250.24(A)(2) Outdoor transformers

[Zane Bailey]

I have a 12KV to 480v step down outdoor transformer that feeds a building with a 480v service distribution panel board. The transformer has a factory installed bonding jumper to the case. The distribution panel board also has a removable bonding jumper connecting the grounded conductors and the grounding conductor bus bars. The Distribution panel board is the building service disconnects means. Is it permissible/required by code as per 250.24(A) (2) NEC for outdoor transformers, to leave the two bonding jumpers permanently installed?
SENOIR ELECTRICAL INSPECTOR: ZANE BAILEY.

 

[JohnJ0906]

Who owns the XFMR? Customer or POCO?

 

[RUWired]

Even if the customer owned the transformer and it was considerd a feeder, article 250.30(A)(1) exception #2 allows multiple system bonding jumpers.

Exception No. 2: A system bonding jumper at both the source and the first disconnecting means shall be permitted where doing so does not establish a parallel path for the grounded conductor. Where 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.

 

[Zane Bailey]

Outdoor transformers 250.30 (A)(1) exception 2

Outdoor transformers 250.30 (A)(1) exception 2

thanks for your reply:
I work for UCSD and we distribute 12k for the campus. The transformer therefore is ours. Would having the systems bonding jumper at both the outdoor transformer and the indoor disconnect means cause a parallel path if you had both a grounded conductor and a equipment grounding conductor run between both the transformer and the building disconnect means? If so it appears that 250.24 (A)(2) allows both connections.

 

[RUWired]

thanks for your reply:
. The transformer therefore is ours. .

When the owner supplies the tranformer down stream of the service point, the secondary conductors are feeders and the system bonding jumper would be installed as per 250.30. Its hard to break the habit of installing the system bonding jumper at the first OCPD to the building when its a pad mount transformer and its owner supplied.Most inspectors would have a hard time agreeing with that. Exception # 2 allows to put the jumper at both location providing you don't create parallel paths. If you install the grounded and the grounding conductor between the two, you will create the parallel path.So if you omit the grounding conductor and bond at both the transformer and the first OCPD and apply exception # 2, it still meets code and satisfies the inspector.

 

[Zane Bailey]

systems bonding jumpers in both locations.

systems bonding jumpers in both locations.

RUwired,
Do you know what conditions my result from have the systems bonding jumpers installed at both the outdoor transformer and the indoor service disconnect means and have the grounding conductors and the grounded conductors installed between both locations?

 

[tom baker]

Creating a parallel path for the neutral current can result in fires, improper operation of electronic equipment and a shock hazard. You end up with neutral current on all metal parts.
Its the reason why mobile homes use a 4 wire feeder.

 

[tryinghard]

thanks for your reply:
I work for UCSD and we distribute 12k for the campus. The transformer therefore is ours. Would having the systems bonding jumper at both the outdoor transformer and the indoor disconnect means cause a parallel path if you had both a grounded conductor and a equipment grounding conductor run between both the transformer and the building disconnect means? If so it appears that 250.24 (A)(2) allows both connections.

Zane,

I think you need to view your high voltage as private service and if you have an EE on contract qualify this through them along with your concerns.

The purpose of controlling the neutral returning current path is to contain this current on the grounded conductor only. In distribution one can visualize returning current on all conductive items when inappropriate bonding has taken place (parallel path), because current will flow on all paths that route to source. But with normal utility service these transformers are bonded and the load service entrance/underground lateral does not include a EGC only a common grounded conductor therefore bonding takes place again at the service disconnect - it is normal to have a parallel path with earth in these situations.

Again I think you should view your high voltage the same as service even though it is private. In my opinion the 12KV transformer should be bonded because the serving utilities do this. At this point I differ from traditional services because I think you should keep your secondary grounded conductor (neutral) separate from the case/frame at the service disconnect, eliminating this parallel, and to do this you need to install an EGC additional with the secondary service. By doing this all neutral current will travel isolated on the neutrals directly to source XO and all fault current will travel directly to source XO as well through equipment grounding, these will only common at the source transformer. Again normally the utility serving transformer is bonded and bonding happens again at the service disconnect, this really is parallel.

I suggest this as a minimum; I will also say utilities prefer plastic non-conductive ducts to metallic. If you apply NEC literally to your application you will not bond your neutral past the service disconnect 250.24(5) & 250.142(B) unless it complies with 250.30(A)(1) or 250.32. You should adopt your own standards and I recommend involving an EE that is familiar with serving high/medium voltages.

 

[Smart $]

RUwired,
Do you know what conditions my result from have the systems bonding jumpers installed at both the outdoor transformer and the indoor service disconnect means and have the grounding conductors and the grounded conductors installed between both locations?

It looks like no one wants to answer your question explicitly... but I will.

With both grounded and grounding conductors run between the disconnecting means and the transformer, you HAVE parallel paths. You either have to remove the jumper or determinate and isolate the EGC.

 

[Smart $]

...it appears that 250.24 (A)(2) allows both connections.

I believe you are reading more into 250.24(A)(2) than what it says... "at least one additional grounding connection shall be made from the grounded service conductor to a grounding electrode, either at the transformer or elsewhere outside the building."

This means you are required to connect the grounded service conductor (at the transformer) to one or more grounding electrodes outside the building. The specified grounding electrodes must be independent of the GES because utilizing any GES electrode creates a parallel path for the grounded service conductor. Typical (IMO) installations do not have an EGC run back to the xfmr. Instead, the grounded service conductor is bonded to the xfmr case (the jumper you refer to) and also to a ground rod or other permissable electrode.

 

[tryinghard]

It looks like no one wants to answer your question explicitly... but I will.

With both grounded and grounding conductors run between the disconnecting means and the transformer, you HAVE parallel paths. You either have to remove the jumper or determinate and isolate the EGC.

Smart $,

Zane's application duplicates a normal utility except they have included the EGC. If Zane really duplicates a utility he would bond the transformer and the service disconnect.

Two questions I have are:

• 
  [1]what makes Zane's application different from a utility?
  [2]should Zane mimic a utility application, if not why?

 

[Smart $]

Smart $,

Zane's application duplicates a normal utility except they have included the EGC. If Zane really duplicates a utility he would bond the transformer and the service disconnect.

Two questions I have are:

• 
  [1]what makes Zane's application different from a utility?
  [2]should Zane mimic a utility application, if not why?

This is correct if you mean bond each to Earth ground (i.e. not to each other)... However, the bonding to Earth ground is done through two seperate grounding electrode systems.

Yes... simply to be code compliant.

On a personal note, I don't believe parallel grounded conductor paths on the line side of the service disconnecting means presents the hazard that is implied by the code. Ocassionally parallel paths are created inadvertently, such as running GRC between transformer and disconnect and there is no major issue made of it. This particular parallel path actually creates a lower impedance and redundant path for clearing ground faults. The major concern is potential currents and the voltage personel could be subjected to... but the way a system is designed to code now has those very same issues. I say if the parallel path is not exposed, such as would be for a "main feeder lateral" (Zane's application?), by all means let it be and encourage it. But then again, I'm all for multi-point grounding (super-equipotential bonding).

 

[tryinghard]

This is correct if you mean bond each to Earth ground (i.e. not to each other)... However, the bonding to Earth ground is done through two seperate grounding electrode systems...

Yes I do mean they each ground to earth with individual electrodes and they each have a bonding jumper, but it really is one grounding system because the grounded conductor ties them together, Zane's EGC is redundant.

If Zane matches a utility installation at minimum there will be a parallel path with earth and if the conduit system is metallic it will be a parallel path. I do suggest matching a utility application at the transformer and not bonding the service disconnect, this will require the EGC to common only at the transformer.

 

[Smart $]

but it really is one grounding system

I agree.

...because the grounded conductor ties them together,...

Yes, it does. It simply a matter of codemakers wanting all current on the grounding conductor.

...Zane's EGC is redundant.

Yes, it is... but also a code violation if bonded to the grounded conductor directly or indirectly (as through a case bonding strap) at the transformer. When bonded it is no longer an EGC but rather a parallel grounded service conductor!

 

[tryinghard]

I agree.


Yes, it does. It simply a matter of codemakers wanting all current on the grounding conductor...

My point is if Zane determines their high voltage distribution to be service rather than a feeder grounding the transformer and service disconnect would be normal and NEC is not the regulation to control this.

 

[Smart $]

My point is if Zane determines their high voltage distribution to be service rather than a feeder grounding the transformer and service disconnect would be normal and NEC is not the regulation to control this.

I believe you will find the code compliant method (NEC/NESC) the same either way.

 

[tryinghard]

I believe you will find the code compliant method (NEC/NESC) the same either way.

Only if Zane installs it the way I'm suggesting and treat it as service but not bond the service disconnect in other words the service entrance will be 277/480 and include A, B, C, N, & GR (5W), and the serving transformer will ground & bond only.

If Zane determines it as service and remians traditional to match utilities then both will ground and a parallel will exsit at minimum with earth. This is not NEC compliant but it is actually how a utility would install it the premises would be handled by an electrician and it would be inspected and accepted.

 

[Smart $]

Only if Zane installs it the way I'm suggesting and treat it as service but not bond the service disconnect in other words the service entrance will be 277/480 and include A, B, C, N, & GR (5W), and the serving transformer will ground & bond only.

If Zane determines it as service and remians traditional to match utilities then both will ground and a parallel will exsit at minimum with earth. This is not NEC compliant but it is actually how a utility would install it the premises would be handled by an electrician and it would be inspected and accepted.

I only see it installed one way, regardless of service point...

 

[tryinghard]

This will eliminate the parallel

 

[Smart $]

This will eliminate the parallel

...but that would only be compliant under NEC if the transformer is a separately derived system on the load side of a service disconnect (i.e. not part of a service). 250.24(B) requires a main bonding jumper within the service disconnect enclosure.