# Neutral to Ground Bonding



## Magnettica (Jan 23, 2007)

The bond is only at the main disconnect to facilitate circuit breakers and fault current. 

Remember that electrons do work and return their source and this is why the bond at the main only is required. 

Hope that helps you. 

Btw, there's no such thing as a 208/480 volt service. It's either 120/208 or 277/480.


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## Vintage Sounds (Oct 23, 2009)

I think you mean 277/480 service. The neutral and ground are only bonded in one place, generally in the first disconnect or the transformer where the supply originates.

The connection between N and G is there for, as I understand, a few reasons. 

The biggest one is safety. People often (wrongly) think when something is "grounded" that they are going to send electricity "into the ground" so that it doesn't shock people in case of a fault. In reality in an AC system current wants to get back to the transformer it came from, not "into the ground". When we "ground" all our boxes and pieces of equipment and whatever we actually are *bonding them *but the terms are used interchangebly in the field which adds to the confusion. 

So all of this equipment is bonded together all the way back to the panel and ultimately the disconnect. Suppose you have a ground fault, where an energised wire shorts to a bonded surface of a cabinet or box or something. Now electricity is flowing on the bond conductor which normally carries no electricity! It ultimately reaches the point at which the "ground"(again, actually bond) conductor meets the neutral. So now this electricity is getting back to where it wants to be. In the mean time, the circuit breaker or fuse supplying the faulted conductor will detect a short circuit condition and the high current flow will cause it to trip or pop the fuse. 

If we didn't have the link between the bond and the neutral, the current from the shorted conductor would energise the bond, but it wouldn't have anywhere to go. Under these conditions the overcurrent protection wouldn't trip. Eventually if another phase shorted to bond, there would be a short across two phases and that would get pretty ugly. Or, if you touched this energised bond, and somehow came in contact with the neutral, you would be blown up by the fault current. There are systems designed to be ungrounded, and they have no neutral but they have detection apparatus that notifies of a fault condition.

Now, the system is not just bonded, but *grounded*. "Grounding" something strictly speaking refers to directly connecting it to the ACTUAL ground, i.e. a ground rod. So the ground rod or plate that you installed outside, gets connected to the same spot where the neutral and bond are joined. This means that the neutral now is at equal potential to the actual earth. If you measure between the now-grounded neutral and the dirt outside you will have 0 volts and this in turn means that no matter what flows on the neutral, its potential to ground isn't going anywhere. The point of that is that the voltage between the neutral and all of the phases becomes predictable. In your case approximately 277v from each phase to neutral. If the neutral was not grounded then the voltage from each phase to the neutral would depend on how much load was placed on each phase, with the most heavily loaded phase having the lowest voltage to neutral and the most lightly loaded phase having the highest. You would not be able to guarantee that a 277v load was really getting 277v and not 265v or 289v or whatever.

I hope that was clear. :thumbsup:


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## pmt8177 (Feb 5, 2012)

Yes I meant 277/480 I got it confused with our step down transformer 120/208 I believe so. Anyways, thank you for that explaination it helped a lot, but where does the electricity go after it goes back to the transformer after a short?


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## 220/221 (Sep 25, 2007)

pmt8177 said:


> Hi fellas,
> 
> I am a first year apprentice and just helped install a 3 phase 208/480 service. And was wondering why the neutral and ground are bonded in panels, troughs, disconnects etc.
> 
> I have a problem understanding because I know the neutral is a current carrying conductor and if it is bonded at the panels wouldn't the grounds be current carrying also?


It may take a while for this to sink in. It did for me.

The neutral is only bonded at the *service* or *service panel* (the first point of disconnect) so, as long as there is a proper neutral service conductor intact, the grounds aren't carrying current.

At any other place in the system, the neutrals and grounds are isolated. The reason behind this is, if a neutral feeder fails, the current would "go back" thru the grounds/bonds and potentially energize any metallic things that were grounded. This could happen on the service conductor also but there are fewer splices therefore less chance.


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## Cletis (Aug 20, 2010)

220/221 said:


> It may take a while for this to sink in. It did for me.
> 
> The neutral is only bonded at the *service* or *service panel* (the first point of disconnect) so, as long as there is a proper neutral service conductor intact, the grounds aren't carrying current.
> 
> At any other place in the system, the neutrals and grounds are isolated. The reason behind this is, if a neutral feeder fails, the current would "go back" thru the grounds/bonds and potentially energize any metallic things that were grounded. This could happen on the service conductor also but there are fewer splices therefore less chance.


*First point of disconnect* OR at *service* or service panel ?? or both? *Any execeptions* to this rule at all or 100% of time ? Just askin


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## Cletis (Aug 20, 2010)

What I mean is how to you classify First point of Disconnect ??


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## Magnettica (Jan 23, 2007)

Great question by the OP by the way...

He should read this for a broader explanation:


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## ohmontherange (May 7, 2008)

The OP made mention of a 480 / 208 step down transformer which constitutes a seperate derived source. Bonding jumper at the secondary of that transformer or first disconnect means as well.


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## Dash Dingo (Mar 3, 2012)

220/221 said:


> It may take a while for this to sink in. It did for me.
> 
> The neutral is only bonded at the service or service panel (the first point of disconnect) so, as long as there is a proper neutral service conductor intact, the grounds aren't carrying current.
> 
> At any other place in the system, the neutrals and grounds are isolated. The reason behind this is, if a neutral feeder fails, the current would "go back" thru the grounds/bonds and potentially energize any metallic things that were grounded. This could happen on the service conductor also but there are fewer splices therefore less chance.


So if the neutral at the transformer is bonded to the ground via a ground rod and the neutral at the service is bonded to the ground via ground rod or water pipe.... Then why doesn't some current flow on the grounding electrodes and grounding electrode conductors back to the transformer?


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## Dennis Alwon (May 9, 2009)

Dash Dingo said:


> So if the neutral at the transformer is bonded to the ground via a ground rod and the neutral at the service is bonded to the ground via ground rod or water pipe.... Then why doesn't some current flow on the grounding electrodes and grounding electrode conductors back to the transformer?


 Depending on soil conditions, distance etc there can be some flow but the impedance of the earth is great and not much will go back thru the earth.


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## Dash Dingo (Mar 3, 2012)

Dennis Alwon said:


> Depending on soil conditions, distance etc there can be some flow but the impedance of the earth is great and not much will go back thru the earth.


What are the reason(s) for the poco bonding the neutral to the soil? For Faults?


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## BBQ (Nov 16, 2010)

Dash Dingo said:


> What are the reason(s) for the poco bonding the neutral to the soil? For Faults?


Mostly lightning. 

But now we get into the confusing part, while the earth has to much impedance to clear a fault at say 120 volts to ground, once you raise the voltage to say 7000 volts to ground things change.

Let's say you have a grounding electrode with 25 ohms of resistance. 

120 volts / 25 ohms = 4.8 amps of current, far to low to trip any breaker on a typical 120 volt circuit

On the other hand

7000 / 25 ohms = 280 amps of current more than enough to open the cut out of a 7000 volt circuit.


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## Cletis (Aug 20, 2010)

Magnettica said:


> Great question by the OP by the way...
> 
> He should read this for a broader explanation:


Is a meter with a handle disconnect considered the first point of disconnect?


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## Rollie73 (Sep 19, 2010)

Cletis said:


> Is a meter with a handle disconnect considered the first point of disconnect?


Is it the "first" disconnect after the POCO's transformer???? That's the easiest way to explain it. Regardless of wether its a meter can with a disconnect, a panel with a main breaker installed, a 100 or 200 amp disconnect switch feeding a distribution panel...........if its the first one after the transformer......bond the neutral.


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## Cletis (Aug 20, 2010)

Rollie73 said:


> Is it the "first" disconnect after the POCO's transformer???? That's the easiest way to explain it. Regardless of wether its a meter can with a disconnect, a panel with a main breaker installed, a 100 or 200 amp disconnect switch feeding a distribution panel...........if its the first one after the transformer......bond the neutral.


Ok then. Now, can you run a 3 wire from that through a 10" nipple to a 200 amp main breaker panel and wire it like a main???????


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## Rollie73 (Sep 19, 2010)

Cletis said:


> Ok then. Now, can you run a 3 wire from that through a 10" nipple to a 200 amp main breaker panel and wire it like a main???????


Sorry....you'll have to wait for someone with more knowledge of the NEC to answer that for you. If I understand what the scenario that you are laying out there......I don't see why not.


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## 220/221 (Sep 25, 2007)

Rollie73 said:


> Is it the "first" disconnect after the POCO's transformer???? That's the easiest way to explain it. Regardless of wether its a meter can with a disconnect, a panel with a main breaker installed, a 100 or 200 amp disconnect switch feeding a distribution panel...........if its the first one after the transformer......bond the neutral.


I have gotten very confused but, Yes


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## Cletis (Aug 20, 2010)

Rollie73 said:


> Is it the "first" disconnect after the POCO's transformer???? That's the easiest way to explain it. Regardless of wether its a meter can with a disconnect, a panel with a main breaker installed, a 100 or 200 amp disconnect switch feeding a distribution panel...........if its the first one after the transformer......bond the neutral.


If that's the case then, when you have a meter with by pass lever do you run a 4 wire through a 12" or less nipple to main breaker panel and make a sub??? How do add an insulated neutral bar to a meter base ?


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## BBQ (Nov 16, 2010)

Cletis said:


> If that's the case then, when you have a meter with by pass lever do you run a 4 wire through a 12" or less nipple to main breaker panel and make a sub??? How do add an insulated neutral bar to a meter base ?


What be the crap yoz is axeking about?


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## Elephante (Nov 16, 2011)

Dash Dingo said:


> So if the neutral at the transformer is bonded to the ground via a ground rod and the neutral at the service is bonded to the ground via ground rod or water pipe.... Then why doesn't some current flow on the grounding electrodes and grounding electrode conductors back to the transformer?


In some cases it does.I did an emergency job where the neutral was disconnected from the service drop and the homeowner was still getting power in his home.I'm not sure if it was going through the neighbors neutral or through the soil straight to the pole. But,it was making it back somehow.


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## Big John (May 23, 2010)

I can see getting *a lot* of mileage out of this picture:


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## BBQ (Nov 16, 2010)

You can use this one as needed


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## 220/221 (Sep 25, 2007)

Elephante said:


> I did an emergency job where the neutral was disconnected from the service drop and the homeowner was still getting power in his home.I'm not sure if it was going through the neighbors neutral or through the soil straight to the pole. But,it was making it back somehow.


Some power would be going "back" via the other leg.

If all the piping was bonded and the water/gas line/system is metallic, most of it was probably going that direction. Cable and phone lines grounds could also be carrying some load.


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## chicken steve (Mar 22, 2011)

pmt8177 said:


> Yes I meant 277/480 I got it confused with our step down transformer 120/208 I believe so. Anyways, thank you for that explaination it helped a lot, but where does the electricity go after it goes back to the transformer after a short?


It takes _all _paths relative to R value pmt

so, if you've multiple MBJ's with their required GEC's under ONE industrial roof , along with multiple mettalic systems common to it, it's divided among them

~CS~


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## Rockyd (Apr 22, 2007)

chicken steve said:


> It takes _all _paths relative to R value pmt
> 
> so, if you've multiple MBJ's with their required GEC's under ONE industrial roof , along with multiple mettalic systems common to it, it's divided among them
> 
> ~CS~


Electricity doesn't taked the path of least resistance....it takes ALL paths to return to it's source. We give it a way to get there via NEC 250.2 Effective Ground-Fault Currrent path.


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## Rockyd (Apr 22, 2007)

The OP may want to take a look at 250.2(A) Grounded systems.

250.2(A)1 Electrical system grounding explains why we ground.


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## chicken steve (Mar 22, 2011)

Rockyd said:


> Electricity doesn't taked the path of least resistance....it takes ALL paths to return to it's source. We give it a way to get there via NEC 250.2 Effective Ground-Fault Currrent path.


yes, and per my post, that's probably the most common 'Kirchoff' misinterpetation ...

~CS~


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