Building Fed from Customer Owned Transformer

[jim dungar]

To be the service disconnecting means, the 3-pole disconnecting device is required to have integral or immediately adjacent ocp. Is such ocp also required?

I see nothing in 230.205 that requires the disconnecting means to be 'integral or immediately adjacent'. In fact in 2008 the NEC specifically addresses the possibility that the service disconnect may not even be readily accessible.

230.208 says the overcurrent protection must be [somewhere] on the load side of the disconnect or integral to it.

 

[Smart $]

I see nothing in 230.205 that requires the disconnecting means to be 'integral or immediately adjacent'. In fact in 2008 the NEC specifically addresses the possibility that the service disconnect may not even be readily accessible.

I was referring to the location of the ocp to the disconnecting means, not the disconnect to the transformer.

230.208 says the overcurrent protection must be [somewhere] on the load side of the disconnect or integral to it.

You are correct since we are talking about MV primary side disconnect... forgot about that and was thinking 230.91 for 600V and less...

...but the point still remains that there has to be ocp between the disconnect and the transformer primary for the disconnect to be considered the service diconnecting means and the transformer a separately derived system.

 

[Smart $]

just recently installed a 5000kva transformer...

...

...

From my understanding of the code the bonding can only take place in 1 spot or the other and the neutral needing to be the same size as the phase conductors on a Y system.

Depends on the location of the service disconnecting means. It appears to be the MV switchgear. There may even be different concerns dependent on which code cycle the installation is under. The general rule is the grounded conductor can only be bonded to the GES in one spot... but there are exceptions.

Without more details, the only point of your comments to which I can reply without reservation is the neutral being required to be the same size as the phase conductors on a "wye" system. The neutral can be reduced in size if all 3 phase conductors are run and the maximum unbalanced neutral current of the loads is less than the ampacity of the phase conductors. Additionally, the neutral [aka grounded] conductor is only required to be sized at 100% of continuous and non-continuous loads for most installations. It is only when [one (unlikely, but I'm throughing it in here) or-] two of three phase conductors are run with a grounded neutral for a so-called single phase system.

Additionally, if you still feel the engineer is in error, it is better that he signs off on it installed per design. Now you can take your concerns to the AHJ and see what happens...

 

[Goroon]

Out Door Transformer ???

Out Door Transformer ???

.....you can only bond in one location, .....

250.24 Grounding Service-Supplied Alternating-Current Systems.

2) Outdoor Transformer. Where the transformer supplying the service is located outside the building, 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.

 

[Smart $]

.....you can only bond in one location, .....

250.24 Grounding Service-Supplied Alternating-Current Systems.

2) Outdoor Transformer. Where the transformer supplying the service is located outside the building, 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.

As stated before, you generally [with emphasis] can only bond the grounded conductor to the GES at one point.

The electrode referred to in your quoted code above is a separate part of the GES. That is to say there is no parallel current path between electrodes bonded at the service disconnecting means and the one (or more) electrodes connected to the grounded conductor [or its terminal] at the transformer. 250.4(B)(4) states, "The earth shall not be considered as an effective fault-current path".

Another generalization [again, with emphasis] is, all bonding on the line side of the service disconnecting means is to the grounded conductor, and to EGCs on the load-side. Keep in mind that if an EGC is run to the outside transformer, it can only be bonded to the transformer enclosure/frame and not the grounded conductor or its electrode conductor(s), and there can be no bonding jumper either.

 

[don_resqcapt19]

.....you can only bond in one location, .....

250.24 Grounding Service-Supplied Alternating-Current Systems.

2) Outdoor Transformer. Where the transformer supplying the service is located outside the building, 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.

The key here is the words "Service-Supplied" in the section you quoted. If this is a campus type distribution system the transformer is not "service-supplied" and is an SDS. In this case the conductors from the transformer are feeder conductors and not service conductors. The rules in 250.30 apply and in general they only permit a neutral to ground bond in one location. There is an exception that would permit a bond at both as long as there are no parallel paths other than the earth between the transformer and the building.

 

[Goroon]

Out of Context?? or ??

Out of Context?? or ??

Original Post
?If the customer owns a padmount transformer, 12.5kV-480/277v, how is the service to the building grounded? The transformer is outdoors and feeds a main distribution switchboard. Do we bond the neutral at the transformer only, or at the switchboard and transformer??

The difference of WHO owns the transformer is immaterial to the installation.


Nunu161
?..Then he had us bond the neutrals and the egcs in both the transformer and the MDP??

I Agree with the engineer, sorry.

Don, Illinois
? In this case the conductors from the transformer are feeder conductors and not service conductors.?

The difference of WHO owns the transformer is immaterial to the installation.
This makes the feeder conductors Service conductors.

"The earth shall not be considered as an effective fault-current path".
True, but that does not stop it from being one.

My Reference may be old but so am I.

Stallcup?s 2002 changes pg 2-61
Stallcup, Designing Electrical Systems 1990 Code pg 182
Mike Holt , Bonding Neutral Vol 5, #4 p119 [has diagram]

Americans Electricians Handbook, 13th edition pg 3.7

Service Equipment is the necessary equipment, ? that is intended to constitute the main control and means of cut off of supply.

Industrtial and Commercial Power Distribution, 1995 chapt. 10

?The System of Secondary Circuits beyond those transformers normally supplied by a utility and provided with independent overcurrent protection are considered Separately Derived.?

With 37 acres of property, 23, 2500KVA 12.5/480 transformers.. all above issues were raised by 3rd Party inspection firm in 2006. Taken to state level.

The determination was: Regardless of who owns the equipment, even if it is an SDS, an outdoor transformer supplying a structure SHALL follow 250.24(2).
EQUAL POTENTIAL GROUNDING --- REQUIRED FOR UTILITY--- FAULT VALUES

 

[RUWired]

Don, Illinois
? In this case the conductors from the transformer are feeder conductors and not service conductors.?

?The System of Secondary Circuits beyond those transformers normally supplied by a utility and provided with independent overcurrent protection are considered Separately Derived.?


The determination was: Regardless of who owns the equipment, even if it is an SDS, an outdoor transformer supplying a structure SHALL follow 250.24(2).
EQUAL POTENTIAL GROUNDING --- REQUIRED FOR UTILITY--- FAULT VALUES

Goroon, 250.24(A)2 is for service supplied systems. For the 3rd party team to come up with the blanket 250.24(A)2 statement is incorrect.

Article 250.30(A)3 supports the need for a GEC/GES at the outdoor transformer 99% of the time.
The option of installing the system bonding jumper at the MDP from a SDS can also be applied 99% of the time if thought out prior to installing the raceways, wiring and GEC/GES between the SDS and the MDP. Attached is a sketch showing the different methods and supporting articles.

Rick

View attachment 3268

 

[Smart $]

Original Post
?If the customer owns a padmount transformer, 12.5kV-480/277v, how is the service to the building grounded? The transformer is outdoors and feeds a main distribution switchboard. Do we bond the neutral at the transformer only, or at the switchboard and transformer??

The difference of WHO owns the transformer is immaterial to the installation.

I agree... but as I go through your comments my response is based on the NEC-qualifying service disconnecting means ("SDM") being on the secondary side of the transformer. The point of contention of others here have been mostly, though seemingly less than explicit, that the "SDM" is on the primary side of the transformer... in which case I will agree with them.

Nunu161
?..Then he had us bond the neutrals and the egcs in both the transformer and the MDP??

I Agree with the engineer, sorry.

The mere fact that EGC's are in the scope of the installation implies the transformer(s) are on the load side of the SDM. It is an NEC violation to bond grounded to grounding conductors in most situations... but not all?the main reason my reply to that post stated I had reservations without more detail.

Don, Illinois
? In this case the conductors from the transformer are feeder conductors and not service conductors.?

The difference of WHO owns the transformer is immaterial to the installation.
This makes the feeder conductors Service conductors.

Again, dependent on the location of the "SDM".

"The earth shall not be considered as an effective fault-current path".
True, but that does not stop it from being one.

Correct... but only in rare cases would the earth be an effective fault-current path. What is of greater importance is not creating a parallel neutral current path on grounding conductors.

My Reference may be old but so am I.

None of your references indicate you are old(er) :grin:

Stallcup?s 2002 changes pg 2-61
Stallcup, Designing Electrical Systems 1990 Code pg 182
Mike Holt , Bonding Neutral Vol 5, #4 p119 [has diagram]

I don't have these references so they mean nothing to me...

Americans Electricians Handbook, 13th edition pg 3.7

Service Equipment is the necessary equipment, ? that is intended to constitute the main control and means of cut off of supply.

I do possess this reference. The context of that definition includes the "SDM"... IMO.

Industrtial and Commercial Power Distribution, 1995 chapt. 10

?The System of Secondary Circuits beyond those transformers normally supplied by a utility and provided with independent overcurrent protection are considered Separately Derived.?

Once again, this is dependent on the location of the "SDM".

With 37 acres of property, 23, 2500KVA 12.5/480 transformers.. all above issues were raised by 3rd Party inspection firm in 2006. Taken to state level.

The determination was: Regardless of who owns the equipment, even if it is an SDS, an outdoor transformer supplying a structure SHALL follow 250.24(2).
EQUAL POTENTIAL GROUNDING --- REQUIRED FOR UTILITY--- FAULT VALUES

Cannot comment here without knowing the facts... especially, guess what??? ...the location of the "SDM".

 

[jbt260]

More Info

More Info

This is a college campus. The customer owns the MV distribution and transformers. Exhibit 230.31 identifies the wiring between the transformer and the building disconnect means as a feeder. That would make the transformer a SDS. Don's reference to 250.30(A)(1)(Ex 2) appears to be the solution. Thanks to all for your input.