# Xo on transformer confuses me.



## ElectricalArtist (Jul 2, 2014)

I should probably ask this at work but u guys will explain better. My confusion is why does bonding the xo to the case and grounding the case not cause a short ?


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## LGLS (Nov 10, 2007)

A short circuit to what? Until you bond XO to ground, the secondary side output from a transformer has no reference to ground. What you're asking is often a very difficult concept for electricians to grasp, the typical reason being you don't fully understand exactly what a transformer does.


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## ElectricalArtist (Jul 2, 2014)

LawnGuyLandSparky said:


> A short circuit to what? Until you bond XO to ground, the secondary side output from a transformer has no reference to ground. What you're asking is often a very difficult concept for electricians to grasp, the typical reason being you don't fully understand exactly what a transformer does.



I'm learning about the transformer. I understand it works on electromagnetism. And depending on how it's set up , the voltage are changed. I do not get why you can put the center tap of voltage to the case and the case to ground and not create a big boom.


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## chicken steve (Mar 22, 2011)

Yeah it's a conceptual thing Sici dude ,we all struggle with it..... think of X0 as a zero volt reference , might be carrying a lotta amps, but zero on the V....~CS~


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## wildleg (Apr 12, 2009)

xo is the neutral, and the earth is used as a ground reference. by bonding the xo to ground in this fashion, you are establishing the earth as a reference voltage. 

It is no different than using the ground as reference at the service, where you are also bonding the neutral to ground, only in the case of the transformer, at the secondary xo, since the neutral is re-derived separately from the primary side. (if that is the xo bond of which you speak)

I am not the best explainer, but hopefully that helps.


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## chicken steve (Mar 22, 2011)

This is gonna be fun when we get to corner grounded delta's......~CS~


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## Bird dog (Oct 27, 2015)

chicken steve said:


> Yeah it's a conceptual thing Sici dude ,we all struggle with it..... think of X0 as a zero volt reference , might be carrying a lotta amps, but zero on the V....~CS~


CS hit it on the head right there.


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## Bird dog (Oct 27, 2015)

chicken steve said:


> This is gonna be fun when we get to corner grounded delta's......~CS~


Or why Jbox covers blow off boxes when the 2nd leg of a floating Delta shorts to ground. :no:


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## ElectricalArtist (Jul 2, 2014)

Xo is a zero volt? I think that concept is something that can help. I'm going to go back to the basics and keep that in mind and hopefully it clicks. I also need a better understanding of bonding and grounding. 


The theory is hard to understand and I'm studying that part right now. I got the Mike holt ultimate training library and I'm studying hard and researching things that I need more help with. This forum helps becuase i tend to get stuck at points until I get a good understanding. When I'm on the field and run into something I learned il go back and re read it or ask questions on here . I'm starting to shy away from asking to much at work becuase i do consider myself a good electrician , I can bend pipe pretty good, I can read prints, I have a decent grasp on fire alarms , I can comfortably work on panels ,ect ect. Problem is I like to learn and I ask alot of questions. People sometimes look at you different when u ask alot of questions. I work with different people all the time and not all of them see what I do know, some just hear the questions so depending whose job site I'm on , il be asking here instead of at work so please have some patients with all the threads I start.


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## emtnut (Mar 1, 2015)

sicilianiggy said:


> Xo is a zero volt?


If you bond it to ground, it will be referenced to 0V.


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## Cl906um (Jul 21, 2012)

The secondary windings have no physical connection to the primary side. Work off of induction. You can ground any part of, and it will not short out on this side and common to ground midpoint of the winding on a delta secondary. Or corner at times.


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## ElectricalArtist (Jul 2, 2014)

Cl906um said:


> The secondary windings have no physical connection to the primary side. Work off of induction. You can ground any part of, and it will not short out on this side and common to ground midpoint of the winding on a delta secondary. Or corner at times.


Thanks, that helps alot. Im actually reading about induction right now as I watch the game. As the current alternates thru a conductor ,it creates and collapses a magnetic field creating voltage.


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## Cl906um (Jul 21, 2012)

I am originally from a suburb of Green Bay , so if my message only seems to make half sense right now, I am watching it too.


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## Cl906um (Jul 21, 2012)

Man that fumble sucked. Ripped it out of his hands. EFF!


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## ponyboy (Nov 18, 2012)

Wait til you do your first corner ground. 


Sent from my iPhone using Tapatalk


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## Drew 64 (Dec 31, 2016)

If you don't bond the XO we call it a floating neutral and you will read voltage from neutral to ground.


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## ElectricalArtist (Jul 2, 2014)

Cl906um said:


> I am originally from a suburb of Green Bay , so if my message only seems to make half sense right now, I am watching it too.



You guys beat my giants, but I'm still going for the Packers. The defense needs to play better


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## emtnut (Mar 1, 2015)

Cl906um said:


> Man that fumble sucked. Ripped it out of his hands. EFF!


From Gronk too .... I was surprised


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## Cl906um (Jul 21, 2012)

*Loo*



emtnut said:


> From Gronk too .... I was surprised


Looks like all the wind just got ripped from their sales.what I thought was gonna be a shootout just became a shootup.


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## HackWork (Oct 2, 2009)

Niggy, in a normal system the only reason you have a "boom" (ground fault) when you touch a hot wire to ground is because ground is bonded to neutral somewhere.

In the case of the transformer you are wiring, that ground to XO connection you are making is that bond that I mentioned above. Nothing blows up because there is no potential (or very little potential) between neutral and ground.

Now people keep mentioning a corner grounded delta being more complicated. It's really just the same line of thinking. When you have a corner grounded delta, there is no neutral, so they ground one of the hots. You may now be thinking "why doesn't it blow up". The answer is- why would it blow up? Since ground isn't bonded to neutral in a corner grounded delta, there is no potential between hot and ground. Remember, the only reason why hot would normal fault to ground is because ground was bonded to neutral. If there is no neutral to bond it to, then there won't be a fault when you touch (or bond) hot to ground. 

Back to the beginning, if you were to remove the bond between neutral and ground in your main panel and at the transformer on the pole (and possibly at you neighbor's houses if they are on that transformer), you can touch hot to ground all day without it blowing up.


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## RePhase277 (Feb 5, 2008)

sicilianiggy said:


> I'm learning about the transformer. I understand it works on electromagnetism. And depending on how it's set up , the voltage are changed. I do not get why you can put the center tap of voltage to the case and the case to ground and not create a big boom.


Before learning about transformers, you should have learned that electricity only flows in a COMPLETE CIRCUIT. The X0 terminal of a transformer is just one pole of the circuit. There is no complete path between it and another phase.

Pretend the transformer is a battery. If you connect one pole of the battery to the case, why isn't there a big boom?


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## Big John (May 23, 2010)

You can ground any system once. All you're doing is creating a reference point.

If you ground a system twice then you're creating a circuit between those two points.

Sometimes it's a high resistance circuit like between the house neutral-ground and the utility neutral-ground and only a couple amps flow.

Sometimes it's a short circuit like if you had a grounded wye and developed a second ground on C phase. Then you have a fire danger.


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## brian john (Mar 11, 2007)

Forget the NEC for a minute.

For a Wye transformer with a 208/120 secondary

In a transformer with a wye secondary and the secondary terminations X1, X2, X3 and X0 are NOT grounded to the case or to any other metallic components serve as ground; IN THEROY the voltage to ground is "0" Volts at NO LOAD. BUT the voltage between the terminations is

X0-X1=120
X0-X2=120
X0-X3=120
X1-X2=208
X2-X3=208
X3-X1=208
X0-Ground=0
X1-Ground=0
X2-Ground=0
X3-Ground=0

Now ground XO

X0-X1=120
X0-X2=120
X0-X3=120
X1-X2=208
X2-X3=208
X3-X1=208
X0-Ground=0
X1-Ground=120
X2-Ground=120
X3-Ground=120

Now lift the X0 Ground and Ground X1 (could be X2 or X3)

X0-X1=120
X0-X2=120
X0-X3=120
X1-X2=208
X2-X3=208
X3-X1=208
X0-Ground=120
X1-Ground=0
X2-Ground=208
X3-Ground=208

And in all three cases everything connected will operate without issue

Google a wye transformer which really in 3- 120 VAC transformers X1-X2, X1-X2, X1-X2 with all the X2's (or X1's) terminated together Y.


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## splatz (May 23, 2015)

sicilianiggy said:


> Xo is a zero volt?


Lot of good info above but if it helps remember no one thing is really "zero volt" - voltage is measured between two things. You wouldn't get a voltage reading with only one lead of your meter touching one terminal, or both leads touching the same terminal.


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## oliquir (Jan 13, 2011)

think of a car battery circuit the negative is tied to the car body


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## ElectricalArtist (Jul 2, 2014)

I have a much better understanding thanks to you guys and studying. The ultimate training library theory course basically builds you up little by little and then gets into greater detail later. Started with electrons , series, parallel, magnetism, ect ect. I'm getting a good basic understanding before I completely understand. Now I'm onto back EMF &#55358;&#56596;LOL this is exciting and frustrating at the same time


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## RePhase277 (Feb 5, 2008)

sicilianiggy said:


> I have a much better understanding thanks to you guys and studying. The ultimate training library theory course basically builds you up little by little and then gets into greater detail later. Started with electrons , series, parallel, magnetism, ect ect. I'm getting a good basic understanding before I completely understand. Now I'm onto back EMF ��LOL this is exciting and frustrating at the same time


Stop doing anything until you understand basic DC circuits. It is vitally important to get the concept of a complete circuit in order to understand ANYTHING else. Believe me, I have seen 20 year electricians that can't understand grounding and bonding because they didn't know/remember basic circuit theory.


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## inetdog (Apr 13, 2016)

I think an earlier post had the right idea with the battery analogy.
If I take two.batteries with no external connections to start off and I connect the - end of one to ground and the + end of the other to ground I now have a +12 terminal and a -12 terminal still unconnected. If I add a wire between the two grounded connections I now have a 24V source I can use.
Now do it another way:
I first tie the two batteries together. That gives me 24V from terminal to terminal. But I do not measure any voltage to ground.
Now I ground the common point between the batteries. Will there be a short or current flow? No. I have just created the same situation as my first example, just assembling it in a different order.

mobile


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## cdslotz (Jun 10, 2008)

Also read up on what a "separately derived system" is, and study it ........

here: http://www.ecmag.com/section/codes-standards/grounding-separately-derived-systems-part-1-basics


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## chicken steve (Mar 22, 2011)

Then move onto earthing systems

~CS~


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## ElectricalArtist (Jul 2, 2014)

I installed a transformer and I'm not sure if I should use the bond screw on the panel. I don't believe so but correct me if I'm wrong

It's a 480 primary 120/208 4 wire secondary . The transformer is grounded to the building steel, I bonded the xo to the transformer case,and ran a egc from the transformer xo to the panel ground bar. Since I have the egc to my panel and my neutral comes from xo which is grounded do I use the bonding screw? I say no


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## brian john (Mar 11, 2007)

ElectricalArtist said:


> I installed a transformer and I'm not sure if I should use the bond screw on the panel. I don't believe so but correct me if I'm wrong
> 
> It's a 480 primary 120/208 4 wire secondary . The transformer is grounded to the building steel, I bonded the xo to the transformer case,and ran a egc from the transformer xo to the panel ground bar. Since I have the egc to my panel and my neutral comes from xo which is grounded do I use the bonding screw? I say no


You either bond the neutral (XO) in the transformer or panel not both.

You bonded/grounded in the transformer DO NOT re-bond in the panel.


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## Cl906um (Jul 21, 2012)

ElectricalArtist said:


> I installed a transformer and I'm not sure if I should use the bond screw on the panel. I don't believe so but correct me if I'm wrong
> 
> It's a 480 primary 120/208 4 wire secondary . The transformer is grounded to the building steel, I bonded the xo to the transformer case,and ran a egc from the transformer xo to the panel ground bar. Since I have the egc to my panel and my neutral comes from xo which is grounded do I use the bonding screw? I say no


I see old journeymen do this wrong. You bond it in xformer, or panel. Never both. You are right, and have to run a neutral and equipment ground conductor to the panel when you connect them together in transformer. Float the neutral then in panel. Throw the bond screw out.


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## Cl906um (Jul 21, 2012)

The only place there is ever a parallel grounded conductor is at the service entrance where metal conduit is used from meter to first breaker. Ever. Bond jumpers or bonding locknuts are required.


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## ElectricalArtist (Jul 2, 2014)

Thanks guys. If I rebond the panel what are the affects? All I know so far from reading and studying is that all the unbalanced current on the neutral will travel through the equipment and building steel. What else happens?

Another question I have is this. I learned a long time ago that if u read 120 on hot to ground and 60 ish on hot to neutral, it's an open neutral. Never asked or looked into why that is?


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## Cl906um (Jul 21, 2012)

ElectricalArtist said:


> Thanks guys. If I rebond the panel what are the affects? All I know so far from reading and studying is that all the unbalanced current on the neutral will travel through the equipment and building steel. What else happens?
> 
> Another question I have is this. I learned a long time ago that if u read 120 on hot to ground and 60 ish on hot to neutral, it's an open neutral. Never asked or looked into why that is?


With an open neutral. I have read to ground 122, and to neutral 115. It depends on the situation. Anything but the same reading hot to ground, hot to neutral should be suspect to something wrong. Capacitive coupling happens with a lot of equipment.


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## brian john (Mar 11, 2007)

ElectricalArtist said:


> Thanks guys. If I rebond the panel what are the affects? All I know so far from reading and studying is that all the unbalanced current on the neutral will travel through the equipment and building steel. What else happens?



You have current on the neutral and current on the ground conductor (If used) and conduit, between the panel and transformer. Not a terrible situation but should be avoided.




> Another question I have is this. I learned a long time ago that if u read 120 on hot to ground and 60 ish on hot to neutral, it's an open neutral. Never asked or looked into why that is?


The voltage can be anywhere 0-240 (on a 240/120 Single phase system) depending on the load.


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## brian john (Mar 11, 2007)

Cl906um said:


> The only place there is ever a parallel grounded conductor is at the service entrance where metal conduit is used from meter to first breaker. Ever. Bond jumpers or bonding locknuts are required.


If you are doing commercial work in buildings with metal studs, metal ducts, rebar, you have had ground current due to improper grounding of neutrals, you may not have measured for it.

90=% of the commercial buildings we work in have neutral ground issues, its may only be a 20 amp circuit but they exist.


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## lightman (Oct 14, 2015)

Transformers are simple and interesting to work with. But only after you understand them. Big John and Brian John did a good job of explaining them. You can ground a transformer winding anyplace that you want to, one time. If you try to ground it a 2nd time it becomes a fault!. Each winding has a voltage value, stated on the name plate. X0 is the mid point of the winding, or what makes a 240 volt winding into two 120 windings. Or makes a 480 volt winding into two 240 volt windings. By grounding the mid point you are making a reference. If you don't ground X0 then your winding has the full voltage named on the name plate. Usually this voltage is not useful unless you are dealing with only a motor load.

Edited to add; Oh yeah, the first guy to ground a hot phase was either very sharp or very gutsy!


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## ElectricalArtist (Jul 2, 2014)

Big John said:


> You can ground any system once. All you're doing is creating a reference point.
> 
> If you ground a system twice then you're creating a circuit between those two points.
> 
> ...


Creating a circuit. I've never looked at it like that. I would be creating a circuit on the grounding system in stead of having the nuetral carry the unbalance load and the ground Carrying overcurrent to earth and the bonding is for a short circuit. Another connection between the egc and the ungrounded conduct would make a circuit. that's the history of it no?


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