Theory Calculation

[theshockdoc]

I know that 277 volts and 120 volts come from two different systems so - if someone could please help me out. In this illustration - how is this voltage calculated?

Thanks - Jimmie

 

[ron]

It is the same reason you don't put two 277V branch circuits of different phases into a common box. The potential difference from Phase A 277V<0 to Phase B 277V<120 is 480V, which is greater than the permitted 300V.

It is adding vectors. The angles make it a little more difficult to understand it intuitively, unless you think about drawing the vector diagrams and adding them on paper (or finding an internet calculator).

 

[mivey]

In this illustration - how is this voltage calculated?

It isn't. It is an approximation. It is more like three-hundred and eighty-something.

277.13@120? - 120@0? = 352.8@137.1?

with an additional 30? phase shift between the sources, you get:

277.13@150? - 120@0? = 385.7@158.9?

I suppose you could even have 277.13@180? - 120@0? = 397.1@180?

At any rate, it is over 300 volts.

 

[LMAO]

It isn't. It is an approximation. It is more like three-hundred and eighty-something.

277.13@120? - 120@0? = 352.8@137.1?

with an additional 30? phase shift between the sources, you get:

277.13@150? - 120@0? = 385.7@158.9?

I suppose you could even have 277.13@180? - 120@0? = 397.1@180?

At any rate, it is over 300 volts.

So they are referring to the worst case scenario voltage deference? i.e. the potential is not necessarily over 300 but could be depending on phase angle between 120 and 277?

 

[mivey]

So they are referring to the worst case scenario voltage deference? i.e. the potential is not necessarily over 300 but could be depending on phase angle between 120 and 277?

It is not specified but it would be reasonable to consider the worse case unless you could prove otherwise.

The lowest I could picture in my head was a 90? difference which gives:
277.13@90? - 120@0? = 302.0@113.4?

Unless I missed it, there may be something below 300 volts but it would be for specifically restricted legs of the supplies.

 

[ron]

It is not specified but it would be reasonable to consider the worse case unless you could prove otherwise.

The lowest I could picture in my head was a 90? difference which gives:
277.13@90? - 120@0? = 302.0@113.4?

Unless I missed it, there may be something below 300 volts but it would be for specifically restricted legs of the supplies.

Wouldn't the lowest be 120V<0 and 277V<0?

Most 120 is created through a Delta to wye XFMR, so it might be 277V<0 and 120V<30.

 

[mivey]

Wouldn't the lowest be 120V<0 and 277V<0?

Most 120 is created through a Delta to wye XFMR, so it might be 277V<0 and 120V<30.

If we look at the worst case, you would pick the highest voltage between any two phases of the two systems. While the two "best" phases might be 277-120=157, that same 120@0? needs to be compared to the other two 277 phase possibilities at 120? and 240?.

 

[Hv&Lv]

I know that 277 volts and 120 volts come from two different systems so - if someone could please help me out. In this illustration - how is this voltage calculated?

Thanks - Jimmie

I believe the easiest way to explain it is to use graph paper, a protractor, and a scale rule. Draw it out on graph paper with the voltages as the length of the lines, use the angles mivey gave to draw the other voltage-length lines and simply measure the results between the voltages.

 

[roger]

See Smart's post #9 in this thread


Roger

 

[don_resqcapt19]

Wouldn't the lowest be 120V<0 and 277V<0?

Most 120 is created through a Delta to wye XFMR, so it might be 277V<0 and 120V<30.

About the only way to get both the 120 and 277 in phase would be to have a high voltage distrubution system with 480/277Y and 208/120Y transformers fed from the high voltage. Most of the time that would not be the case and you would have the 30? phase shift for the 120.

 

[theshockdoc]

Thanks

Thanks

I know it has been a few weeks since I posted but I just want to say thanks to all that replied. It has been a couple of years since I had been to the website - I forgot what a wealth of information (and opinion) is hear.

BTW - thanks to Roger that actually sent me to another thread on the subject where someone had a graph which helps to visualize the answer.

Thanks again - Jimmie