Neutral bond in transformer

[kwired]

You have a separately derived system whether your "load" requires or neutral or not.
The X0 in bonded to the enclosure and you ground the X0 as directed by the NEC. The line conductors and a EGC is pulled to the load. Should there be a L-G fault the fault current will flow back to the X0 causing the OCPD to trip. Since the secondary is a 208Y/120 any L-G ground fault will be 120v regardless if there is a neutral pulled to a load or not. To even consider grounding a phase conductor in leu of the X0 does not make sence. If one elects to ground one of the 208v lines a ground fault will now be 208v. What anvantage is that? Is that better than 120v? When there is no choice other than grounding a phase such as corner grounding a 240 or 480v delta I find it unreasonable to even consider grounding one of the phases of a wye. One of the guys said "Just because you can doesn't mean you should."

If you have a 480 volt corner grounded system and need a backup generator would you say it is unsafe to use a generator with wye connected windings, float the neutral and ground a phase? I would not have a problem with it.

Why would the situtation with the transformer be any different.


Alternating-Current Systems to Be Grounded. 250.20(B)(2):

Where the system is 3-phase, 4-wire, wye connected in which the neutral conductor is used as a circuit conductor

If the neutral conductor is not used as a circuit conductor it does not have to be the conductor that is grounded. It is usually wise to still use it as the grounded conductor if that is possible.

 

[teco]

If you are using the neutral you must ground it. If you don't bond anything you have an ungrounded system. You do however need to use a ground detection system and still need to bond all non current carrying metal enclosures, raceways, etc. with the usual equipment grounding conductor and install a grounding electrode system.

Grounding a phase on a wye secondary is probably not too common but could be done. One example of where you may do this is if you need to replace a transformer that was originally a corner grounded delta, but other equipment is not being changed.

This is more to the question I'm asking. It was asked not to bond or ground the X0 terminal, not the enclosure. Still grounding and bonding the metal enclosure with a grounding electrode and bonding metal parts likely to become energized.

Again the question is does anyone know why they would ask for this? Is it an ungrounded system? I don't think so. They did not ask to ground a phase either. How would the secondary recognize a fault? I would brush this off if this was one person saying this. But there is a few that asked for this. Problem is I can't ask any of them myself. It blew me away, I feel like I'm missing something.

 

[kwired]

This is more to the question I'm asking. It was asked not to bond or ground the X0 terminal, not the enclosure. Still grounding and bonding the metal enclosure with a grounding electrode and bonding metal parts likely to become energized.

Again the question is does anyone know why they would ask for this? Is it an ungrounded system? I don't think so. They did not ask to ground a phase either. How would the secondary recognize a fault? I would brush this off if this was one person saying this. But there is a few that asked for this. Problem is I can't ask any of them myself. It blew me away, I feel like I'm missing something.

If done as you say it is an ungrounded system. But ungrounded systems have to have a ground detection system installed in the system to indicate there is a fault. See 250.21(B) (2008 NEC)

(B) Ground Detectors. Ungrounded alternating current systems as permitted in 250.21(A)(1) through (A)(4) operating at not less than 120 volts and not exceeding 1000 volts shall have ground detectors installed on the system.

So in general you have to have either a solidly grounded system (any conductor of the system - if there is a neutral it is likely the conductor required to be grounded), a high impedance ground system (and associated fault detection equipment) or an ungrounded system (with associated fault detection equipment).

 

[templdl]

If you have a 480 volt corner grounded system and need a backup generator would you say it is unsafe to use a generator with wye connected windings, float the neutral and ground a phase? I would not have a problem with it.

Why would the situtation with the transformer be any different.


Alternating-Current Systems to Be Grounded. 250.20(B)(2):



If the neutral conductor is not used as a circuit conductor it does not have to be the conductor that is grounded. It is usually wise to still use it as the grounded conductor if that is possible.

With all do respect am I missing the intent of your reply?
I am to understand that this is this how you yourself would install and ground a 208Y/120 transformer by leaving the X0 unconnected and grounding one of the line conductors as one would a corner grounded delta where the loads don’t require a neutral? Is this what you would recommend that the OP do in your option?
If a qualified person were to run into an installation such as this he would strongly question the compentancy of the designer as it would not be customery, reasonable as one would never expect the a line conductor to be used as a grounded conductor one a wye transformer.
Are you saying that 120 v L-G and 208v L-G are just as safe as one another and that it is easier to ground the line instead of the X0? For what practical reason would one ever ground the secondary of a wye transformer on this way?
Interesting.
Again, "Just because you can doesn't mean you should."

 

[kwired]

With all do respect am I missing the intent of your reply?
I am to understand that this is this how you yourself would install and ground a 208Y/120 transformer by leaving the X0 unconnected and grounding one of the line conductors as one would a corner grounded delta where the loads don?t require a neutral? Is this what you would recommend that the OP do in your option?
If a qualified person were to run into an installation such as this he would strongly question the compentancy of the designer as it would not be customery, reasonable as one would never expect the a line conductor to be used as a grounded conductor one a wye transformer.
Are you saying that 120 v L-G and 208v L-G are just as safe as one another and that it is easier to ground the line instead of the X0? For what practical reason would one ever ground the secondary of a wye transformer on this way?
Interesting.
Again, "Just because you can doesn't mean you should."

I would question the competence of someone that finds a system with no neutral and tries to connect a line to neutral load to said system. If a phase conductor has white insulation isn't that supposed mean the phase is grounded? Does it matter if it is a wye or delta transformer as long as correct number of phases and correct voltage is delivered? True it is not likely to run into this situation very often but it is certainly possible and code compliant if done correctly.

This situation is more likely with a 480 volt system as you will not easily find a 240 volt system that is derived from a wye.

 

[iceworm]

kwired -
For 208/120, 250.20.B.1 requires that the neutral be grounded. 250.20(B)(2) does not apply. I can't think of a single case where it would be desireable or legal to ground one phase of 208/120.

And I'm thinking you already knew that.

ice

 

[texie]

This is more to the question I'm asking. It was asked not to bond or ground the X0 terminal, not the enclosure. Still grounding and bonding the metal enclosure with a grounding electrode and bonding metal parts likely to become energized.

Again the question is does anyone know why they would ask for this? Is it an ungrounded system? I don't think so. They did not ask to ground a phase either. How would the secondary recognize a fault? I would brush this off if this was one person saying this. But there is a few that asked for this. Problem is I can't ask any of them myself. It blew me away, I feel like I'm missing something.

Teco,
You are right to be concerned. If this is a 208 wye secondary transformer we are talking about here, X0 must be bonded/grounded- period, do not pass go, etc.-see NEC 2011 250.20 (B). And forget about grounding a phase conductor. If you guys have an instructor saying you can float X0, it mat be time for a new instructor.

 

[texie]

Teco,
You are right to be concerned. If this is a 208 wye secondary transformer we are talking about here, X0 must be bonded/grounded- period, do not pass go, etc.-see NEC 2011 250.20 (B). And forget about grounding a phase conductor. If you guys have an instructor saying you can float X0, it mat be time for a new instructor.

I should have added that the fact that you do or don't need a neutral for the load is not relevant, it still needs to be brought to the first disconnecting means, which as you know, is the fault return path.

 

[kwired]

kwired -
For 208/120, 250.20.B.1 requires that the neutral be grounded. 250.20(B)(2) does not apply. I can't think of a single case where it would be desireable or legal to ground one phase of 208/120.

And I'm thinking you already knew that.

ice

No argument on whether or not it would ever be desirable to ground a phase of a 208 volt wye. Notice I said 208 volt wye and not 208/120 volt wye.

In order for a wye connected set of windings to work you must tie one lead of each winding together to form the X0 point. Does this mean you have to use XO as a system conductor? Pole mounted transformers using 120/240 pots to make a delta bank have unused center taps on 2 of the pots and if corner grounded all three have an unused tap. Just because there is a tap does it mean it needs to be used? If you use the center tap for a circuit conductor on single phase or a wye then 250.20 makes it clear that it will be the conductor that is to be grounded.

If a POCO replaces a transformer bank (for whatever reason) that was a 480 volt corner grounded delta and chooses to use 277 volt transformers and connect in wye configuration and not bond XO but instead bond a phase - there will be no difference in what is needed for service equipment or in voltage readings on system conductors. The neutral for all practical purposes does not exist as a system conductor - it is just a connection within the transformer bank that is necessary to make it work. A little like having extra leads that can be swapped for dual voltage purposes.

 

[iceworm]

... Notice I said 208 volt wye and not 208/120 volt wye. ...

No, I didn't notice. And I don't understand the nuiance. How could you get a "208Y" and not have "208/120 Wye"? Color me clueless on this issue.

... In order for a wye connected set of windings to work you must tie one lead of each winding together to form the X0 point. Does this mean you have to use XO as a system conductor? ...

Depends on what you mean by "system conductor". You don't have to have 120v loads. But you do have to bond this neutral to ground, per 250.20.B.1

... Pole mounted transformers using 120/240 pots to make a delta bank have unused center taps on 2 of the pots and if corner grounded all three have an unused tap. Just because there is a tap does it mean it needs to be used? If you use the center tap for a circuit conductor on single phase or a wye then 250.20 makes it clear that it will be the conductor that is to be grounded. ...

... If a POCO replaces a transformer bank (for whatever reason) that was a 480 volt corner grounded delta and chooses to use 277 volt transformers and connect in wye configuration and not bond XO but instead bond a phase - there will be no difference in what is needed for service equipment or in voltage readings on system conductors. The neutral for all practical purposes does not exist as a system conductor - it is just a connection within the transformer bank that is necessary to make it work. A little like having extra leads that can be swapped for dual voltage purposes.

You left me confused with these last two. I don't see any connection to the OP.

Where are you going with this?

ice

 

[teco]

No argument on whether or not it would ever be desirable to ground a phase of a 208 volt wye. Notice I said 208 volt wye and not 208/120 volt wye.

In order for a wye connected set of windings to work you must tie one lead of each winding together to form the X0 point. Does this mean you have to use XO as a system conductor? Pole mounted transformers using 120/240 pots to make a delta bank have unused center taps on 2 of the pots and if corner grounded all three have an unused tap. Just because there is a tap does it mean it needs to be used? If you use the center tap for a circuit conductor on single phase or a wye then 250.20 makes it clear that it will be the conductor that is to be grounded.

If a POCO replaces a transformer bank (for whatever reason) that was a 480 volt corner grounded delta and chooses to use 277 volt transformers and connect in wye configuration and not bond XO but instead bond a phase - there will be no difference in what is needed for service equipment or in voltage readings on system conductors. The neutral for all practical purposes does not exist as a system conductor - it is just a connection within the transformer bank that is necessary to make it work. A little like having extra leads that can be swapped for dual voltage purposes.

250.20(B)2 the wording that stands out to me is " in which the neutral conductor is used as a circuit conductor" and we are not using the neutral as a circuit conductor. That sentence must be there for a reason. I've also been looking at 250.26(3) and (5). If we have not used the X0 then do we have a wire common to all phases? Do we count the winding connections themselves?

 

[david luchini]

Depends on what you mean by "system conductor". You don't have to have 120v loads. But you do have to bond this neutral to ground, per 250.20.B.1

I don't see that 250.20(B)(1) says that you have to ground the neutral, only that the system has to be grounded.

 

[iceworm]

I don't see that 250.20(B)(1) says that you have to ground the neutral, only that the system has to be grounded.

Yes you are certainly right about the wording. The Op concerns 208/120Wye. So pick out a different wire than the neutral that would be useable.

And I'm still thinking everyone still knows this

ice

 

[texie]

250.20(B)2 the wording that stands out to me is " in which the neutral conductor is used as a circuit conductor" and we are not using the neutral as a circuit conductor. That sentence must be there for a reason. I've also been looking at 250.26(3) and (5). If we have not used the X0 then do we have a wire common to all phases? Do we count the winding connections themselves?

Teco,
250.20.(B) (2) is there to cover the possibilty of a 480Y system, that could be ungrounded, if it met other requirements, but not if the neutral is being used as a circuit conductor, such as a 277 volt light circuit.
250.20.(B) (1) says any system that can be grounded to less than 150 volts to ground must be. It makes no mention of using the neutral. And as a practicle matter, it the grounded conductor will be the mid point of the Y as other wise you would have more than 150 volts to ground.

 

[templdl]

Teco,
250.20.(B) (2) is there to cover the possibilty of a 480Y system, that could be ungrounded, if it met other requirements, but not if the neutral is being used as a circuit conductor, such as a 277 volt light circuit.
250.20.(B) (1) says any system that can be grounded to less than 150 volts to ground must be. It makes no mention of using the neutral. And as a practicle matter, it the grounded conductor will be the mid point of the Y as other wise you would have more than 150 volts to ground.

That is my thought texie,
It appears as though an argument is attempted to be made as to whether or not you can ground a phase conductor on the secondary of a 208Y/120 connected transformer. Someone mentions a 208Y transformer which omits the 120v. I venture to say that a transformer most likely would never be supplied with such a name plate. With all of the back and forth that has been going on nobody has presented any positive advantage of using a phase conductor as the grounded conductor.
If what is being described or proposed it would be commonly done and would like for those to present some field examples of configurations that they are describing.
Basically, suitably ground the X0 period as one would as a separate source and bring your EGC along with the line conductors.
What is the issue here? Are the arguments presented here practical or just for the sake of an argument or debate?
Your L-G short circuit voltage will be 120v and not 208v unless there are those who find that 208v L-G fault is superior to 120v.
Would it be prudent to advise the original OP that grounding one of the line conductors rather than the X0
is an acceptable practice?

 

[kwired]

No, I didn't notice. And I don't understand the nuiance. How could you get a "208Y" and not have "208/120 Wye"? Color me clueless on this issue.


Depends on what you mean by "system conductor". You don't have to have 120v loads. But you do have to bond this neutral to ground, per 250.20.B.1




You left me confused with these last two. I don't see any connection to the OP.

Where are you going with this?

ice

OP:

Hello,

Can someone tell me why one should not ground or bond the neutral (X0) on a Delta to Wye transformer if the neutral is not going be used?

Thanks

OP did not specify voltage. Based on some comments that have been presented I may buy into your argument that you can not do this with a 120/208, but definately disagree for 277/480. I have also said there is likely not much need or desire to do so with 120/208, but there would be for a 480 volt system.

250.20(B)2 the wording that stands out to me is " in which the neutral conductor is used as a circuit conductor" and we are not using the neutral as a circuit conductor. That sentence must be there for a reason. I've also been looking at 250.26(3) and (5). If we have not used the X0 then do we have a wire common to all phases? Do we count the winding connections themselves?

I don't see that 250.20(B)(1) says that you have to ground the neutral, only that the system has to be grounded.

Both these statements support what I have been saying here.

Yes you are certainly right about the wording. The Op concerns 208/120Wye. So pick out a different wire than the neutral that would be useable.

And I'm still thinking everyone still knows this

ice

OP did not mention voltage - OK he did a few posts later. Again most people would suggest that you do ground the neutral, but we have no definite answer yet telling us you have to if there are no loads that use the neutral.

That is my thought texie,
It appears as though an argument is attempted to be made as to whether or not you can ground a phase conductor on the secondary of a 208Y/120 connected transformer. Someone mentions a 208Y transformer which omits the 120v. I venture to say that a transformer most likely would never be supplied with such a name plate. With all of the back and forth that has been going on nobody has presented any positive advantage of using a phase conductor as the grounded conductor.
If what is being described or proposed it would be commonly done and would like for those to present some field examples of configurations that they are describing.
Basically, suitably ground the X0 period as one would as a separate source and bring your EGC along with the line conductors.
What is the issue here? Are the arguments presented here practical or just for the sake of an argument or debate?
Your L-G short circuit voltage will be 120v and not 208v unless there are those who find that 208v L-G fault is superior to 120v.
Would it be prudent to advise the original OP that grounding one of the line conductors rather than the X0
is an acceptable practice?

If you have 208 volts phase to phase in a wye you obviously will have 120 to the neutral point. If you do not use the neutral point as a circuit conductor do you have to use it as a grounded conductor? From some of what has been mentioned here it is acceptable for 277/480 (but not 120/208??). I agreed time and time again that there is not much practical use for doing so with 208 volts. I also agree that 250.20(B)(1) probably prohibits so with 208 volts, but I have a hard time figuring out why it is acceptable to do so with 480 volts but not if you are capable of having 150 volts or less to ground?

Sorry I threw this conversation off by suggesting grounding a phase. If we go back to what was said in the OP it was not about grounded phases but was about ungrounded systems. Unfortuneately 250.21(4) sends us back to 250.20(B) for systems allowed to not be grounded which maybe leaves the OP needing to ground his system in question. I will say no more as to which conductor must be grounded in the OP system since we do not totally agree on that.

 

[teco]

Teco,
250.20.(B) (2) is there to cover the possibilty of a 480Y system, that could be ungrounded, if it met other requirements, but not if the neutral is being used as a circuit conductor, such as a 277 volt light circuit.
250.20.(B) (1) says any system that can be grounded to less than 150 volts to ground must be. It makes no mention of using the neutral. And as a practicle matter, it the grounded conductor will be the mid point of the Y as other wise you would have more than 150 volts to ground.

I don't believe that 250.20.(B)(1) only says any system that can be grounded must be. It does say that, but also says under any of the following conditions. I think it does make mention of using the neutral in condition 2 which is part of the article stating where a 3-phase wye system where the neutral is used as a circuit conductor. In our case, it is not being used as a circuit conductor. If you look at the 2011 handbook it shows a diagram specifically of a 120/208 wye system in the 250.20.B explanation. It goes on to state that this system is covered by 250.20.B(2). It states this system must be grounded because of the line to neutral loads. Hence my question, being that there is no line to neutral loads must it still be grounded? Is this the reason why these people preferred not to ground the XO terminal? BTW, I am not asking about grounding a phase and never did. I am looking for a code reference that prohibits not grounding this XO terminal so I could share it with my crew. The fact that the 250.20.(B) says where the neutral is used as a circuit conductor, and the explanation in the handbook, has thrown myself and my crew off a bit when trying to come up with the answer, otherwise I would agree this is the code reference we are looking for. Anyway thanks for all the input from all.

 

[kwired]

I don't believe that 250.20.(B)(1) only says any system that can be grounded must be. It does say that, but also says under any of the following conditions. I think it does make mention of using the neutral in condition 2 which is part of the article stating where a 3-phase wye system where the neutral is used as a circuit conductor. In our case, it is not being used as a circuit conductor. If you look at the 2011 handbook it shows a diagram specifically of a 120/208 wye system in the 250.20.B explanation. It goes on to state that this system is covered by 250.20.B(2). It states this system must be grounded because of the line to neutral loads. Hence my question, being that there is no line to neutral loads must it still be grounded? Is this the reason why these people preferred not to ground the XO terminal? BTW, I am not asking about grounding a phase and never did. I am looking for a code reference that prohibits not grounding this XO terminal so I could share it with my crew. The fact that the 250.20.(B) says where the neutral is used as a circuit conductor, and the explanation in the handbook, has thrown myself and my crew off a bit when trying to come up with the answer, otherwise I would agree this is the code reference we are looking for. Anyway thanks for all the input from all.

I am with you on your comments, but will add that if you do not ground the neutral you still have to ground something or if you leave it as an ungrounded system then you must provide ground detection, you can't just leave it ungrounded and no detection. 250.21(B).

 

[texie]

I don't believe that 250.20.(B)(1) only says any system that can be grounded must be. It does say that, but also says under any of the following conditions. I think it does make mention of using the neutral in condition 2 which is part of the article stating where a 3-phase wye system where the neutral is used as a circuit conductor. In our case, it is not being used as a circuit conductor. If you look at the 2011 handbook it shows a diagram specifically of a 120/208 wye system in the 250.20.B explanation. It goes on to state that this system is covered by 250.20.B(2). It states this system must be grounded because of the line to neutral loads. Hence my question, being that there is no line to neutral loads must it still be grounded? Is this the reason why these people preferred not to ground the XO terminal? BTW, I am not asking about grounding a phase and never did. I am looking for a code reference that prohibits not grounding this XO terminal so I could share it with my crew. The fact that the 250.20.(B) says where the neutral is used as a circuit conductor, and the explanation in the handbook, has thrown myself and my crew off a bit when trying to come up with the answer, otherwise I would agree this is the code reference we are looking for. Anyway thanks for all the input from all.

The handbook just didn't choose to show your situation.
In 250.20(B)(1), you are missing the part about 150 volts to ground. Can your system be gounded so that the maximum voltage to ground is less than 150 volts? Yes. The use of the neutral is irrelevant at this point. Your system must be grounded.
And just to be clear, the grounded conductor shall be the common conductor of the phases per 250.26(3).

 

[templdl]

It appears as though there are those who are trying to justify other ways to accomplish grounding of a wye secondary. Would any of them install a transformer that way?
I provided and example of a 480v MCC that only has line to line loads when no neutral is required. As such pulling a neutral from the wye secondary of a transformer is not required only the ground. It is very common to build and ship an MCC without a neutral bar. Neutral bars have to be specified.
I would like to see one example where the X0 of the transformer supplying the MCC is not grounded but one of the phase conductors is used as the grounded conductor unless it is being supplied by a corner grounded delta. It just isn't done.
The X0 is grounded and the EGC is pulled with the line conductors. As such anybody who would use a phase conductor as a grounded conductor with a 208/120 secondary instead of the X0 would be doing completely contrary to standard installations.