# bonding a new transformer?



## heavyduty73 (Jan 11, 2008)

was wondering the proper way to bond a new transformer inside of a building? what we have is a main 480 volt distribution center with one bucket feeding a 400 amp transfer switch that feeds a 400 amp emer panel. out of that a 100 amp breaker is feeding a 75 kva trans, then into a 200 amp 120 /208 panel. in the past i have done the bonding in the transformer and ran the bond to the nearest building steel. on this one the boss wants to bond in the trans, but use gronding path back to the main 480 volt gear for the bonding. that ground is already tied to the water/building steel. this is all in the elec room and there is no building steel exposed due to concrete construction. also will run into the same issue in a penthouse in same building. with 480 volt emer panel feeding panel in pent house ,breaker to trans to 120/208 panel. 

Is this plan legal?'
does the bonding wire need to be continous from trans back to ground bar in the the main 480 volt gear or can it be attached to a ground bar in one of the 480 volt panels?
Proper size of bonding wire? (copper)


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## nap (Dec 26, 2007)

size of conductor is based upon the derived phase conductors just as in a typical service. You're talking a 200 amp panel so you have I would suppose 3/0. If so, then your GEC needs to be #4 or larger.

250.64C controls the GEC and I believe it states it must be continuous (back to the original GEC/neut bond) or made continuous with listed ( in that section)connections

If I understand all this, the plan is acceptable as long as the above is followed.


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## RePhase277 (Feb 5, 2008)

250.30(A)(3) covers the grounding electrode requirements for separately derived systems. You must have an electrode, the equipment ground in the 480 panel is not going to get it. You either need to hit building steel or some other electrode.

And remember when bonding the neutral, that you can only do this in one location, either inside the transformer or in the first secondary disconnect, but not both. And where ever you do this, it is also where your GEC must bond.

InPhase277


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## goose134 (Nov 12, 2007)

Does this building have a high rise common grounding electrode system? Maybe it's just a Chicago thing (sometimes called a Hogan ground). Basically it's just a series of copper bus details in boxes that run up the building. Each box is connected to GEC. Requirements are put in for bonding to building steel at certain intervals and so on. Anyways, if you have one of those, bring the transformer ground to the detail.


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## nap (Dec 26, 2007)

those are nice when used. It saves a lot of work.

We had the same situation in a school I did a few years back. There were many many x-formers that needed to be bonded directly to the GEC system.


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## raider1 (Jan 22, 2007)

InPhase277 said:


> 250.30(A)(3) covers the grounding electrode requirements for separately derived systems. You must have an electrode, the equipment ground in the 480 panel is not going to get it. You either need to hit building steel or some other electrode.
> 
> And remember when bonding the neutral, that you can only do this in one location, either inside the transformer or in the first secondary disconnect, but not both. And where ever you do this, it is also where your GEC must bond.
> 
> InPhase277


I agree.

You could not use the EGC for the primary circuit to abtain your connection to the grounding electrode system.

I would suggest carefully reviewing section 250.30 of the NEC for grounding and bonding of a transformer.

Chris


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## sparks134 (Jan 30, 2009)

For seperatly derived systems you need to use the hogan ground or go all the way back to the water pipe. DO NOT TIE BACK INTO THE PRIMARY SYSTEM!!!


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## Bob Badger (Apr 19, 2009)

sparks134 said:


> For seperatly derived systems you need to use the hogan ground


What is a 'Hogan ground'?





> or go all the way back to the water pipe. DO NOT TIE BACK INTO THE PRIMARY SYSTEM!!!


I agree the NEC prohibits it but if the primary EGC was large enough to meet the GEC size requirements can you provide an electrical reason we should not use it?


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## sparks134 (Jan 30, 2009)

because its a separete system!!! also the nec says you have to!!!


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## Bob Badger (Apr 19, 2009)

sparks134 said:


> because its a separate system!!!


That is really not an explanation but yes it is an SDS. 

What is the intended function of the GEC from an SDS? 




> also the nec says you have to!!!


Actually the NEC does not really say that right now, it looks like it will specifically say that for 2011.


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## brian john (Mar 11, 2007)

sparks134 said:


> because its a separete system!!! also the nec says you have to!!!


Separate system is not a decent argument. Ground is ground and there is nothing that can be done to change that. All grounds are common.


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## Jcates455 (10 mo ago)

Bob Badger said:


> What is a 'Hogan ground'?
> 
> 
> 
> ...


I’ve upsized the equip ground before by direction of the local authority…and used it as the ground electrode conductor, but that was a few codes ago…
They are much more concerned these days about making sure any ground faults return to its source …(primary fault would return to breaker feeding it) and not be directed to the transformer . 
In days gone by it was enough that everything was grounded together weather through earth or building steel ect. 
They are using phrases like objectionable current and concerned about it being pulled through secondary windings….I would not attempt the old trick of upsizing the equip ground to serve as the ground electrode conductor without very specific written and provable direction from an engineer.


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## backstay (Feb 3, 2011)

Jcates455 said:


> I’ve upsized the equip ground before by direction of the local authority…and used it as the ground electrode conductor, but that was a few codes ago…
> They are much more concerned these days about making sure any ground faults return to its source …(primary fault would return to breaker feeding it) and not be directed to the transformer .
> In days gone by it was enough that everything was grounded together weather through earth or building steel ect.
> They are using phrases like objectionable current and concerned about it being pulled through secondary windings….I would not attempt the old trick of upsizing the equip ground to serve as the ground electrode conductor without very specific written and provable direction from an engineer.


2009


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