120v 277v insame 3ph box
- Thread starter terrytc
- Start date
- Status
- Location
- Illinois
- Occupation
- retired electrician
Re: 120v 277v insame 3ph box
Terry,
Can you give us more details? You can't get 120 and 277 from the same panel.
Don
Terry,
Can you give us more details? You can't get 120 and 277 from the same panel.
Don
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: 120v 277v insame 3ph box
Let me try and rephrase the question. You come upon a 3-phase panelboard. You measure voltage from each of the three main lugs to ground. What you get is 120 volts from Phase A to Ground, 120 volts from Phase B to Ground, and 277 volts from Phase C to Ground. The question is this: "Can this panelboard have been connected properly, in accordance with the NEC, and still get these three readings?"
terrytc: Did I get your question right?
To the rest of the membership: I am not familiar with "high leg delta," and that might not even be the right name. But if you have a delta secondary, with phase to phase secondary voltage at 240volts, and if you connect the center of one secondary winding to ground, will this not give you the voltage readings that terrytc is asking about?
Let me try and rephrase the question. You come upon a 3-phase panelboard. You measure voltage from each of the three main lugs to ground. What you get is 120 volts from Phase A to Ground, 120 volts from Phase B to Ground, and 277 volts from Phase C to Ground. The question is this: "Can this panelboard have been connected properly, in accordance with the NEC, and still get these three readings?"
terrytc: Did I get your question right?
To the rest of the membership: I am not familiar with "high leg delta," and that might not even be the right name. But if you have a delta secondary, with phase to phase secondary voltage at 240volts, and if you connect the center of one secondary winding to ground, will this not give you the voltage readings that terrytc is asking about?
Re: 120v 277v insame 3ph box
sounds like high leg delta with the high leg on the wrong phase. some counties that I know of in NW penna the utility requires the high leg be on C phase for metering, and some inspectors say, if it comes in the building on C phase, leave it on C phase everywhere.
sounds like high leg delta with the high leg on the wrong phase. some counties that I know of in NW penna the utility requires the high leg be on C phase for metering, and some inspectors say, if it comes in the building on C phase, leave it on C phase everywhere.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: 120v 277v insame 3ph box
Don't get confused by which phase is which. I was just trying to understand the question. I selected the A, B, C at random. The OP did not tell us which phase had the higher voltage.
jimwalker
Senior Member
- Location
- TAMPA FLORIDA
Re: 120v 277v insame 3ph box
Check your meter ,just almost no way you can get this output.If you do then some hack has been there before you
Check your meter ,just almost no way you can get this output.If you do then some hack has been there before you
- Location
- Illinois
- Occupation
- retired electrician
Re: 120v 277v insame 3ph box
The high leg of a 3 phase 4 wire delta is 208 volts to ground not 277. The voltages in that system are 240 phase to phase, two legs at 120 to the grounded conductor and the third at 208.
If you really have 2 phases from a 120/240 volt panel and the third from a 277/480 volt wye system, that is not right.
Don
The high leg of a 3 phase 4 wire delta is 208 volts to ground not 277. The voltages in that system are 240 phase to phase, two legs at 120 to the grounded conductor and the third at 208.
If you really have 2 phases from a 120/240 volt panel and the third from a 277/480 volt wye system, that is not right.
Don
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: 120v 277v insame 3ph box
I did it as a vector addition, and got an answer of 317 volts. So either I did it wrong, or there is still even more more to it.
Does anyone know what voltage you should get, on a high leg delta, from the high leg to ground, if the voltage to ground on the other two legs is 120V?
EDIT: I see that Don answered my question while I was typing my question.
[ December 27, 2005, 04:41 PM: Message edited by: charlie b ]
There is more to it. Using my earlier description, the voltage from the ground point (halfway along the A-B secondary winding) to the A point is 120 volts. The voltage between C and A is 240 volts. One might guess that the voltage between C and ground would therefore be 120 plus 240, or 360 volts. However, the voltages are not in phase with each other. So you have to use trigonometry or vectors or phasor addition, in order to get the voltage from C to ground.
I did it as a vector addition, and got an answer of 317 volts. So either I did it wrong, or there is still even more more to it.
Does anyone know what voltage you should get, on a high leg delta, from the high leg to ground, if the voltage to ground on the other two legs is 120V?
EDIT: I see that Don answered my question while I was typing my question.
[ December 27, 2005, 04:41 PM: Message edited by: charlie b ]
rcarroll
Senior Member
- Location
- Fort Collins, Colorado
Re: 120v 277v insame 3ph box
I think I used to measure the high leg 197 to 208 volts to ground. If you measure phase to phase on a 120/240 delta system, you should read 240 volts.
I think I used to measure the high leg 197 to 208 volts to ground. If you measure phase to phase on a 120/240 delta system, you should read 240 volts.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Re: 120v 277v insame 3ph box
It is simple trigonometery involving right triangles, or even easier, the Pythogrean Theorem (A^2 = B^2 + C^2). The high leg value is always 208V (well actually closer to 207.846097V) just like the line-line is always 240V.
All electricians should be able to get this value just like they be able to get 208Y/120 or 480Y/277. Learn the math, learn to memorize, or like I do, learn to look it up.
It is simple trigonometery involving right triangles, or even easier, the Pythogrean Theorem (A^2 = B^2 + C^2). The high leg value is always 208V (well actually closer to 207.846097V) just like the line-line is always 240V.
All electricians should be able to get this value just like they be able to get 208Y/120 or 480Y/277. Learn the math, learn to memorize, or like I do, learn to look it up.
- Location
- Illinois
- Occupation
- retired electrician
Re: 120v 277v insame 3ph box
Charlie,
Don
Charlie,
You have a right triangle with a hypotenuse of 240 and one side of 120. Solve for the third side.
Don
gvmv
Member
- Location
- Travelling Electrician
Re: 120v 277v insame 3ph box
Dunno but my instructor always says:
120 x square root of 3 = 207.846
your high leg voltage and he beats this into us over and over again.
"remember 1.73" He says, "you will need this number all the time."
Delmar's Standard book of Electricity pg.776
"This line is known as a high leg, because the voltage between this line and the nuetral conductor will be higher than the voltage between the neutral and either of the other two conductors. The High-Leg voltage can be computed by increasing the single-phase center-tapped voltage value by 1.732."
Dunno but my instructor always says:
120 x square root of 3 = 207.846
your high leg voltage and he beats this into us over and over again.
"remember 1.73" He says, "you will need this number all the time."
Delmar's Standard book of Electricity pg.776
"This line is known as a high leg, because the voltage between this line and the nuetral conductor will be higher than the voltage between the neutral and either of the other two conductors. The High-Leg voltage can be computed by increasing the single-phase center-tapped voltage value by 1.732."
- Location
- Windsor, CO NEC: 2023
- Occupation
- Hospital Master Electrician
Re: 120v 277v insame 3ph box
That said, we've got some issues here. We have a separately derived system, which means we have a neutral that's supposed to be used for the 277 leg, and a different neutral that is to be used for the 120 leg.
I'll bet you only have one neutral in that panel: That's a violation of 300.3(B).
If there are two neutrals, and the neutral bars are separated, then I believe this wholly unholy installation is legal, until a two-pole circuit breaker is installed across a 120 & 277 phase. That breaker won't be rated for whatever voltage that unholy marriage will result in.
If no 2-poles are installed, in my uneducated opinion I cannot find anything definitive against code in this installation.
If possible, undo whatever horrible things have been done.
That's horrible! Terrible! Ugh!
That said, we've got some issues here. We have a separately derived system, which means we have a neutral that's supposed to be used for the 277 leg, and a different neutral that is to be used for the 120 leg.
I'll bet you only have one neutral in that panel: That's a violation of 300.3(B).
If there are two neutrals, and the neutral bars are separated, then I believe this wholly unholy installation is legal, until a two-pole circuit breaker is installed across a 120 & 277 phase. That breaker won't be rated for whatever voltage that unholy marriage will result in.
If no 2-poles are installed, in my uneducated opinion I cannot find anything definitive against code in this installation.
If possible, undo whatever horrible things have been done.
Re: 120v 277v insame 3ph box
Terry, the only way that I can see this panel being supplied as you describe it is by tying together the grounded conductors of 2 different grounded systems at or before a single neutral bar. If nothing else this would violate 250.6(A) [2002] as it would place neutral current on the GEC's of both systems - if they are correctly sharing a common premise grounding system.
But then George raised the possibility of 2 separate neutral bars in the panel, which makes me a bit woozy just thinking it...
What type of systems are being used to supply this panel? The 277v sounds like it might be a single phase (w/neutral) of a 480/277v wye-connected system. I would guess that the 120v is 2 phases (w/neutral) of a 208/120v wye-connected system. What's the phase-to-phase voltage of the 2 120v phases? And how did you happen to come across this?
Terry, the only way that I can see this panel being supplied as you describe it is by tying together the grounded conductors of 2 different grounded systems at or before a single neutral bar. If nothing else this would violate 250.6(A) [2002] as it would place neutral current on the GEC's of both systems - if they are correctly sharing a common premise grounding system.
But then George raised the possibility of 2 separate neutral bars in the panel, which makes me a bit woozy just thinking it...
What type of systems are being used to supply this panel? The 277v sounds like it might be a single phase (w/neutral) of a 480/277v wye-connected system. I would guess that the 120v is 2 phases (w/neutral) of a 208/120v wye-connected system. What's the phase-to-phase voltage of the 2 120v phases? And how did you happen to come across this?
Re: 120v 277v insame 3ph box
FIrst I'd like to see the wiring system incorporated so I can label as a "never do"
then I must tell you to FIRE the person/company that did the work. For no other reason than, this is dangerous. You have two totally different wiring systems sharing the same panel. How the two neutrals are "combined" sounds interesting. What it looks like is a nightmare, I'm sure.
FIrst I'd like to see the wiring system incorporated so I can label as a "never do"
then I must tell you to FIRE the person/company that did the work. For no other reason than, this is dangerous. You have two totally different wiring systems sharing the same panel. How the two neutrals are "combined" sounds interesting. What it looks like is a nightmare, I'm sure.
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