bonding in the transformer and main panel

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Hello I'm new so I'll try my best here, today I was overhearing between a contractor and an inspector about bonding the transformer and the main service panel the transformer was feeding. The inspector said both had to be bonded and the contractor thought it was either or. I also thought it was either or but usually in the transformer. Can I get some insight on this?

I'm sure this post has been brought up here so I apologize if it's a repeat question.
 

iwire

Moderator
Staff member
Location
Massachusetts
One or the other is all that the NEC requires.

There is an exception that permits bonding at both locations under certain conditions but this is a choice not a requirement.
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
... a contractor and an inspector about bonding the transformer and the main service panel the transformer was feeding. The inspector said both had to be bonded and the contractor thought it was either or. ....

Take a look at 250.24.A.2 Grounding Service-Supplied Alternating-Current Systems, System Grounding Connection, Outdood Transformer

My translation is:
For a utility provided, grounded AC service, the transformer is utility provided (service point is on the xfm secondary), xfm located outdoors, grounding electrode connections are required at the xfm (or somewhere else outside) and at the panel.

Does this fit your installation?

Note: Per my translation, if the xfm belongs to the premisis owner, the service is on the xfm primary (or further upstream), the secondary is a feeder and not a service. The feeder from the xfm to the panel is five wire. Bond is in one place only - the panel, xfm, or anywhere in between. This is as it should be - service or feeder.

However, for a service, the code mandates a connection that will provide neutral current ove the EGC/ground mat/any metalic connection. :sick: I have no clues as to what risk/danger the code is attempting to mittigate. This is not one of my favorite code sections.

ice
 
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Sorry should of been more descriptive It is a transformwr feeder for a 208 3 phase panel both ends bonded, so my understanding with both sides bonded the neutral would be a current carry neutral through the pipe?

Again I thank you guys for your input
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
Hello I'm new so I'll try my best here, today I was overhearing between a contractor and an inspector about bonding the transformer and the main service panel the transformer was feeding. The inspector said both had to be bonded and the contractor thought it was either or. I also thought it was either or but usually in the transformer. Can I get some insight on this?

I'm sure this post has been brought up here so I apologize if it's a repeat question.

Sorry should of been more descriptive It is a transformwr feeder for a 208 3 phase panel both ends bonded, so my understanding with both sides bonded the neutral would be a current carry neutral through the pipe?

Again I thank you guys for your input

Yes, it is good to be descriptive

Is it a feeder or is it a service?

What code (date) was it built to?

Is the transformer outdoors?

ice
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
It's a service, indoors, 2011 nec
sorry still a bit confused as to what you have... The Code treats service transformers differently from customer SDS units.
In a somewhat standard scenario with a utility transformer supply a service, with the transformer outside, 250.24(A)(2) requires a ground electrode connection at the transformer and the utility will bond XO to the case. The grounded conductor will be bonded to the service equipment per 250.24(B) and there will be no equipment bonding jumper between the utility transformer and the service.

On a customer SDS, 250.30 becomes the applicable section and the basic rule is to install a system bonding jumper at the source (transformer) OR at the first disconnecting means but not both.
(In the event you bond at both points, you could introduce unwanted current flow thru an unwanted path such as the metal conduit connecting the transformer and the panel.)
There are exceptions in 250.30
An equipment bonding jumper is required between the transformer and panel to assure bonding of the two.

Quite bhonestly I do not know how one handles the situation where the utility transformer is inside the premises. Perhaps someone will address that.
 
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