non simutanious water heater load

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Dbronx

Senior Member
Location
Colorado
water heater specs
120 gallon
4500 watt rated non simtanious with 2 elements
208 volts

I was told by an inspector at a arm forces camp that the 20 amp 2p CB is under rated and the 2p 60 amp disconnect was over sized for a 4500 watt rated non simontanious water heater. As I caculated it 4500 watts @ 208 volts to be 12.5 amps that is 62.5% of 20 amps CB. It was told to install a 60 amp 2p CB to comply. Can someone enlighten me if I am wrong
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
4500W @ 208V = 21.6 Amps, not 12.5A. Per 422.13, the circuit would be considered continuous load, so you would need a 30A branch circuit.

You would need a 60A circuit if the two 4500W elements could operate simultaneously.
 

Strathead

Senior Member
water heater specs
120 gallon
4500 watt rated non simtanious with 2 elements
208 volts

I was told by an inspector at a arm forces camp that the 20 amp 2p CB is under rated and the 2p 60 amp disconnect was over sized for a 4500 watt rated non simontanious water heater. As I caculated it 4500 watts @ 208 volts to be 12.5 amps that is 62.5% of 20 amps CB. It was told to install a 60 amp 2p CB to comply. Can someone enlighten me if I am wrong
Just out of curiosity, what formula or math did you use to arrive at 12.5 amps? The formula is P/E=I or 4500/208
 

Dbronx

Senior Member
Location
Colorado
I used P/E=I is there a step I missed or a different formula to use. I used a amp probe to measure amps and when I switched it on it only drew 12-14.5 amps and did not trip over night and the water was hot.
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
I used P/E=I is there a step I missed or a different formula to use.
Using I=P/E, you should get 21.6 Amps, not 12.5 Amps. I=4500/208 = 21.6

I used a amp probe to measure amps and when I switched it on it only drew 12-14.5 amps and did not trip over night and the water was hot.
Is it possible that this is a 4500W, 240V rated water heater connected to a 208V circuit? You'd expect to see approx. 16.2A draw in that case.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector
What voltage does the nameplate indicate ?
A common rating on an element is 4500 watt at 240 volts.. If you are using these on a 208 volt system your current will be less.
(also, double check your nameplate.. a 5000/4500 is highly unusual)

(david, I was typing as you posted..sorry.. same info basically)
 
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charlie b

Moderator
Staff member
Location
Seattle, WA
Occupation
Electrical Engineer
I don't know if anyone actually manufactures a water heater that meets this description, and that takes three phase power. If it did exist, then the math would work out as follows: I = (4500) / (208 * 1.732) = 12.5 amps. Perhaps that is the source of the OP's original calculation error.
 
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charlie b

Moderator
Staff member
Location
Seattle, WA
Occupation
Electrical Engineer
I was told by an inspector (that) . . . the 2p 60 amp disconnect was over sized. . . .
That, at least, is an error on the part of the inspector. Put a disconnect rated at 4000 amps in this application, if you like. That might prove unnecessarily expensive, and perhaps a bit awkward looking. But the only code requirement is that the disconnect be rated at least as high as the rating of the circuit in which it is installed.
 

kwired

Electron manager
Location
NE Nebraska
If element is rated 4500 watt at 240 volts it will only draw 3380 watts if 208 volts is what is applied.

4500 watts @ 208 volts = 21.6 amps

3380 watts @ 208 volts = 16.3 amps

What is voltage rating of element?

What is supply voltage?

If your meter showed only 14 amps you may have lower than 208 volts being applied, or the element is not within tolerances, is marked wrong or something of that nature.

3000 watts seems to be what I recall most 4500's to be marked as for a 208 volt rating. This is not what the true resistive load calculates to be but maybe the load is not truly 100% resisitve?

3000 watts @ 208V gets very close to your 14 amp reading.
 

jumper

Senior Member

broadgage

Senior Member
Location
London, England
The calculated current of 16.3 amps and the measured value of 14 amps are not that much different.
16.3 amps at 208 volts would be less than 15 amps if the utility voltage is low and there is voltage drop between the service and the heater.
If the heater is only 95% of the nominal wattage, which is an entirely reasonable tolerance, then that is 14.25 amps, or within reasonable instrument error.

I would measure the actual voltage at, or close to the heater terminals.
If it is only drawing 14 amps on a generous 208 volts, then it may be faulty or wrongly marked.
If the actuall supply is about 190 volts, then 14 amps is about what should be expected.
 

Strathead

Senior Member
I don't know if anyone actually manufactures a water heater that meets this description, and that takes three phase power. If it did exist, then the math would work out as follows: I = (4500) / (208 * 1.732) = 12.5 amps. Perhaps that is the source of the OP's original calculation error.
How did you get so smart?
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Electrical Contractor
If the top element is 5000 watts and the lower 4500 watts then you must use the 5000 watts as you calculation. Again is the 4500 watt and 5000 watt nameplate at 240V or is that at 208V.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Electrical Contractor
That is a baby unit, try 15K and larger if you want to get serious about heating something;)
My point was that they make small units even with 480V. I would have expected a lot larger units with 3 phase 480V. I have wired a 26 or 28 kw unit that was 3 phase so I know they make them bigger.
 

Dbronx

Senior Member
Location
Colorado
David,

I will check the name plate I'm TDY right now so it will be a day or so please stay in touch

Thanks

Using I=P/E, you should get 21.6 Amps, not 12.5 Amps. I=4500/208 = 21.6



Is it possible that this is a 4500W, 240V rated water heater connected to a 208V circuit? You'd expect to see approx. 16.2A draw in that case.
 
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