# Transformer Bonding



## shunt trip (Jan 15, 2009)

When bonding a transformer that supplies a secondary panel, is the bond to be located in the panel or at the transformer?


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## mollydog (Jan 9, 2009)

Panel


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## mollydog (Jan 9, 2009)

years ago we did both, i'd have to see if that changed, not really sure. someone will look it up:blink:


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## shunt trip (Jan 15, 2009)

*bonding*

Thanks, Makes sense, since its easier to see... I have seen inspectors require it both ways though.


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## brian john (Mar 11, 2007)

Either is acceptable (panel or transformer NEVER BOTH) it is better (IMO) to bond in the transformer, most transformers come with the XO bonded already, if you re-bond in the panel and do not remove the factory bond you set up parallel paths for current with the neutral conductor and EGC (equipment grounding conductor).


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## mollydog (Jan 9, 2009)

250.104 is seperatly derived systems bonding. But its only saying supply side of service. I dont have time to look it up right now but I am gonna guess and say that its to be done in the MDP, because the frame of the transformer is grounded by a 3 or 4 wire feeder. I would say the panel & keep em seperate. Thats a guess and dont go by me tonight, I dont feel like looking at the good book this evening. Its colder than a witches tit here and me bones are KILLING me....... Its somewhere in 250 tho.

Code question of the day: Based on NEC, What (if any) is the ampacity deduction amount for 5 2 wire romex cables through the same bored wood member where it has been filled with fire stopping material ?

Just give percentage.......:thumbup:


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## mollydog (Jan 9, 2009)

If you are answering tonight it's: 50 % 310.15(B)(2)(a) 2008 NEC page: 70-146 Table

I have to figure out how to choose my font, any help ?


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## manchestersparky (Mar 25, 2007)

It could be in either one - your choice - But only in 1 or the other not both
I like to see it in the transformer as they are set up that way from the factory.
The jurisdiction I work for we want it in the transformer not the panel , just our way of "keeping it all the same"


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## RePhase277 (Feb 5, 2008)

Either is acceptable, but remember that your grounding electrode must originate from wherever the bond happens. So, if you bond in the transformer, then you take the GEC from there. If you bond in the panel, your GEC comes from there.


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## Megawatts (Jan 12, 2009)

*X-former Bonding*

Always bond it in the transformer. Run the bond to the building steel. make sure you scrape the paint off the steel for a good electrical connection.


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## brian john (Mar 11, 2007)

Megawatts said:


> Always bond it in the transformer.


While I feel this is better it is not MANDATORY as noted above.



> Run the bond to the building steel.


Building steel is not always available...So following the NEC is the way to gooooooo.


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## Megawatts (Jan 12, 2009)

brian john said:


> Building steel is not always available...So following the NEC is the way to gooooooo.


I'm assumming it's a commerical building. But if no steel is available you may use the H2O line, sprinkler main, and whatever the code states you can use. 

And on some x-former ( Sq D has it) in the instruction they send with it. It states to do the bonding in the x-former and not at the MDP, or this voids the warranty. But again, the inspector (who think they are GODS and know all) can do whatever they like. Prime example. I was told durnig the inspection to put the ground bonding screw in the panel. I said no because you are Par. the neu/grounds. He goes on saying I want it bonded there also. So I did to shut him up. After it was approved, I removed the screw.


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## brian john (Mar 11, 2007)

Megawatts said:


> . But again, the inspector (who think they are GODS and know all) can do whatever they like. Prime example.


Not all inspectors think they are god, they are people like you and me, some good some bad.



> I was told durnig the inspection to put the ground bonding screw in the panel. I said no because you are Par. the neu/grounds. He goes on saying I want it bonded there also. So I did to shut him up. After it was approved, I removed the screw.


Now I am assuming you mean a SUB-panel? What I would do in that circumstance is attempt to educate the inspector (depending on his personality).


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## heel600 (Oct 31, 2007)

Brian John said:
Now I am assuming you mean a SUB-panel? What I would do in that circumstance is attempt to educate the inspector (depending on his personality).


WOW! You obviously never worked in New Jersey!

Most inspectors here are 'my way or the highway'.


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## Pierre Belarge (Feb 3, 2007)

Has anyone read 250.30?

There is a lot of information in this section number. 
In regards to the OP's question, see 250.30(A)(1) & 250.30(A)(7).


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## sherman (Sep 29, 2008)

Pierre Belarge said:


> Has anyone read 250.30?
> 
> There is a lot of information in this section number.
> In regards to the OP's question, see 250.30(A)(1) & 250.30(A)(7).


Hi Pierre good to see you. Your favorite cow Sherman. Oh yeah I vote either or but not both.


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