Single phase load on 208/120Y calculation req'd by Oreg

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Re: Single phase load on 208/120Y calculation req'd by Oreg

I'm not a mathematician and I'm very poor in that department but I do no that we are dealing with 208 volts and not 240 volts it was converted to 208 volts because of the three phase wye configuration. If you take a meter and measure from line to line you will have a nominal voltage of around 208 volts not 240 volts which the constant of 1.154 gives you.And we are not using the third leg with single phase.


Its as simple as I=P/E

My editing keeps changing on me? I edited to add leg then seen the 1.154 was wrong and then my leg disapeared? :)

[ February 17, 2005, 08:37 PM: Message edited by: ronaldrc ]
 
Re: Single phase load on 208/120Y calculation req'd by Oreg

Well, what can I say. My original thought was that it did not make a bit of sense. But I felt that I needed to give the idea the benefit of the doubt. I researched and researched and could not find one tiny bit of information to back it up. You guys have confirmed my understanding of the issue, I just wanted to make sure I was not missing something. I sure appreciate all of you input. It really puzzles me how someone of such high position can make this stand. I will continue to monitor this thread.
 
Re: Single phase load on 208/120Y calculation req'd by Oreg

I think what is really happening here is a failure to communicate. I would like to see the exact wording of a test question for which that specific formula is described as being required by the administrators of Oregon?s Electrical Supervisor?s Exam. I think it likely that the actual test question will not match Ed?s description:
Originally posted by Ed MacLaren: If the calculation is for the current drawn by a 208 volt single phase load supplied from a three phase 208/120Y system, the learned gentleman is wrong.
Click to expand...
Ed is right. It?s that simple. That formula will give the right answer to a different question.

Specs: Can you ask your ?learned gentleman? (I would not let anyone describe me that way! ;) ) to give you, in writing, the exact wording of ?the question? for which this formula gives the ?only acceptable answer??
 
Re: Single phase load on 208/120Y calculation req'd by Oreg

The formula is expected when calculating a service or feeder calculation.
Always used in this manner...

6kva 1 phase 208vac toaster. The expected answer for current on line 1 is 25amps and line 2 is 25amp. Forget about any demand factors or derates for the sake of discussion.

In my way of thinking the answer would be 29amp on line 1 and line 2. If someone gives this answer on the test it would be marked wrong.
 
Re: Single phase load on 208/120Y calculation req'd by Oreg

25A is correct for 240V. 29A is correct for 208V. I still do not get it, and I think no one on this forum gets it! Someone needs to talk to the Governor about this!
 
Re: Single phase load on 208/120Y calculation req'd by Oreg

I do I have seen it all through this forum most everyone thinks you apply Sqrt(3) or 1.73 to everything derived from a threephase source and its simply not true you need the third hot leg.

Ronald :)
 
Re: Single phase load on 208/120Y calculation req'd by Oreg

ronaldrc,

Sqrt(3)x120=208, but otherwise it is a single phase problem. The phase voltages will be out of phase with the load current, but that is of no matter to us.
 
Re: Single phase load on 208/120Y calculation req'd by Oreg

Yes I know Rattus its 208 instead of 240 because of the phase angle of the Wye configuration and the alternator rotor is not creating a full 120 voltage in the phase that is offset from it.The system is acting like a auto transformer.

Ronald :)
 
Re: Single phase load on 208/120Y calculation req'd by Oreg

Ronald,
I beg to differ.

The two legs of the wye produce 120Vrms each separated by 120 degrees. To get 208V, we SUBTRACT, not ADD, the two vectors. The third winding is unused. This has nothing to do with the alternator rotor position. That would imply that we are working in the time domain which we are not.

Furthermore, I see no autotransformer here. There are distinct primary and secondary coils wound on separate cores.

[ February 18, 2005, 03:39 PM: Message edited by: rattus ]
 
Re: Single phase load on 208/120Y calculation req'd by Oreg

Rattus

Yes Rattus they are separated in time by 120 degrees.

But the winding are physically hook together in a delta formation. Now image that the alternator is turning and the Phase A is in perfect aligment with the A phase coil at this point the maximum voltage output is being produce in Phase coil A. but since B coil is tied together with A phase coil there is a voltage being conducted through it also ths is where I get the autotransformer effect.

I never said the vectors add.I said the next phase was offset.

Ronald :)
 
Re: Single phase load on 208/120Y calculation req'd by Oreg

Ronald, you have lost me. We are talking of two 120V secondary windings of a wye configuration. That is all we need to know about this particular problem.

None of us so far has been able to understand the 1.154 factor in the formula. We know that it is the ratio of 240 to 208, but we do not understand the reasoning for including it in the formula for current.

[ February 18, 2005, 09:07 PM: Message edited by: rattus ]
 
Re: Single phase load on 208/120Y calculation req'd by Oreg

None of us so far has been able to understand the 1.154 factor in the formula. We know that it is the ratio of 240 to 208, but we do not understand the reasoning for including it in the formula for current.
Click to expand...
My guess is that the person who came up with that formula was not familiar with 208 volt single phase loads.

He probably thought that if a load is rated at 240 volts, the current would be determined by the 240 volt rating.

Ed
 
Re: Single phase load on 208/120Y calculation req'd by Oreg

Sorry about that Rattus

I guess I did get off track when I seen everyone trying to apply 1.73 to substitute for 1.145 and 1.145 made no sense this upset me.



My thinking is the person that came up with this idea thinks if you don't use the third leg then it figures with 240 volts.

Ronald :)
 
Re: Single phase load on 208/120Y calculation req'd by Oreg

Gentlemen, I work with specs and I also had a hard time trying to figure out just where this test writer was getting the information from. I deal in the math end of teaching electrical apprenticeship along with the code and could not find a rational reason for the use of the "factor" in a single phase loading.

I found a wonderful graphing program on the internet and proceded to use it to try to graph the results of two 120v vectors, 120 degrees apart, added, to make a combined, possibly distorted waveform that would explain the use of a factor in a single phase load on a 120/208 3ph Y system. Alas, I could not generate a distorted waveform that would account for the "factor". What I DID find was that the addition of the two vectors defined a very pure sine wave that is 60 degrees shifted from them both.

My conclusion is: The ONLY time sqt(3) is needed is to account for the 3 vectors of 120v added in such a way to give 208v on any two legs. Once that conversion is done, just use Ohms law to find the line current for the single phase load. NO other factors needed.

I also am beliving that the "learned gentleman" from Oregon is in error. Just my two cents. GRL
 
Re: Single phase load on 208/120Y calculation req'd by Oreg

GRL,

What you say is correct except for the 60 degree phase angles, and this can be a sticky wicket as they say somewhere.

Draw vectors for the two legs of a wye including the arrowheads,

Va = V @ 0 (due east)

Vc = V @ -120 (south by southwest)

Then to find Va relative to Vc, we SUBTRACT Vc from Va,

Vac = Va - Vc = V @ 0 - V @ -120

= V @ + V @ 60 = 1.732 x V @ 30

The differences in phase are then 30 and 150 degrees.

What I am saying is that you must consider the direction of the vector (arrowheads) and reference the angles to the horizontal x=axis.
 
Re: Single phase load on 208/120Y calculation req'd by Oreg

Edited and deletied because I posted in wrong tread :eek:

[ February 20, 2005, 04:04 PM: Message edited by: ronaldrc ]
 
Re: Single phase load on 208/120Y calculation req'd by Oreg

Gentlemen,
Your comments are valued.

grlsound,
I am interested in the graphing program you were using. Could you give us the URL for this program.
 
Re: Single phase load on 208/120Y calculation req'd by Oreg

Hi All, Yes, the graphing program is at http://www.ournet.md/~fngraph it is freeware, and works great. The program generates graphs only within its own program, so I cannot insert an image here.

When I spoke of the 60 degree offset, maybe I should have said, the combined waveform is 60 degrees BETWEEN the two original waves. I used the function: sin(x)+sin(x-2.1) to generate the graph. The 2.1 is the linear distance to get 120 degrees of rotational offset. That probably makes no sense whatever unless you use the FNGraph software.

Hope that helps. I hope to setup an experiment on Monday to check that the math works in reality. I'll post the findings here after the experiment.

Garrett
 
Re: Single phase load on 208/120Y calculation req'd by Oreg

Thanks so much for the information.

"It is not what you know, but who you know."
 
Re: Single phase load on 208/120Y calculation req'd by Oreg

Garrett,

Were you able to do your experiment? Do you have any results?
 
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