Grounding/Bonding

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ken44

Senior Member
Location
Austin, TX
I need some input from the community on the following matter:
A contractor recently install an outdoor pad mounted transformer with a 12,500 volt primary (3 phase, 4 wire) and a 480 volt secondary (3 phase, 5 wire), all of the conductors are in PVC which is encased in concrete. I discovered that while they connected the primary grounds, they failed to connect any of the grounds on the secondary side to anything-they are just taped up and laying inside the transformer, and when I called them out on the matter, they stated that the following in an email:

"The grounding conductors were called for on the drawings but they are not required. We feel they should be neatly coiled up and placed inside each enclosure and not be connected. Connecting these wires could potentially create a parallel path to ground. The ground wires could be pulled out however we feel that this is an unnecessary disruption to the building and building tenants.

We feel the installation complies with NEC 2014 section 250.30 (a)(1), therefore a supply side bonding jumper is not required per the exception for section 250.30 (a)(2). This is typical for pad mount transformer installations.

We do not recommend connecting these ground wires to the electrical distribution system.

I have attached a simple diagram showing how the installation complies with code.

I am available to discuss this issue in more detail if needed."
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
I need some input from the community on the following matter:
A contractor recently install an outdoor pad mounted transformer with a 12,500 volt primary (3 phase, 4 wire) and a 480 volt secondary (3 phase, 5 wire), all of the conductors are in PVC which is encased in concrete. I discovered that while they connected the primary grounds, they failed to connect any of the grounds on the secondary side to anything-they are just taped up and laying inside the transformer, and when I called them out on the matter, they stated that the following in an email:

"The grounding conductors were called for on the drawings but they are not required. We feel they should be neatly coiled up and placed inside each enclosure and not be connected. Connecting these wires could potentially create a parallel path to ground. The ground wires could be pulled out however we feel that this is an unnecessary disruption to the building and building tenants.

We feel the installation complies with NEC 2014 section 250.30 (a)(1), therefore a supply side bonding jumper is not required per the exception for section 250.30 (a)(2). This is typical for pad mount transformer installations.

We do not recommend connecting these ground wires to the electrical distribution system.

I have attached a simple diagram showing how the installation complies with code.

I am available to discuss this issue in more detail if needed."
I don't know exactly what you mean by "grounds".

As I understand it, if the SBJ is downstream of the transformer, you need a SSBJ to the xfmr.

If the SBJ is at the transformer, you need an EGC to the xfmr.

The only time the SSBJ would create an issue would be if the SBJ existed both at the xfmr and downstream somewhere.

maybe you can post a sketch so we all know exactly what it is we are talking about.
 

Smart $

Esteemed Member
Location
Ohio
A separately derived 12.5kV—480Y/277 3Ø 4W system is an atypical installation for a multi-tenant location.

Assuming this is a separately derived system and not a service transformer, it can go either way under 2014 Code. However, without a supply-side bonding jumper, there must be a system bonding jumper installed at either the transformer or the disconnecting means and a neutral-ground bonding jumper installed at the other. In other words, the secondary grounding system must be bonded to the transformer frame in one manner or another

Note compliance with 250.30(C) is also required.

Please post the diagram referred to in the email.
 

ken44

Senior Member
Location
Austin, TX
Diagram request

Diagram request

See attached diagram. Also, to clarify, the "grounds" that I am referring to are the ones that were pulled inside the PVC with the other 480 volt wiring. Although they may be considering this as the utility transformer, I actually would consider it to be the feeders as the utility is ahead of all of this transformer. The power is fed overhead from the PoCo, then comes down a pole into a manhole, then over to the 12,500 volt switching station, then it goes from the switch at the 12,500 volt switching station over to the transformer. Hope this helps.
 

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don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
That is permitted by the code as long as there are no metallic paths between the transformer and the building. With PVC conduit it is unlikely that there would be an additional path.

There would be no problem using the conductors as the supply side bonding jumper, assuming they are correctly, and they connect only to the equipment bonding bus at the building. The system bonding jumper would be at the transformer and there would be no neutral to grounding connection at the building.
 

kwired

Electron manager
Location
NE Nebraska
Though the contractor may have installed the transformer it may still be on utility side of the "service point". If so the conductors leaving that transformer are service conductors and not feeders to the NEC. The service point needs to be verified before one can determine exactly what rules apply to the load side conductors of that transformer.

There are times when a contractor installs equipment but POCO takes over maintenance and operation of that equipment whether it is utility owned or customer owned doesn't matter, where it is in relation to the service point determines whether NEC applies to it or not.

There should be a service disconnecting means on the primary side somewhere if the transformer is on the customer side of the service point.
 

Smart $

Esteemed Member
Location
Ohio
Though the contractor may have installed the transformer it may still be on utility side of the "service point". If so the conductors leaving that transformer are service conductors and not feeders to the NEC. The service point needs to be verified before one can determine exactly what rules apply to the load side conductors of that transformer.

There are times when a contractor installs equipment but POCO takes over maintenance and operation of that equipment whether it is utility owned or customer owned doesn't matter, where it is in relation to the service point determines whether NEC applies to it or not.

There should be a service disconnecting means on the primary side somewhere if the transformer is on the customer side of the service point.
One of the reasons an SDS is permitted to be wired like this is because this is basically how a pad-mount service transformer is wired. So for the most part, it does not matter whether it is a service or consumer SDS transformer as wired.
 

kwired

Electron manager
Location
NE Nebraska
One of the reasons an SDS is permitted to be wired like this is because this is basically how a pad-mount service transformer is wired. So for the most part, it does not matter whether it is a service or consumer SDS transformer as wired.
Did I miss something? If it is a SDS, then there has to be a service ahead of it, regardless of what the voltage may be, the secondary could be bonded at the first disconnecting means but a supply side bonding jumper would have to run back to the transformer. Either way you need EGC or SSBJ between SDS and first disconnect.

If it is service transformer then it can (and probably will be) bonded at both transformer and at service equipment.

Location of "service point" is what will determine if this is service transformer or not even if POCO didn't install it.
 

Smart $

Esteemed Member
Location
Ohio
Did I miss something? If it is a SDS, then there has to be a service ahead of it, regardless of what the voltage may be, the secondary could be bonded at the first disconnecting means but a supply side bonding jumper would have to run back to the transformer. Either way you need EGC or SSBJ between SDS and first disconnect.

...
Yes.

By exception, the SSBJ is not required under 2014 Code... but an SBJ is required at both SDS and disconnecting means... and grounding pretty much mimics that of a service transformer.
 
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