Ground Wire From 3 Phase Pad Mounted Utility Transformer

Status
Not open for further replies.
Sparky2791

Sparky2791

Senior Member
Location
Northeast, PA
Occupation
Electrical Design
I am having a friendly disagreement with a client regarding a ground wire requirement.


The utility company brings 13.2 KV to the exterior pad mounted transformer and customer pulls 480/277V into the building from the secondary side. I believe a separate ground wire is required to be pulled from the three phase pad mounted utility transformer to the switchgear as an equipment bonding jumper. System bonding happens inside at the switchgear. My client feels no ground is required to be pulled into the building only one tied from the transformer enclosure to the ground ring around the transformer so only a neutral wire is required into the building (other than the phase conductors).

I would like the specific NEC reference to confirm this requirement if I am correct. If I am not I will gladly revise my way of design and move forward. Thanks for your time.
 
jumper said:
Who owns the transformer?

Are the secondary conductors still part of POCOs service?

Where is the first disconnecting means to the building?
Click to expand...


The utility company takes ownership of the conductors once they are installed.


The utility company owns the transformer. Although I ask why that would matter?


First disconnecting means is @ the switchgear where the GES is created
 
LibertyEngineering said:
The utility company takes ownership of the conductors once they are installed.


The utility company owns the transformer. Although I ask why that would matter?


First disconnecting means is @ the switchgear where the GES is created
Click to expand...

No extra equipment bonding jumper needed from tranny to SWGR. Neutral only.

Edit: adding a EBJ is also probably not allowed since it be a parallel path for a fault/objectionable current,
 
Last edited:
LibertyEngineering said:
I am having a friendly disagreement with a client regarding a ground wire requirement.


The utility company brings 13.2 KV to the exterior pad mounted transformer and customer pulls 480/277V into the building from the secondary side. I believe a separate ground wire is required to be pulled from the three phase pad mounted utility transformer to the switchgear as an equipment bonding jumper.
Click to expand...

Negative, an equipment bonding jumper in this case would simply become a parallel neutral conductor. The neutral is bonded to the transformer by the utility and the neutral is bonded again at the service disconnect as required by the NEC.

Consider a simple service at a home, two ungrounded, one grounded and zero grounding conductors. The grounded conductor performs two functions, it is a the neutral current circuit conductor and it is the grounding means between the transformer and the service disconnecting enclosure. Regardless of the difference in size between this simple home service and the large service you are doing the grounding bonding rules are the same.
 
iwire said:
Negative, an equipment bonding jumper in this case would simply become a parallel neutral conductor. The neutral is bonded to the transformer by the utility and the neutral is bonded again at the service disconnect as required by the NEC.

Consider a simple service at a home, two ungrounded, one grounded and zero grounding conductors. The grounded conductor performs two functions, it is a the neutral current circuit conductor and it is the grounding means between the transformer and the service disconnecting enclosure. Regardless of the difference in size between this simple home service and the large service you are doing the grounding bonding rules are the same.
Click to expand...

Mike Holt calls it the white wire with the imaginary green stripe.
 
In this case where the utility owns the transformer and the conductors, they are obviously "service conductors" and the switchgear is the "service equipment"; there is no need for a 5th conductor and, as jumper and iwire point out the installation of such could well introduce a violation of parallel conductors.
As to why ownership matters.......
The scenario changes when the transformer, conductors, etc, are customers and the "service point" is ahead of the transformer.
The install is then a SDS and the rules, IMO, are a bit muddied depending on the Code cycle. In 2011 the NEC added in 250.30(A)(2) that a "supply side bonding conductor" be installed between the transformer and the disconnecting means. This was, however, in conflict with 250.30(A)(1) Exception 2.
I believe in 2104 an exception to 250.30(A)(2) was added to address that conflict.
 
So, in theory is the service point is ahead of the customer owned transformer (and the metering done there), there should be a ground neutral bond at the transformer primary (assuming that there is a disconnect there, yes?) and the secondary conductors would be either SDS or non-SDS but not a service anymore?

I can see the service point being further down the wiring from any POCO owned and controlled wiring, at the POCO's discretion, but can the service point also be upstream of other POCO owned equipment? What if the customer "owns" the transformer, but POCO maintains it? Or vice-versa, if that is possible?
 
Last edited:
GoldDigger said:
So, in theory is the service point is ahead of the customer owned transformer (and the metering done there), there should be a ground neutral bond at the transformer primary (assuming that there is a disconnect there, yes?) and the secondary conductors would be either SDS or non-SDS but not a service anymore?

I can see the service point being further down the wiring from any POCO owned and controlled wiring, at the POCO's discretion, but can the service point also be upstream of other POCO owned equipment? What if the customer "owns" the transformer, but POCO maintains it? Or vice-versa, if that is possible?
Click to expand...

I've been through similar nightmares a time or two. IMO, you have to forget all about "ownership" and base the decision on an agreed "service point" which is decided by the utiilty and customer.
 
GoldDigger said:
Or more likely by the utility on their own. :)
Click to expand...
Even in that case, Gus' statement is correct. POCO decides where the service point will be, and customer decides to agree with their decision so they can get power... :p

Anyway, you cannot technically have a customer-owned service transformer on the customer side of the service point... but there is no requirement for it to be wired as SDS. Connect POCO MGN to secondary neutral and it is non-SDS... and it can be treated just like a service transformer.
 
Last edited:
iwire said:
Negative, an equipment bonding jumper in this case would simply become a parallel neutral conductor. The neutral is bonded to the transformer by the utility and the neutral is bonded again at the service disconnect as required by the NEC.

Consider a simple service at a home, two ungrounded, one grounded and zero grounding conductors. The grounded conductor performs two functions, it is a the neutral current circuit conductor and it is the grounding means between the transformer and the service disconnecting enclosure. Regardless of the difference in size between this simple home service and the large service you are doing the grounding bonding rules are the same.
Click to expand...

Not exactly, if the pad mount is customer owned it becomes a separately derived source. And objectionable current can be ruled in this case as well.
 
Just answered this here
iceworm said:
Take a look at 250.24.A.2 Grounding Service-Supplied Alternating-Current Systems, System Grounding Connection, Outdoor Transformer
Click to expand...

This is one of the more goofed up code sections - truly bizzare. But that is what it says.

in my experience:
The service point,(for industrial customers - read "big"), is negotiated. And yes, the POCOs is generally not completely stupid.

Consider:
The MV/HV disconnect, transformer are customer owned. The metering may be on the secondary side. The xfm/disconnect maintenance may be a negotiated contract with the utility.

The service point is where ever the customer and the utility agree it is. I don't recall an AHJ ever telling them they are wrong. The AHJ just wants to know where it is. On one side the electrons behave according to the NESC and according to the NEC on the other side.
 
iceworm said:
Just answered this here


This is one of the more goofed up code sections - truly bizzare. But that is what it says.

in my experience:
The service point,(for industrial customers - read "big"), is negotiated. And yes, the POCOs is generally not completely stupid.

Consider:
The MV/HV disconnect, transformer are customer owned. The metering may be on the secondary side. The xfm/disconnect maintenance may be a negotiated contract with the utility.

The service point is where ever the customer and the utility agree it is. I don't recall an AHJ ever telling them they are wrong. The AHJ just wants to know where it is. On one side the electrons behave according to the NESC and according to the NEC on the other side.
Click to expand...


Ive always wondered, why do we have 2 codes? Why not a unified version of the NESC and the NEC?
 
mbrooke said:
Ive always wondered, why do we have 2 codes? Why not a unified version of the NESC and the NEC?
Click to expand...
Well, for a start, one is mandatory and the other is advisory, I believe.
And the NESC is intended to be used as guidance by engineers while the NEC is a cookbook.
 
mbrooke said:
Ive always wondered, why do we have 2 codes? Why not a unified version of the NESC and the NEC?
Click to expand...

I hear talk off and on of having the two be more in tune with each other but I don't see that as a natural fit. We (I worked for a poco) on the line side of the meter were governed by the NESC but we had to follow the NEC (load side) in some instances. The S in NESC is Safety of course. It was our bible for horizontal and vertical clearances from our high voltage lines to roads, buildings, signs, waterways, and much more. It can be a cumbersome document but it's a lot thinner than the NEC. The main reason I joined this forum in my retirement years is to attempt to increase my very limited NEC knowledge. It's been a great resource this last month and a half. Thanks to all.

Ben
 
mbrooke said:
Ive always wondered, why do we have 2 codes? Why not a unified version of the NESC and the NEC?
Click to expand...

Well, I have a copy of the 1901NEC

c
CLASS​
​
​
​
A​
STATIONS AND DYNAMO ROOMS

INCLUDES CEI'ITRAL STATIONS, DYNAMO, MOTOR, AND STORAGE-BATTERY ROOMS,​
TRANSFORMER SUBSTATIONS, ETC.

CLAss​
​
​
​
c.INSIDE WORK​
ALL SYSTEMS AND VOLTAGES.
Click to expand...

Same code covered both then. Things change. Different rules for 276KV as opposed to house roping.

Of course, one might also say there are different rules for grandma' A1 Abrams factory as opposed to grandpa' cottage on the lake. But the NEC applies the same code to both.

ice
 
mbrooke said:
Not exactly, if the pad mount is customer owned it becomes a separately derived source. And objectionable current can be ruled in this case as well.
Click to expand...

I don't believe 250.6 can ever be cited or rules as it is not defined in anyway at all.

Beyond that it is the service point that will be the deciding factor, not the ownership of the equipment.
 
mbrooke said:
Ive always wondered, why do we have 2 codes? Why not a unified version of the NESC and the NEC?
Click to expand...

In my opinion the nature of the work is different enough to have different codes or standards. The work is also typically done by different workers with different training.

I guess what I am saying is don't confuse me with a ton of rules I will never have to know about.:p If I need to find the answer I know where to look.
 
Status
Not open for further replies.
Top