common practice or code?

[roger]

Thanks....

You're welcome.

Roger

 

[kwired]

interesting point... I am curious myself. I know that the words Phase and Time are actually the same thing as they relate to voltage but I was under the impression that the two systems were in synch. Any opinions?

Those images were posted while I was writing what I did before. Important thing I left out (and mostly because this is something I never have to deal with) that was pointed out is displacement. That I'm assuming is the difference in angle between the primary and secondary as drawn in that image. Get the displacement and draw same thing I described with the displacement factored in and you get same thing that was drawn there. Again the grounded conductor is common to both systems if anything because they are bonded together, though I'm sure there is a little more then that to it.

 

[xformer]

I will give you a description that will hopefully help you draw your own.

Both the 277 and 120 volt systems will have a common point at the grounded conductor.

Establish that point then draw a line 277 units long. Then draw another line 120 units long from that same point at a 120 angle (the phase angle) If on same phase you will draw the 120 volt line in the same direction as the 277 volt line. The distance between the ends of these two lines (in same units of measurement) will be same as the voltage would be. The same phase application would be simplest to figure out 277-120= 157 volts.

I can see this may be more complicated by wye-wye transformer compared to delta-wye transformer or even a single phase transformer connected to two phase conductors on the primary as compared to connecting it phase to neutral, as the vectors are probably a little more complex to figure out, but hopefully this gives enough understanding to know why the voltages turn out the way they do. Most field electricians would never need to know much more then this on a regular basis.

That is totally understandable. but wouldnt that only occur if the two voltages were "in-phase"?

 

[xformer]

Those images were posted while I was writing what I did before. Important thing I left out (and mostly because this is something I never have to deal with) that was pointed out is displacement. That I'm assuming is the difference in angle between the primary and secondary as drawn in that image. Get the displacement and draw same thing I described with the displacement factored in and you get same thing that was drawn there. Again the grounded conductor is common to both systems if anything because they are bonded together, though I'm sure there is a little more then that to it.

Sorry Kwired... I was writing a response as well... this explains everything... Thank you greatly....

 

[charlie b]

Phase A from the 480 system and phase A from the 208 system.

Things do not work that way, especially since most of the step down transformers we use are configured as Delta-Wye. The secondary side Phase A winding sits opposite a primary winding that is connected to the 480 volt Phases A and B. So there really is no correlation between what is called Phase A on the primary and secondary sides. In addition, the Delta-Wye creates a 30 degree shift in phase, so that the primary and secondary sides are not in phase with each other.

 

[iwire]

It seemed Winnie agrees with me and Charlie does not.

I am left extremely confused. :?

 

[xformer]

Things do not work that way, especially since most of the step down transformers we use are configured as Delta-Wye. The secondary side Phase A winding sits opposite a primary winding that is connected to the 480 volt Phases A and B. So there really is no correlation between what is called Phase A on the primary and secondary sides. In addition, the Delta-Wye creates a 30 degree shift in phase, so that the primary and secondary sides are not in phase with each other.

Would 2 delta-Wyes create the same phase shift when comparing secondary to secondary?

 

[roger]

I am left extremely confused. :?

Don't worry Bob, think "engineer" and how a single winding transformer can create a thread of 32,000 posts.

Roger

 

[charlie b]

Would 2 delta-Wyes create the same phase shift when comparing secondary to secondary?

As long as you started with the same primary source, and you get the correlation of which phase is which correct, then yes. Keep in mind that changes in loading conditions can also cause phase shifts, so my comments are based on unloaded conditions.

 

[xformer]

As long as you started with the same primary source, and you get the correlation of which phase is which correct, then yes. Keep in mind that changes in loading conditions can also cause phase shifts, so my comments are based on unloaded conditions.

Thank you, Sir.

 

[charlie b]

It seemed Winnie agrees with me and Charlie does not. I am left extremely confused.

Winnie and I are not in disagreement. If a 277V signal and a 120V signal are indeed in phase with each other, or are nearly so, then the voltage between them would be below 300, as you correctly surmised, and as Winnie confirmed. My point is that you can't really look at what is named "Phase A" on the 480/277V side and what is named "Phase A" on the 120/208V side and say that they are cousins (i.e., that they will be close to being in phase with each other).

 

[kwired]

In addition, the Delta-Wye creates a 30 degree shift in phase, so that the primary and secondary sides are not in phase with each other.

Isn't that the earlier mentioned "displacement". Once you figure out what the displacement is, draw your two systems on top of one another with the applicable angle differential between them. If done to scale the distance between any two points is going to be representative of the actual voltage between those points

 

[kwired]

Winnie and I are not in disagreement. If a 277V signal and a 120V signal are indeed in phase with each other, or are nearly so, then the voltage between them would be below 300, as you correctly surmised, and as Winnie confirmed. My point is that you can't really look at what is named "Phase A" on the 480/277V side and what is named "Phase A" on the 120/208V side and say that they are cousins (i.e., that they will be close to being in phase with each other).

kind of answers what I just brought up.

We should be able to determine that with no displacement (though probably not really possible) we will have a minimum possible voltage between the two systems, and that 60 degree displacement should yield maximum possible voltage between the two systems.

 

[iwire]

Winnie and I are not in disagreement. If a 277V signal and a 120V signal are indeed in phase with each other, or are nearly so, then the voltage between them would be below 300, as you correctly surmised, and as Winnie confirmed. My point is that you can't really look at what is named "Phase A" on the 480/277V side and what is named "Phase A" on the 120/208V side and say that they are cousins (i.e., that they will be close to being in phase with each other).

I can look at any way I would like to.

You must have assumed I was saying in phase when I was saying the phase.

There are both 480 and 208 conductors labeled phases A, B & C. the voltage between the same letter phases will be less than 300.

How would you have worded it in laymen terms?

 

[iwire]

Don't worry Bob, think "engineer" and how a single winding transformer can create a thread of 32,000 posts.

Roger

The engineers version of ground up or down.

 

[xformer]

I can look at any way I would like to.

You must have assumed I was saying in phase when I was saying the phase.

There are both 480 and 208 conductors labeled phases A, B & C. the voltage between the same letter phases will be less than 300.

How would you have worded it in laymen terms?

I am going to have side with Charlie on this one...
when comparing the 480V Primary A to the 208V Primary A your voltage may or may not be less than 300 It depends on the voltage vectoring. IMHO...

 

[winnie]

There are both 480 and 208 conductors labeled phases A, B & C. the voltage between the same letter phases will be less than 300.

How would you have worded it in laymen terms?

"The voltage between two phase conductors of these two different system can be less than 300V or more than 380V, depending on the specific phase angle difference between the two systems."

The point that charlie is making is that the phase labeled "480V A" and the phase labeled "208V A" have no specified phase relation. They might be 0 degrees apart, they might be 150 degrees apart.

Hell, if the 480V system was mains derived, and the 208V system came from an isolated generator running on its own speed regulator, then the phase angle between 'phase A' of the two systems would vary over time.

-Jon

 

[jtinge]

So not knowing the phase angle relationship between the 120V and 277V in the switch box, it would be correct to require a barrier between the switches because the "possibility" to exceed 300V between the switches exists.


Sent from my iPhone using Tapatalk

 

[steve66]

So not knowing the phase angle relationship between the 120V and 277V in the switch box, it would be correct to require a barrier between the switches because the "possibility" to exceed 300V between the switches exists.


Sent from my iPhone using Tapatalk

You can always measure the difference with a voltmeter. But I also agree with the comment that phases may be changed in the future, so installing a divider would be a good practice. (Our spec. typically requires the contractor to balance phases after everything is installed and powered. You wouldn't want to have to go back and re-measure and add dividers at the end of the project.)

But I'm not sure an inspector can force anyone to install a divider just because someone may move the phases later, resulting in exceeding the 300v limit.

 

[iwire]

I am going to have side with Charlie on this one...

The point that charlie is making is that the phase labeled "480V A" and the phase labeled "208V A" have no specified phase relation. They might be 0 degrees apart, they might be 150 degrees apart.

Well I know when to throw in the towel, I would be lying if I said I fully understood what you all are telling me but I have to accept it regardless.

so installing a divider would be a good practice.

For sure.