Building Fed from Customer Owned Transformer

[jbt260]

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? I know how it's done when the utility owns the transformer. Thanks in advance.

 

[Smart $]

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? I know how it's done when the utility owns the transformer. Thanks in advance.

Do it the same way. Just because the customer owns the transformer does not change the fact that it is [part of] a service. Only difference here is that the transformer falls under NEC purview... but for the most part, requirements are the same whether under NEC or NESC.

 

[jim dungar]

The secondary conductors of the transformer fall under article 240 not 230, it is not a service.

Treat it the same as you would a general purpose 480-208Y/120 transformer. It gets bonded in only one place, at the TX or the disconnect.

In this part of the world, bonding at the main panel is most common.

 

[brian john]

The secondary conductors of the transformer fall under article 240 not 230, it is not a service.

Treat it the same as you would a general purpose 480-208Y/120 transformer. It gets bonded in only one place, at the TX or the disconnect.

In this part of the world, bonding at the main panel is most common.

And here we bond at the transformer.

 

[Smart $]

The secondary conductors of the transformer fall under article 240 not 230, it is not a service.

Treat it the same as you would a general purpose 480-208Y/120 transformer. It gets bonded in only one place, at the TX or the disconnect.

In this part of the world, bonding at the main panel is most common.

...and that makes it different from a service how?

If the secondary is not a service, where is the service disconnect? Many customer owned xfmrs around here have no [customer owned] service disconnect on the primary side. The primary is fed direct from the distribution pole.

 

[IMM_Doctor]

I am curious - different local requirements

I am curious - different local requirements

When an EC installs a Customer Owned Pad Mounted XFMR, and typically uses multiple PVG underground conduits to get the secondary power (three phase four wire) into the building structure:

How many conductors are in each conduit?
A. (4) Three ungrounded phase conductors and one grounded conductor

or

B. (5) Three ungrounded phase conductors, one grounded conductor, and one grounding conductor.


If you contemplate the above, for situation A. then one would need to draw a "schematic" indicating what the path (including the source and the OCPD), of a major ground fault within the building would be.

If you only 4 conductors in the pvc, and bond only in the transformer, how would a ground fault in the premisies cause the OCPD to open?

If you have 5 conductors in the pvc, I know you would only bond in the transformer, otherwise the grounding conductor would carry return current between the switchboard and the transformer. (not desired).

Is this a NATIONAL code, or is delegated down to the local AHJ?

 

[jbt260]

This is a 2500KVA transformer. There will be 10 conductors per phase and neut. If I ground the neutral in the main switchboard and isolate the neutral from ground in the transformer, then I would need to provide an EGC in each conduit from the transformer to the main switchboard. Where do you land 10 EGC's in a padmount transformer compartment? Are EGC's required between the transformer and main switchboard?

 

[don_resqcapt19]

This is a 2500KVA transformer. There will be 10 conductors per phase and neut. If I ground the neutral in the main switchboard and isolate the neutral from ground in the transformer, then I would need to provide an EGC in each conduit from the transformer to the main switchboard. Where do you land 10 EGC's in a padmount transformer compartment? Are EGC's required between the transformer and main switchboard?

If you take advantage of 250.30(A)(1) Exception #2, you would wire this installation just like a service with a bonding connection at both the transformer and the building disconect. This assumes non-metallic conduit. If the conduit is metallic, you can only bond in one location, but you could use the conduit at the required EGC between the two locations.
If you only bond at one location, you must use EGCs of some type between the two locations.

 

[jbt260]

Very useful info! Thank you Don.

 

[tryinghard]

If you take advantage of 250.30(A)(1) Exception #2, you would wire this installation just like a service with a bonding connection at both the transformer and the building disconect. This assumes non-metallic conduit. If the conduit is metallic, you can only bond in one location, but you could use the conduit at the required EGC between the two locations.
If you only bond at one location, you must use EGCs of some type between the two locations.

I agree here, avoid parallel neutral path.

 

[Smart $]

If you take advantage of 250.30(A)(1) Exception #2, you would wire this installation just like a service...

Why are you (et al) so quick to say the secondary is not a service???

Unless there is a service disconnecting means established on the primary side of the transformer (e.g. fused MV switchgear), the secondary IS a service. The xfmr being customer owned does not change the definition or equipment makeup of a service.

If there is a qualifying NEC service disconnecting means on the primary side, then I'll concede the transformer is a separately derived system.

 

[don_resqcapt19]

Why are you (et al) so quick to say the secondary is not a service???

Unless there is a service disconnecting means established on the primary side of the transformer (e.g. fused MV switchgear), the secondary IS a service. The xfmr being customer owned does not change the definition or equipment makeup of a service.

If there is a qualifying NEC service disconnecting means on the primary side, then I'll concede the transformer is a separately derived system.

It is my opinion that customer owned equipment does not exist on the line side of the service point. There is no requirement that the service disconnect be located at or even close to the service point. The serivce point is defined by the utility and not by the code.

 

[don_resqcapt19]

... The xfmr being customer owned does not change the definition or equipment makeup of a service. ...

I think it does. Look at the definiton of "service point".

Service Point. The point of connection between the facilities of the serving utility and the premises wiring.

If the customer owns the transformer it is not a "facility of the serving utility" and the service point has to be somewhere on the line side of the transformer primary. Maybe even on the primary terminals of the transformer.
That being said, I expect that this is a "campus" type installation and the service point is somewhere upstream on the 12.5 kV distrubution.

 

[Smart $]

I think it does. Look at the definiton of "service point".

Service Point. The point of connection between the facilities of the serving utility and the premises wiring.

If the customer owns the transformer it is not a "facility of the serving utility" and the service point has to be somewhere on the line side of the transformer primary. Maybe even on the primary terminals of the transformer.
That being said, I expect that this is a "campus" type installation and the service point is somewhere upstream on the 12.5 kV distrubution.

The "service point" does not determine where the service ends. It is only the point where ownership changes hands, and even then that is not exclusively so. For example, there are many, many (let me emphasize... many :grin instances where the meter and meter enclosure are POCO owned, yet it is installed load-side of the service point. The "end" of a service is exclusively determined by the service disconnecting means.

It is my opinion that customer owned equipment does not exist on the line side of the service point. There is no requirement that the service disconnect be located at or even close to the service point. The serivce point is defined by the utility and not by the code.

Exactly my point!!!

 

[don_resqcapt19]

The "service point" does not determine where the service ends. It is only the point where ownership changes hands, and even then that is not exclusively so. For example, there are many, many (let me emphasize... many :grin instances where the meter and meter enclosure are POCO owned, yet it is installed load-side of the service point. The "end" of a service is exclusively determined by the service disconnecting means.

I can see your point but still don't agree. The premises wiring starts at the service point and not at the sevice disconnect. If you apply the defintion of sevice and of premises wiring, the service stops at the service point, but you still have service conductors (per the defintion of service conductors) to the disconnect.

Exactly my point!!!

I don't see how it is your point...it is mine. If the utility is telling you how to make that part of the installation it is on the line side of the service point with the exception of the meter equipment.

 

[Smart $]

...If the utility is telling you how to make that part of the installation it is on the line side of the service point with the exception of the meter equipment.

I do not disagree with this.

...The premises wiring starts at the service point and not at the sevice disconnect.

I do not disagree with this.

If you apply the defintion of sevice and of premises wiring, the service stops at the service point, ...

This is where we disagree.

As I see it, the "service" always extends to the service disconnecting means, regardless of where the service point is. This is the very reason we have "service conductors" per NEC definitions and are required to use "service-rated equipment".

If this transformer is on the line side of the service disconnecting means, is it not required to be service-rated? If it is, doesn't it follow that it should be connected as service equipment and not as a separately derived system?

...but you still have service conductors (per the defintion of service conductors) to the disconnect.

So in essence you are saying the transfomer is not service equipment, but conductors into it are POCO/NESC service conductors and out of it are NEC/Premises Wiring System Service Conductors. That don't make any sense at all...!!!

 

[nunu161]

just recently installed a 5000kva transformer of the same voltage and there was a MV disconnect right outside the parking lot feeding 2 indoor dry-type 5000kva's and one outdoor oil-filled 5000kva transformer. the engineer had us pull a 3MV cables and a system bonding jumper from the MV disconnect to each transformer and then the 2 indoor ones had 4 conductor busduct from them to the MDP's which the bonding took place in MDP. As for the outdoor transformer the enginner had us pull the same MV feeder but had us put in an EGC in each pipe feeding the MDP off the transformer.

(now this is where it got screwy no idea how the engineer came up with this set-up with its wire sizing and bonding)

This transformer is feeding a 5000 amp 480/277Y switchgear with parrelled 500's for the phase conductors, A #2 for the Neutral and a 1/0 for the Egc. Then he had us bond the neutrals and the egcs in both the transformer and the MDP. At the time when getting ready to install and wire the system i warned my foreman about the violations of the size of the neutral and the bonding in both the transformer and MDP. the foreman contacted the engineer and and told him the violations and the code references the engineer told him to install it as figured and he wrote off on it.

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.

 

[Pierre C Belarge]

this is an interesting topic.
I believe the answer would be different for different utility companies.

Is it not the utility tariff that usually will determine the case of service/utility - service/private property - no service/private property.




In our area, if the utility sells the transformer to a property owner, there still would be required a service disconnect on the secondary side, if there is no service disconnect on the primary side. Regardless of how far the cables and transformer may be inside the private property.

I am not sure, but I also believe the size of the transformer has something to do with the tariff.

 

[jim dungar]

In Wisconsin, if the customer owns the transformer there must be a 3-pole disconnecting device on the primary side, unless the installation has been 'grandfathered in'.

 

[Smart $]

In Wisconsin, if the customer owns the transformer there must be a 3-pole disconnecting device on the primary side, unless the installation has been 'grandfathered in'.

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?