Help with grounding a typical 15 kV primary utility pad mounted transformer

[uberflex]

I've been pouring over the NEC requirements for grounding and bonding related to service transformers but have not found what I'm looking for. I'm trying to develop a good "typical" detail drawing showing the grounding and bonding requirements for a typical utility transformer which serves, say, a commercial building like a school or hospital. 15kV primary and 480/277 or 208/120 secondary. I realize each utility company has it's own standards for transformer grounding but I'm wondering if there is an overall agreed-to design which is accepted by most AHJ and utilites.

Attached is a drawing I've been working on. I'm sure there are some "issues" with the design so I'm looking for feedback from you folks on how it should look. The key here is to reference code(s) in order to cover ourselves but I'm not finding any good code references related to primary/secondary grounding and bonding.

Help!!

 

[roger]

The grounding looks fine per the drawing, what I question is why you are transitioning to metal into the transformer, you should use PVC with bell ends or something similar.

Roger

 

[petersonra]

4/0 does not need a "#" in front of it.

 

[uberflex]

The grounding looks fine per the drawing, what I question is why you are transitioning to metal into the transformer, you should use PVC with bell ends or something similar.

Roger

Ask my boss. I honestly do not know. His request. I'll ask when he's back in the office.

 

[uberflex]

Let me ask you guys this: Is it acceptable to bond the primary and secondary steel conduits together like I show?

If you had to reference codes, what would you put on the drawing? For example, "Provide buried 4/0 ground ring per XXX code..."

 

[roger]

Ask my boss. I honestly do not know. His request. I'll ask when he's back in the office.

The problem with bringing metal into a padmount transformer is, you're just introducing another possible catastrophic event maker.

Roger

 

[roger]

Let me ask you guys this: Is it acceptable to bond the primary and secondary steel conduits together like I show?

Yes it is and it is done through the transformer enclosure even if you don't do it directly.

If you had to reference codes, what would you put on the drawing? For example, "Provide buried 4/0 ground ring per XXX code..."

Actually you would only need a #6 per 250.66(A)

Roger

 

[dicklaxt]

I've done many of these design installations over the years,,,,you need to make some corrections/additions. The ground ring probably only needs 2 rods.The taps from the ring via the exothermic welds should go to opposite corners of the transformer base frame,field to verify location of ground pads.A Bonding jumper can then be installed from the pad to the ground bus with in the chamber to tie off the grounding conduit bushings to. You could also run another tap to the grounding bus. Do you have a vendor cut sheet showing the HV and LV connections on the same side(it's not the norm for a power transformer in an industrial application. The RSG conduit riser should also have a RSG sweep with the transition to PVC at the bottom of the sweep. so that the pulling rope will not cut thru the bend.The Cabinet you show is usually called a High Voltage Chamber and is on the opposite end from the Low Voltage Bushings.The use of stress cones also needs to be addressed with detailed installation instructions being careful to allow for clearances and provisions for cleaning of the skirts. Shield drain wires of the cones need to be tied off to the ground bus as well.

If you need a fence enclosure there are clearances from transformer surfaces to the fence that need to be maintained and bonding of personnel and equipment gates back to the ground ring.You also need to provide a conduit for protective relaying wiring another for 120 Vac requirements,heaters,cooling fans etc.If this is a Y-wound secondary you may have a grounding resister required.

The underground installation is not usually routed thru the transformer foundation but into the bottom of the chambers,make sure the chambers have rodent and snake protection deterant and not just pure open bottom enclosures.


The old man

 

[Hv&Lv]

I've been pouring over the NEC requirements for grounding and bonding related to service transformers but have not found what I'm looking for. I'm trying to develop a good "typical" detail drawing showing the grounding and bonding requirements for a typical utility transformer which serves, say, a commercial building like a school or hospital. 15kV primary and 480/277 or 208/120 secondary. I realize each utility company has it's own standards for transformer grounding but I'm wondering if there is an overall agreed-to design which is accepted by most AHJ and utilites.

Attached is a drawing I've been working on. I'm sure there are some "issues" with the design so I'm looking for feedback from you folks on how it should look. The key here is to reference code(s) in order to cover ourselves but I'm not finding any good code references related to primary/secondary grounding and bonding.

Help!!

You are wanting to add ground ring and rods to the utility transformer? Who is to install the ground ring? Thuis has to be for an instance where electricians build the pad and the utility just comes in and sets the XF if you are worried about NEC requirements.

 

[dicklaxt]

Sorry I missed the "Utility Type" which makes some of my comments wrong.Read it to fast,my bad.


The Old Man

 

[roger]

Read it to fast,my bad.


The Old Man

Don't worry, I seem to be doing that more and more.


Roger

 

[uberflex]

Thanks everyone for the comments.

I should clarify something.......the transformer detail is a "general" detail in which it would be used for either a utility-owned transformer OR a customer-owned transformer (i.e. school campus, hospital campus, etc). The intent is to have a master detail drawing in which we can modify or otherwise change the info to suit the project. I want to make sure ALL of the grounding and bonding details are there for editting.

Thanks!!

 

[uberflex]

Lets assume a customer owned pad mount transformer where the service point is made at the service drop from the utility pole. Let's say 15kV primary and 480/277V secondary Wye. The contractor is now responsible for providing grounding and bonding for a 500 kVA service transformer. Considering this type of installation, what kind of grounding is required at the transformer to meet all required codes? I know NEC applies but is there any other code or standard I'm missing besides state specific codes? Does NECS apply at all? What exactly does NEC require when it's a customer owned transformer fed by 15kV service?



And to clarify something earlier.....the rigid steel conduit sweeps are preferred to PVC due to abrasion resistance espcially when pulling heavier gauge cable thorugh multiple bends. Yes, I understand it introduces metal within the transformer but it's a common practice in this part of the country.

 

[uberflex]

Here's an updated detail drawing. Comments, suggestions, etc are welcome.

 

[Smart $]

Thanks everyone for the comments.

I should clarify something.......the transformer detail is a "general" detail in which it would be used for either a utility-owned transformer OR a customer-owned transformer (i.e. school campus, hospital campus, etc). The intent is to have a master detail drawing in which we can modify or otherwise change the info to suit the project. I want to make sure ALL of the grounding and bonding details are there for editting.

Thanks!!

Many POCO's publish their requirements online, from which you can glean many of the details...

https://www.google.com/webhp?source....&fp=f3803001d38086d7&ion=1&biw=1108&bih=1127

If your work is primarily in an area served by one to a few POCO's, refine the search term with their name.

 

[uberflex]

Many POCO's publish their requirements online, from which you can glean many of the details...

https://www.google.com/webhp?source....&fp=f3803001d38086d7&ion=1&biw=1108&bih=1127

If your work is primarily in an area served by one to a few POCO's, refine the search term with their name.

Yeah I get that. Let's disregard the "utility transformer" part of this for a minute. Assume a customer owned transformer. I need to understand the code requirements for this type of installation. Is a ground ring required? How many ground rods? What bonding is required? etc.....

 

[Smart $]

Yeah I get that. Let's disregard the "utility transformer" part of this for a minute. Assume a customer owned transformer. I need to understand the code requirements for this type of installation. Is a ground ring required? How many ground rods? What bonding is required? etc.....

The NEC doesn't really have a much to say about grounding of outside pad mount transformers specifically. The only one I'm aware of is regarding outside service transfomers: 250.24(A)(2)

(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.

Aside from this, everything else is the general grounding and bonding requirements of Article 250.

 

[uberflex]

Ok thanks. That's pretty much what I came up with. State codes usually just point back to the NEC. I don't have a copy of the NESC book so not sure what it covers in terms of grounding.

 

[georgeswe]

I would set up a meeting with your utility inspector. I have seen different requirements over the years from different utility providers. I have had to use tinned copper with exothermic welds for the ground rings, and some have had me take out ground busing in above ground switchgear in "their pull sections". I would assume that a utility company will be inspecting your work and since you are looking at a boiler plate detail on an engineered blueprint ,it my differ from what the utility inspector wants. Also, if this is the for a utility company you can throw the NEC out the window.