Bonding in transformer and panel?

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Ibew journeyman

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Philadelphia pa
I have a question about if you bond in the transformer, then why would you also bond in the panel? I was always told and understood that you bond once at the transformer where the service starts and nowhere else. If you bond at transformer and panel then you are paralleling the neutral and ground. It's like running two neutrals down to panel from transformer. Any thoughts? Thanks
 
Since the POCO service transformer secondary neutral is bonded to POCO ground, you might think that you should not have another bond at your service disconnect.
But POCO does not carry a grounding wire to your panel, just the grounded neutral. If you do not bond your building EGCs to the POCO neutral there will not be an effective fault clearing path back to the transformer if there is a hot to ground fault on the premises.
The earth electrodes do not form a low enough impedance connection to clear a fault.
 
Ibew journeyman said:
I have a question about if you bond in the transformer, then why would you also bond in the panel? I was always told and understood that you bond once at the transformer where the service starts and nowhere else. If you bond at transformer and panel then you are paralleling the neutral and ground. It's like running two neutrals down to panel from transformer. Any thoughts? Thanks
Click to expand...

This is a service and not just an SDS correct?

Welcome to the Forum. :)
 
infinity said:
This is a service and not just an SDS correct?

Welcome to the Forum. :)
Click to expand...
Yes some clarification is needed on that.

If he is talking about services - unless your service is medium voltage level the transformer is not subject to NEC it is controlled by the POCO. POCO's typically use a MGN type of system -there is a grounding electrode at nearly every structure they have on the system and it connects to the MGN at all of them.
 
Sorry guys, hears a little more info.. transformer is in a building being fed off of MDP. 480 primary/ 208 secondary to feed panel. I bonded in the transformer and was told that I needed to bond also in the panel. I have never bonded twice in two separate locations. My argument is that the panel would be considered like a sub panel and that you don't bond in there because I already established a bond at the transformer and bonding in the panel as well would parallel the neutral and ground wire. It would be like running two neutrals down to the panel with no effective ground. Am I correct?
Thanks
 
Ibew journeyman said:
Sorry guys, hears a little more info.. transformer is in a building being fed off of MDP. 480 primary/ 208 secondary to feed panel. I bonded in the transformer and was told that I needed to bond also in the panel. I have never bonded twice in two separate locations. My argument is that the panel would be considered like a sub panel and that you don't bond in there because I already established a bond at the transformer and bonding in the panel as well would parallel the neutral and ground wire. It would be like running two neutrals down to the panel with no effective ground. Am I correct?
Thanks
Click to expand...

You are correct. You bond in only one location, can be either one, your choice.
 
Ibew journeyman said:
Transformer is 75kva so that exception would not apply in this instance right?
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The size of the transformer doesnt matter. Generally, the only time one might use this exception is between detached structures. An application where this would be useful is where you have a long wire run where you want to step up voltage to say 480V or 600V. You would ground one of the secondary conductors and bond at both ends, thus you would only need 2 conductors for the wire run.
 
electrofelon said:
The size of the transformer doesnt matter. Generally, the only time one might use this exception is between detached structures. An application where this would be useful is where you have a long wire run where you want to step up voltage to say 480V or 600V. You would ground one of the secondary conductors and bond at both ends, thus you would only need 2 conductors for the wire run.
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I don't have the code in front of me but when I looked it up it had mentioned about transformer being less then 1000va for exception to be used
 
Ibew journeyman said:
I don't have the code in front of me but when I looked it up it had mentioned about transformer being less then 1000va for exception to be used
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Here's the exception that roger mentioned:

250.30(A)(1)Exception No. 2: If a building or structure is supplied by a
feeder from an outdoor transformer, a system bonding
jumper at both the source and the first disconnecting means
shall be permitted if doing so does not establish a parallel
path for the grounded conductor. If a grounded conductor
is used in this manner, it shall not be smaller than the size
specified for the system bonding jumper but shall not be
required to be larger than the ungrounded conductor(s).
For the purposes of this exception, connection through the
earth shall not be considered as providing a parallel path.
Click to expand...
 
Ibew journeyman said:
Thanks.. I was looking at wrong one. It says though it's permitted IF it does not establish a parallel path. How do you determine that? Do u have to test it with a multi meter? Thanks
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Are there any present or possible metallic paths between the panel and transformer? If there are or will be you will have multiple neutral current paths.

Roger
 
roger said:
Are there any present or possible metallic paths between the panel and transformer? If there are or will be you will have multiple neutral current paths.

Roger
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Just the 3 inch emt that changes over to Greenfield to carry down the conductors. That would be considered as a path for multiple neutral paths right?. I don't want to drag this out it's just I've never bonded twice, I see no reason ever to do it
 
Ibew journeyman said:
Just the 3 inch emt that changes over to Greenfield to carry down the conductors. That would be considered as a path for multiple neutral paths right?.
Click to expand...
Yes, this would be a parallel path and you are correct that it would be a violation.

Roger
 
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