high leg voltage

[jwelectric]

I did a search but did not find the answer I need.
In the text book it shows to find the high leg of a 240/120 delta system find the square root of the line voltage squared minus the phase voltage squared.
On a 240 volt system this will equal 207.84 volts.

I have some electrical students that say the high leg in the plant where they are working is a lot higher than this. They say it is up as high as 219 volts.

How do I explain this to them? I have never seen a high leg on a 240 volt system that high in my entire experience. Could it be their meter is that far off and if it was wouldn?t it give a higher voltage than 240 and 120?

 

[jim dungar]

I did a search but did not find the answer I need.
In the text book it shows to find the high leg of a 240/120 delta system find the square root of the line voltage squared minus the phase voltage squared.
On a 240 volt system this will equal 207.84 volts.

I have some electrical students that say the high leg in the plant where they are working is a lot higher than this. They say it is up as high as 219 volts.

How do I explain this to them? I have never seen a high leg on a 240 volt system that high in my entire experience. Could it be their meter is that far off and if it was wouldn?t it give a higher voltage than 240 and 120?

The laws of physic are not up for debate. Your method is correct.

The only possible explanation is: If they are seeing 219V, then their 120/240V nominal system is also high (i.e. they have 126/252V).

 

[broadgage]

Yes, as posted above, if the high leg voltage is higher than expected, then the other voltages are almost certainly higher in proportion.

There may be some weird and in practice exceedingly unlikely circumstances of grossly unbalanced load or improbable power factor that would give rise to unexpected voltages, but in the real world it simply means that the voltage of either the POCO supply, or customer owned transformer is on the high side.

Both POCO and private supplies are often around 105% of nominal close to the transformer in order to give the maximum possible allowance for voltage drop.

 

[Hv&Lv]

I did a search but did not find the answer I need.
In the text book it shows to find the high leg of a 240/120 delta system find the square root of the line voltage squared minus the phase voltage squared.
On a 240 volt system this will equal 207.84 volts.

I have some electrical students that say the high leg in the plant where they are working is a lot higher than this. They say it is up as high as 219 volts.

How do I explain this to them? I have never seen a high leg on a 240 volt system that high in my entire experience. Could it be their meter is that far off and if it was wouldn?t it give a higher voltage than 240 and 120?

219 / 1.73=126.5 volts. I agree, this is too high. where are your students experiencing this? Duke, Progress or Randolph?

 

[kwired]

Right around 215 volts on wild leg is what is normal around here. This puts the other lines at about 124/248, which is about what is typical with minimal load anyway.

 

[broadgage]

If the actual high leg voltage is 215 volts, then an instrument error of less than 2% could give a reading of 219 volts.
Or it might be actually 219 volts, which is a little higher than is strictly correct, but not that bad.