at the same time therefore the load would be 4500/480 v . No?
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
Your answer is correct.
4500 happens to be most common size for household tank style water heaters but is generally single phase 240 volts. I think people are getting too hung up on that instead of looking at the question. But at this site we are really good at picking apart things and occasionally do make fools of ourselves when we run into something not too common, other times we catch a person not fully understanding what they have.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
I looked at the question I just questioned the question.
Once I realized it may be a test question and they make 3 phase 4500 watt elements then I agree with the answers given
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
I looked at the question I just questioned the question.
It is not all that likely to find a 4.5 kW domestic water heater that is 480 volt three phase. It is more likely to find some industrial equipment that may have an element with this rating. Replacement element is not going to be something you find at local hardware store either. Also OP's location was Korea so who knows what he may have.
Sorry, member "transient" was located in Korea. I thought I was replying to the OP.
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I've seen three phase immersion heaters. As a matter of fact, we used one as a dummy load when we were on site re-commissioning some rectifiers. From memory it was 7.5kW and essentially three 2.5kW elements connected in star.
Anyway, FWIW, I agree with the calculated current of 5.41A for the example given by the OP.
Out of curiosity, why do you guys call current Amperage?
Anyway, FWIW, I agree with the calculated current of 5.41A for the example given by the OP.
Out of curiosity, why do you guys call current Amperage?
- Location
- New Jersey
- Occupation
- Journeyman Electrician
jeremysterling
Senior Member
- Location
- Austin, TX
The OP states there are two elements. These two elements share a common leg. Is this three phase open delta?
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
Typically an electric water heater has two similar elements, the lower one cuts in when the incoming cold water reduces the temperature at its thermostat. The top element cuts in when the water which is going to be output from the tank goes below the set temperature.
Normally the upper thermostat is N-pole, double throw and in its off position energizes the bottom thermostat.
That means that the two elements are fed from the same circuit. Energizing the upper element leads to faster heating of smaller volume of water near the top, making it more demand responsive. The lower element kicks in instead to heat the whole tank when water is not being heavily used.
In more complex cases, the top thermostat will be fed from a full-time circuit/meter while the lower thermostat is directly fed from a limited off-peak circuit.
That way the top element will heat water that absolutely has to be used during the day while the bottom element will heat all of the water in the tank at lower electric rates.
For solar PV water heating/pre-heating, the lower element would be fed by PV only while the top element would be fed from POCO.
Bottom line: If both elements were three phase, it could be wye, or either open or closed delta, and the thermostat would be triple pole or a contractor would be used. They need not share a common leg.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
Bad use of language skills like most anything else.
There could be a common leg and other two phases controlled either by 2 pole thermostat or contactor delta or wye doesn't really matter, unless there would be a neutral to the element like 277 volt elements connected in wye configuration with supply neutral connected to mid point.
wallypiper
Member
- Location
- Atlanta, GA, USA
Why even consider non-simultaneous operation of such a small load. Even with simultaneous operation, this is less than 12 amps total load. Most places I have been don't allow power wiring below #12 is allowed anyway, regardless of the actual load. (Not code, but owner rule. Assume this is some kind of industrial installation since it's 3 phase.)
I use many 3 phase electric heaters below 10 KW. Immersion heater in industrial applications.
Many heater applications have to be treated as a continuous load. Can't really tell what this application is.
Immersion heaters, by the way, generally have to be counted as a continuous load whether that is really the case or not. 427.4
I use many 3 phase electric heaters below 10 KW. Immersion heater in industrial applications.
Many heater applications have to be treated as a continuous load. Can't really tell what this application is.
Immersion heaters, by the way, generally have to be counted as a continuous load whether that is really the case or not. 427.4
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
When the words "water heater" are used many automatically envision a typical household water heater with dual 4500 watt elements. Even a single 1500 watt 120 volt unit gets overlooked as being one of the 4500 watt units in most cases in general discussion.
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