"Bonding" in 2500kva Padmount Transf.

[ld]

We have a 2500Kva Padmount Transformer. Primary 12,470, with a 480/277v secondary. The "service entrance" conductors consisted of 8 paralleled 500MCM copper conductors with a full size neutral going to a 3000Amp Main Breaker, with a "number" of "fusible" branch feeders. We established the "bond" in the switchboard at the neutral bar, by running a seperate 3/0 copper to EACH of the following points. 1. Incoming water pipe. 2. To the Building Steel. 3. Rebar steel in the footer of the building.
My question is this: In the 2500kva transformer "hanging" from the Neutral Bushing is a "strap" with a hole in the end of the strap. Since we are "bonded" in the SWBD at the Neutral don't we let this strap in the transformer stay in the "disconnected" position?

 

[don_resqcapt19]

Re: "Bonding" in 2500kva Padmount Transf.

This is a problem area of the code. The transformer is an SDS and a structure. What type of racecway was used between the transformer and the switchgear?
Don

 

[hassaf]

Re: "Bonding" in 2500kva Padmount Transf.

The point of connection for the separately derived system neutral-to-ground bond should be made at the same location where the separately derived grounding electrode conductor terminate, NEC 250.(A)(2)(a), OR it can be made at the source or the first disconnecting mean, NEC 250.30 (A)(1).

So it seems that you're OK and no need the bond at the transformer and the strap shall remain disconnected.

 

[explorer]

Re: "Bonding" in 2500kva Padmount Transf.

You are still required to install a grounding electrode and bond it to the grounded service conductor at the XO terminal in the pad mount transformer outside of the building per Art. 250.24 (A) (2).

State Electrical Inspector
John Cosmo

 

[bphgravity]

Re: "Bonding" in 2500kva Padmount Transf.

Originally posted by explorer:
You are still required to install a grounding electrode and bond it to the grounded service conductor at the XO terminal in the pad mount transformer outside of the building per Art. 250.24 (A) (2).

State Electrical Inspector
John Cosmo

You would then violate 250.30(A)(1) considering the bonding has already been performed at the switchboard.

 

[don_resqcapt19]

Re: "Bonding" in 2500kva Padmount Transf.

John,
This is not a service and 250.24(A)(2) does not apply.

The best solution to this type of installation would be to use nonmetallic raceways between the transformer and the bulding. Then grounding electrode systems would be installed and bonded at both locations.

Don

 

[explorer]

Re: "Bonding" in 2500kva Padmount Transf.

Don,
I thought that is what I said. I agree you would be best to use PVC as your conduit between your transformer and your main panel. Connect the grounding electrode conductor for the building at the service disconnect. I was just stating that you must also, install a grounding electrode at the transformer outside and connect it to the grounded service conductor in the transformer or elsewhere outside the building per Article 250.24 (A)(2). Please tell me if I'm missing something here. I have been wrong before.
Thanks

John Cosmo
State Electrical Inspector

 

[don_resqcapt19]

Re: "Bonding" in 2500kva Padmount Transf.

John,

I was just stating that you must also, install a grounding electrode at the transformer outside and connect it to the grounded service conductor in the transformer or elsewhere outside the building per Article 250.24 (A)(2).

In this case the transformer and feeder is not part of a service. The equipment is on the load side of the service point. In my opinion, 250.24(A)(2) only applies to transformers on the line side of the service point. Since this customer owned equipment, it is a SDS and the transformer is also a structure. Compliance with 250.30 and 250.32 is required, but compliance with 250.24 is not required and in some cases would be prohibited by 250.30.
This is the exact reason that my first post in this thread said that this is a problem area in the code. The required installation methods change based on who owns the equipment. How do the electrons know who owns the equipment?? In my opinion, the code needs to be changed so the intallation is the same for both cases.
Don

[ June 17, 2004, 09:21 AM: Message edited by: don_resqcapt19 ]

 

[hornetd]

Re: "Bonding" in 2500kva Padmount Transf.

Don
I cannot argue with your interpretation of the code as written since I have no experience with installations that are that large. But I have to agree very strongly that the electrons will behave the same in both cases. If the code really does require two separate approaches when there is no physical difference between the two systems that makes no sense at all. Perhaps part of the difficulty is caused by what definition of building we apply. I find the present NEC definition of building to be wholly inadequate because to be a building a structure should be accessible for interior access by people. A transformer casing just doesn't seem like a building to me.
--
Tom Horne

[ June 17, 2004, 09:20 AM: Message edited by: hornetd ]

 

[don_resqcapt19]

Re: "Bonding" in 2500kva Padmount Transf.

Tom,
Even if we don't call the transformer a structure, it is still an SDS and 250.30 only permits bonding of the grounded conductor at one location if a possible parallel path exists. The code has a number of problems when dealing with large "campus" type distribution systems. For all practical purposes these are the same as services, but because they are on the load side of the service point, the rules treat them differently than services. The code needs to have an Article that deals with this type of distribution system.
Don

 

[hornetd]

Re: "Bonding" in 2500kva Padmount Transf.

Originally posted by don_resqcapt19:
Tom,
Even if we don't call the transformer a structure, it is still an SDS and 250.30 only permits bonding of the grounded conductor at one location if a possible parallel path exists. The code has a number of problems when dealing with large "campus" type distribution systems. For all practical purposes these are the same as services, but because they are on the load side of the service point, the rules treat them differently than services. The code needs to have an Article that deals with this type of distribution system.
Don

What I want to avoid calling it is a building. I have no problem treating it as an SDS although I think that it would be a lot simpler if we treated it as a service. As you already pointed out the two installations only differ by there legal owners rather than how they behave electrically.
--
Tom Horne

 

[don_resqcapt19]

Re: "Bonding" in 2500kva Padmount Transf.

Tom,
It is my opinion that a transformer is clearly a structure per Article 100. However it doesn't make any difference as the bonding rules for 2 or more structures from a common service and for a SDS are almost identical.

Structure. That which is built or constructed.

Don

 

[explorer]

Re: "Bonding" in 2500kva Padmount Transf.

Don,

His original post stated "service entrance conductors ". That leads me to believe this is a service and not a feeder. He didn't mention in his post about the transformer being customer owned

I would agree with you if this is a feeder that the code has different rules than services. I also hope you would agree that is can be difficult sometimes to properly assess the actual code requirements over the Internet.

John Cosmo
State Electrical Inspector

 

[don_resqcapt19]

Re: "Bonding" in 2500kva Padmount Transf.

John,
I went back and re-read the original post and he did say service entrance conductors. I guess, I'm just basing my comments on how it works around here. If the transformer is utility owned, the contractor does not make any terminations at the transformer. The original post and my experiences made me assume that this was a customer owned transformer. Sorry for the confusion.
Don