# Bonding neutral



## dronai (Apr 11, 2011)

I have an equipment room with a 480V service to a large disco. From there it goes thru a gutter to several smaller discos feeding motors, and one small 1.5 kw transformer to a small panel, for 120/208V loads (lights, service receptacles, etc.) 

The small panel after the 1.5 kw transformer, requires neutral to ground bonding yes ?


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## Hippie (May 12, 2011)

dronai said:


> I have an equipment room with a 480V service to a large disco. From there it goes thru a gutter to several smaller discos feeding motors, and one small 1.5 kw transformer to a small panel, for 120/208V loads (lights, service receptacles, etc.)
> 
> The small panel after the 1.5 kw transformer, requires neutral to ground bonding yes ?


Yeah, separately derived system


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## jmerf76 (Nov 15, 2010)

It is true that it is a seperately derived system. However, you'd have to open up the transformer because the ground and the neutral could already be bonded in there.


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## dronai (Apr 11, 2011)

jmerf76 said:


> It is true that it is a seperately derived system. However, you'd have to open up the transformer because the ground and the neutral could already be bonded in there.


This transformer is small, the size of a buck boost. Yes I think it's is bonded in there as well. But I still think you would also need to bond at the panel.


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## KGN742003 (Apr 23, 2012)

dronai said:


> This transformer is small, the size of a buck boost. Yes I think it's is bonded in there as well. But I still think you would also need to bond at the panel.


No, you need to check it. If it is bonded in the transformer and you bond it in the panel, you create a parallel neutral.


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## dronai (Apr 11, 2011)

KGN742003 said:


> No, you need to check it. If it is bonded in the transformer and you bond it in the panel, you create a parallel neutral.




Found it NEC 250.30


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## erics37 (May 7, 2009)

Neutral gets bonded anywhere between derived system and first disconnect, but only gets bonded once. Needs an electrode connection to underground water pipe or structural steel electrodes too, if available, or another grounding electrode if not. GEC and neutral bond need to be in the same place.


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## dronai (Apr 11, 2011)

erics37 said:


> Neutral gets bonded anywhere between derived system and first disconnect, but only gets bonded once. Needs an electrode connection to underground water pipe or structural steel electrodes too, if available, or another grounding electrode if not. GEC and neutral bond need to be in the same place.



If the service is 480V with only a disconnect, I don't know if there's a neutral in there. If I shut it off to look, the Generator will turn on ! 

My transformer is a separately derived system, and as someone said I have to check if that small transformer is already bonded to determine if my panel needs bonding. So when you say once, I am not following you.

If my service before the transformer was bonded, I still might a second bonding, if the Transformer is not right ?


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## KGN742003 (Apr 23, 2012)

I think part of the problem you're having is there are a few different ways to do things and people are trying to include other optional ways to do it. Let's start from scratch.

You have 480 coming into the transformer, the phases will land H1, H2 and H3 respectively. Out of the transformer, from the X1, X2 and X3 lugs you have your phases going to the 208/120 panel. Now from XO run a neutral and an EGC to your panel landing on the separated neutral and ground busses respectively. From XO you also need to run a GEC to building steel or a ground rod and also a bonding jumper to the transformer case.

There are other ways to do this but from what you have given us this is the simplest imo.


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## erics37 (May 7, 2009)

dronai said:


> If my service before the transformer was bonded, I still might a second bonding, if the Transformer is not right ?


Think of it this way. Treat your separately derived system like you would a service. It kind of is a new little mini-service anyway. So your derived neutral at the secondary of the transformer needs to be grounded just like the neutral on a regular service.

Check 250.30


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## Mshea (Jan 17, 2011)

You said it looked like a buck boost connection? Are you sure it isn't a b/b transformer? we often connect these as Autotransformers and they do NOT get regrounded as the primary and secondary windings are electricaly connected. these are not Isolation transformer connections or inother words the neutral runs continuously through the transformer so regrounding it is a violation. 

Ensure it is not an autotransformer and proceed as advised previously.


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## dronai (Apr 11, 2011)

Mshea said:


> You said it looked like a buck boost connection? Are you sure it isn't a b/b transformer? we often connect these as Autotransformers and they do NOT get regrounded as the primary and secondary windings are electricaly connected. these are not Isolation transformer connections or inother words the neutral runs continuously through the transformer so regrounding it is a violation.
> 
> Ensure it is not an autotransformer and proceed as advised previously.



I just mean for size comparison. It's for a few service plugs, and some lights for the equipment only. I read the schematic for the transformer, and it's 480V primary/120/208V secondary with two windings not one. Thanks for the other information though.


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## Jlarson (Jun 28, 2009)

I didn't read the whole thread but a single phase step down with a 120/208 secondary? Sure you don't mean 120/240 dronai?


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## dronai (Apr 11, 2011)




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## McClary’s Electrical (Feb 21, 2009)

So j's right


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## dronai (Apr 11, 2011)

mcclary's electrical said:


> So j's right


Yep ! :jester: It's also 3KW Also from that drawing it looks like neutral is not bonded. Neutral is from x2 I think.


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## erics37 (May 7, 2009)

dronai said:


> Yep ! :jester: It's also 3KW Also from that drawing it looks like neutral is not bonded. Neutral is from x2 I think.


You are the one that decides if the neutral gets bonded or not, the diagram won't necessarily reflect that. In this case it needs to be bonded as a separately derived system according to NEC. The transformer manufacturer doesn't care what you do with it.


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## glen1971 (Oct 10, 2012)

dronai said:


> Yep ! :jester: It's also 3KW Also from that drawing it looks like neutral is not bonded. Neutral is from x2 I think.


If you are bonding the neutral, it would be from X2...


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## RGH (Sep 12, 2011)

KGN742003 said:


> I think part of the problem you're having is there are a few different ways to do things and people are trying to include other optional ways to do it. Let's start from scratch.
> 
> You have 480 coming into the transformer, the phases will land H1, H2 and H3 respectively. Out of the transformer, from the X1, X2 and X3 lugs you have your phases going to the 208/120 panel. Now from XO run a neutral and an EGC to your panel landing on the separated neutral and ground busses respectively. From XO you also need to run a GEC to building steel or a ground rod and also a bonding jumper to the transformer case.
> 
> There are other ways to do this but from what you have given us this is the simplest imo.


If I follow you correctly this is with NO nuetral-to-case connection in transformer correct?


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## Lighting Retro (Aug 1, 2009)

That's a lot of discos. I didn't think they were in business any more.


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