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If the low voltage transformers are fed from the high voltage system then the 3 phases will be in the same time voltage plane.
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Not if the transformers are delta/wye...there is a 30 degree shift in that case, and this is the most common type of transformer that is used to change from 480 to 208/120.
Don
 
Hmmmmm...........:confused:

So bottom line is the graphic is incorrect in stating that the max voltage is 380?

Also there could be a scenario were the max voltage could be around 157 (thats if they were the same phase ). In this case it wouldn't be a violation if you put them together in the same device box without a barrier?

ibew441dc
 
ibew441dc said:
Hmmmmm...........:confused:

So bottom line is the graphic is incorrect in stating that the max voltage is 380?

ibew441dc
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It's not stating 380 volts, it's stating 386v and it would still need a barrier.

The easieast way to see this for yourself is to check an online 480 v Delta to 208y/120v wye transformer on one of your jobs.

Using a Volt Meter, check all the combinations and make list of the numbers per combination of phases.


Roger
 
1008709181_2.gif
 
roger said:
It's not stating 380 volts, it's stating 386v and it would still need a barrier.

The easieast way to see this for yourself is to check an online 480 v Delta to 208y/120v wye transformer on one of your jobs.

Using a Volt Meter, check all the combinations and make list of the numbers per combination of phases.


Roger
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roger,

the graphic above is the one I'm refering to.

ibew441dc
 
ibew441dc said:
roger,

the graphic above is the one I'm refering to.

ibew441dc
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Oooops, I misunderstood what you were refering to and I appologize.

I think Mike Holts graphic is close enough when speaking in the terms of "nominal" numbers.

You are correct that in one combination the voltage would be below 300v and would not technically need a barrier, but if I were an inspector I would want to see this were the case before I approved it.

Roger
 
roger said:
I think Mike Holts graphic is close enough when speaking in the terms of "nominal" numbers.
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Agreed. When I realized that the 6 volts became the debate . . . :roll:

You are correct that in one combination the voltage would be below 300v and would not technically need a barrier, but if I were an inspector I would want to see this were the case before I approved it.
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Agreed again. Were it that I was doing such a job, while the one combination could excuse a barrier, I'd install them everywhere the separate systems shared a box anyway.
 
Actually if the 208/120 and the 480/277 transformers are both supplied from a higher voltage you could have legs 180 degrees out of phase and would have 397 volts.
Don
 
Everyone

Thanks fot the feedback. This I have a firmer grasp in how to figure this type of problem out, this was just one of those things that was driving me crazy because I could'nt figure it out.

ibew441dc
 
hang in there

hang in there

As soon as the oportunity arises I will certainly test this theory I have long ago sold my oscilliscopes however as soon as the opportunity arrises I will test this theory with a true rms meter and post back I will be able to determine vectorially the phase shift. will probably be within a month I am installing a couple of xformers and delta to wye main distribution panels just have to schedule the install with gc.
 
30 degree phase shift

30 degree phase shift

don_resqcapt19 said:
Not if the transformers are delta/wye...there is a 30 degree shift in that case, and this is the most common type of transformer that is used to change from 480 to 208/120.
Don
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Allright here are the results of a delta wye 480----120/208 150kvaxformer I just put into service today. No load results A-B 485 A-C 486 B-C 485 A-a179 A-b378 A-c292 /B-a294 B-b178 B-c377 / C-a382 C-b297 C-c184 Key Capital letters are 480 delta small letters are 120/208 phase terminals can someone please crunch the numbers and determine the phase shift on the 120/208 output>>??
 
quogueelectric said:
allright I bite ........what does rattus mean is this a knock knock joke??
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No. Rattus is the residential theory guru. He's the one who will point out stuff like "the average value of a symmetrical sine wave is zero."
 
Rattus

Rattus

LarryFine said:
No. Rattus is the residential theory guru. He's the one who will point out stuff like "the average value of a symmetrical sine wave is zero."
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k I understand now. Am a little rusty with my calculus these days out of college about 30 yrs now.
I will ask the kids tommorrow they are 1 and 3 maybe they know. Tell Don I did not find any 386 today maybe it is because it is in New York
 
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quoqueelectric,
The calclulations show that the voltages with a 30 degree phase shift would be 188, 305 and 388. This is based on line to ground of the 480 volt side being 280 volts (486/1.73). If I change the phase shift from 30 to 25, I get 176, 311, and 382. As with a lot of electrical calculations, they don't always exactly match the real physical world.
Don
 
phase shift

phase shift

I will crunch this weekend too busy now 11pm eastern and just got home. I will get more measurements tommorrow. Phase to neutral
 
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