# Neutral/ Bonding in a Transformer



## Firefighter102 (Dec 7, 2011)

So all the journeyman at work are at each others throats over where you have to use bonding bushings in panels and transformers.

The other hot topic of argument is where the neutral is actually derived In an 3 phase building. Some are saying from the inner windings of the coils in the transformers and others are saying from the building steel, I can't find anything good on google that I trust and I'm still trying to learn the code book. (ive only been in the trade 9 months) 

So any insight would be great and references to code sections would be amazing. Thanks


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## Magnettica (Jan 23, 2007)

The neutral is derived from the transformers of 3ø supply. 

250.20(B) is the requirement.

The building steel should never have any current flowing on it.

250.6


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## Magnettica (Jan 23, 2007)

As for the bonding bushing question.... take a look at 250.92.


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## Big John (May 23, 2010)

A neutral point is always derived at the transformer or generator. 

Think of it like one of the terminals on a battery: It doesn't matter if you connect your 9 volt to the earth, because unless you have a complete circuit between the two terminals, no current will flow.

Same thing with electrical service: Unless you have a connection between at least two points on the transformer or generator winding, you will not have current flow. The neutral is just one of those points.

-John


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## brian john (Mar 11, 2007)

Magnettica said:


> The neutral is derived from the transformers of 3ø supply.


 
Of a 3 phase 4-wire transformer, on the secondary at the XO, which should be bonded either in the transformer or at the first OCP.



> The building steel should never have any current flowing on it.
> 
> 250.6


Except for minimal leakage current and fault current.


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## Firefighter102 (Dec 7, 2011)

Okay, so maybe on a more technical side to it, is the neutral made from the center of the windings or??? A delta wye gets you a brown orange yellow but no neutral, and then allOf a sudden there's the x legs black red blue with a neutral so I guess that's what I'm really confused about, why isn't there your high leg neutral but there is a low leg neutral.


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## Arc'n'Spark (Jul 21, 2011)

Firefighter102 said:


> Okay, so maybe on a more technical side to it, is the neutral made from the center of the windings or??? A delta wye gets you a brown orange yellow but no neutral, and then allOf a sudden there's the x legs black red blue with a neutral so I guess that's what I'm really confused about, why isn't there your high leg neutral but there is a low leg neutral.


The short answer is: because it's unnecessary with a delta-wye connection. To the primary, the transformer is just another 3ph load.


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## Firefighter102 (Dec 7, 2011)

Okay that makes sense that it's a 3 phase load, that's very simple, so the only remaining question is where does the neutral come from or why do the electrons want to flow back to the neutral.
And i want to thank everyone for helping me out with this.


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## erics37 (May 7, 2009)

Firefighter102 said:


> Okay that makes sense that it's a 3 phase load, that's very simple, so the only remaining question is where does the neutral come from or why do the electrons want to flow back to the neutral.
> And i want to thank everyone for helping me out with this.


The electrons will flow wherever there is a difference of potential and a complete circuit.

If you complete a circuit between a phase leg and a neutral, electrons will flow.

If you complete a circuit between a phase leg and another phase leg, electrons will flow.


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## cuba_pete (Dec 8, 2011)

Firefighter102 said:


> Okay that makes sense that it's a 3 phase load, that's very simple, so the only remaining question is where does the neutral come from or why do the electrons want to flow back to the neutral.
> And i want to thank everyone for helping me out with this.


Google an image of a delta-wye...the neutral is derived from the "center" of the wye which is actually at the base of the three inductors. This sets the reference point for each of the three phases. The electrons want to "flow back" to the reference set up through the inductor which has its field set up by the delta windings. The neutral of the derived service is fed only to its panels, however all neutrals will ultimately reference the service entrance through proper grounding and bonding (this is assuming that there is only one service).

There are other ways to get neutral (split-phase, for instance), but the above is pretty common and straightforward.


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## RIVETER (Sep 26, 2009)

Firefighter102 said:


> Okay, so maybe on a more technical side to it, is the neutral made from the center of the windings or??? A delta wye gets you a brown orange yellow but no neutral, and then allOf a sudden there's the x legs black red blue with a neutral so I guess that's what I'm really confused about, why isn't there your high leg neutral but there is a low leg neutral.


I have never heard of a low leg neutral. For a three phase electrical system to work it does not need a neutral if all of the loads are three phase loads. In a delta/wye configuration where you would want the lower voltage you must bond/ground the XO to the building steel or other GOOD ground. It is not because it has to be to make the system work. It is to protect the building from aberrant voltages. It limits the voltage in the building to ground.


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## RePhase277 (Feb 5, 2008)

Basically, voltage causes electrons to move, which means that there are more electrons in one place than another. The place with more electrons has a negative charge, and the place devoid of electrons has a positive charge. If an external circuit connects these two places, the electrons will rush through that circuit trying to equalize the positive charge, and drive a load while they're at it.

A neutral and phase wire are just such a separation. If given the chance through a circuit, a current will flow.


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## RIVETER (Sep 26, 2009)

InPhase277 said:


> Basically, voltage causes electrons to move, which means that there are more electrons in one place than another. The place with more electrons has a negative charge, and the place devoid of electrons has a positive charge. If an external circuit connects these two places, the electrons will rush through that circuit trying to equalize the positive charge, and drive a load while they're at it.
> 
> A neutral and phase wire are just such a separation. If given the chance through a circuit, a current will flow.


I gotta say I am not sure why you are saying this but I have to say...as well...you are correct. Electricity is of no use unless there is a difference of potential...and a load to use it up.


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## yungun (Oct 7, 2011)

The neutral is bonded to the ground at the transformer, or if the transformer is in the utility side the neutral is bonded to ground at the main panel or service that it feeds.

As far as grounding bushings, I can't quote the code but where you are terminating a metal pipe into a box using factory punched knock outs(concentric or eccentric) you need a grounding bushing. You'd have to look at the UL listing but if I understand correctly some connectors such as hub style connectors are rated to ground the pipe without the need of a busing. If you are entering a box using a hole you punched yourself then 2 lock rings rated for this purpose will count as well. Any wire bigger than #4 needs some sort of protection as well, such as a plastic bushing, grounding bushing, or a UL listed connector rated for this purpose. I will try to find the code ref.s


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## cuba_pete (Dec 8, 2011)

*electron "flow"*



InPhase277 said:


> Basically, voltage causes electrons to move, which means that there are more electrons in one place than another.


Ack!!! I wish we wouldn't over-simplify things like this. Electron "flow" is not the main influencer in electical energy. The energy state of the atom's particles has a much greater influence...as in the energy state of the electrons which are "moving" through the circuit. In a simple circuit, a battery and a wire, the electrons are not going to move any faster than physics allows. You can't speed up energy by applying more but you can change its level of energy by applying a higher potential.

You cannot change the speed of light by applying more energy, it just changes the energy level of the photon(s) involved.

Simple electron flow is usually applied to solid metals...current flow in the human body is effected as ion flow...both positive and negative flow.

In conventional current flow electrons don't even move.


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## MaxFuse (Oct 23, 2011)

A 3 phase Transformers is fed on the primary side with three phases and just a ground. Then the neutral is created for the secondary output.
I allways bond the neutral XO to the shell ground inside the transformer as well to building steel.
keeps the floating neutral syndrome unaffected by outside sources.
Then also bond at the first disconnect on the secondary side.


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## wesleydnunder (Mar 19, 2012)

As stated, the neutral in a wye secondary is derived at the junction of the three windings. It is grounded for two reasons:
1. To hold the neutral at 0 volt potential
2. To give fault current on the equipment ground a path back to the neutral to activate the OCP.

Mark


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## wesleydnunder (Mar 19, 2012)

MaxFuse said:


> A 3 phase Transformers is fed on the primary side with three phases and just a ground. Then the neutral is created for the secondary output.
> I allways bond the neutral XO to the shell ground inside the transformer as well to building steel.
> keeps the floating neutral syndrome unaffected by outside sources.
> Then also bond at the first disconnect on the secondary side.


This is a code violation. By bonding the neutral to the ground at both locations you have effectively turned the ground into a parallel neutral, and the ground becomes a current-carrying conductor. The ground should only carry fault current. The bond in this case should be at one or the other, not both.

Mark


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## brian john (Mar 11, 2007)

wesleydnunder said:


> As stated, the neutral in a wye secondary is derived at the junction of the three windings. It is grounded for two reasons:
> 1. *To hold the neutral at 0 volt potential*
> 2. To give fault current on the equipment ground a path back to the neutral to activate the OCP.
> 
> Mark


To keep the neutral as close to "0" as feasible, depending on the the loads on the system, impednace of the system and the voltage drop on the secondary distribution.


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## brian john (Mar 11, 2007)

MaxFuse said:


> A 3 phase Transformers is fed on the primary side with three phases and just a ground. Then the neutral is created for the secondary output.
> I allways bond the neutral XO to the shell ground inside the transformer as well to building steel.
> keeps the floating neutral syndrome *unaffected by outside sources*.


Not sure what that means?



> Then also bond at the first disconnect on the secondary side.


And a Wesley noted a NEC violation, one place or the other not both.


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## Bulldog1 (Oct 21, 2011)

brian john said:


> And a Wesley noted a NEC violation, one place or the other not both.



The bonding to the building steel may be his GEC?


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## brian john (Mar 11, 2007)

Bulldog1 said:


> The bonding to the building steel maybe his GEC?


He said he bonds the neutral in the transformer and Fused Safety Switch


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## Bulldog1 (Oct 21, 2011)

brian john said:


> He said he bonds the neutral in the transformer and Fused Safety Switch



I'm wondering if he is really connecting a GEC to the building steel and not bonding it. Maybe he is doing what he posted.


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## RIVETER (Sep 26, 2009)

Bulldog1 said:


> I'm wondering if he is really connecting a GEC to the building steel and not bonding it. Maybe he is doing what he posted.


It is required that he tie the building steel to the service and that wire is the GEC. That requirement only satisfies the "aberrant" voltage possibility.. It has not a thing to do with making the system work.


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## MaxFuse (Oct 23, 2011)

I confronted the higher up's about this.
The reply was ;
1- It is in the specifications-Bond shall be independent and Protected from outside sources-aka first disconnect.
even on 1-line drawings.
2-If a one wire is bonded to another at any point-it is bonded together as one.
If you bond it a million times more at different points -it does not change any characteristics of the bond.aka paralell neutral.
3- Whatever D.O.D. wants- D.O.D. gets.

I did not enjoy that .


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## cuba_pete (Dec 8, 2011)

*DoD?*



MaxFuse said:


> 3- Whatever D.O.D. wants- D.O.D. gets.
> 
> I did not enjoy that .


So I take it you work for the Department of Defense? If so I don't know where your higher-ups get their info from. Just because some goofy PE put that in the specs doesn't mean people in the know do it because it's spec'd that way.

We don't do it that way in my (Navy) building...no parallel neutrals that is...no way...no how.


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