Customer Owned MV XFMR - Is Sec. Ground Conductor Required?

[Sunny_92]

Can someone please confirm my thinking? Here's the situation:

Large industrial facility w/ multiple buildings. Buildings are fed by customer-owned MV overhead distribution and pad mount transformers. Each pad mount transformer has its own ground ring, and each building has its own grounding electrode system.

Two questions:

1. Are the underground service conductors (conductors between XFMR secondary and main service disconnect) required to have a ground conductor run with them?

2. If a ground conductor is installed with the service conductors, would it technically be considered a "supply side bonding jumper"?

My guess is that a ground conductor is not required with the service conductors as long as there's a neutral (grounded conductor, technically), and the neutral is bonded to ground at both the transformer and main disconnect. Is this correct?

Thanks in advance!

 

[david luchini]

Can someone please confirm my thinking? Here's the situation:

Large industrial facility w/ multiple buildings. Buildings are fed by customer-owned MV overhead distribution and pad mount transformers. Each pad mount transformer has its own ground ring, and each building has its own grounding electrode system.

Two questions:

1. Are the underground service conductors (conductors between XFMR secondary and main service disconnect) required to have a ground conductor run with them?

2. If a ground conductor is installed with the service conductors, would it technically be considered a "supply side bonding jumper"?

My guess is that a ground conductor is not required with the service conductors as long as there's a neutral (grounded conductor, technically), and the neutral is bonded to ground at both the transformer and main disconnect. Is this correct?

Thanks in advance!

It looks like under the 2017 NEC, the supply-side bonding jumper would not be required if the installation is made in compliance with 250.30(A)(1) Ex. No. 2.

 

[jumper]

That neutral should not be rebounded to ground at the building disco.

The building GEC should be in the ground bar, and the neutral bar and the ground bar should be separate.

View attachment 18050

 

[jumper]

It looks like under the 2017 NEC, the supply-side bonding jumper would not be required if the installation is made in compliance with 250.30(A)(1) Ex. No. 2.

Is it the same as the 2014?

Exception No. 2: If a building or structure is supplied by a
feeder from an outdoor transformer, a system bonding
jumper at both the source and the first disconnecting means
shall be permitted if doing so does not establish a parallel
path for the grounded conductor. If a grounded conductor
is used in this manner, it shall not be smaller than the size
specified for the system bonding jumper but shall not be
required to be larger than the ungrounded conductor(s).
For the purposes of this exception, connection through the
earth shall not be considered as providing a parallel path.

 

[david luchini]

Is it the same as the 2014?

Yes, the same for 2014 and 2017

That neutral should not be rebounded to ground at the building disco.

I don't think that is correct. Without the SSBJ, I think you need bond the neutral at both ends.

 

[jumper]

Yes, the same for 2014 and 2017



I don't think that is correct. Without the SSBJ, I think you need bond the neutral at both ends.

I was referring to an install like the graph I posted that has a SSBJ and a SBJ at the tranny, not an install using the exception you posted.

 

[david luchini]

I was referring to an install like the graph I posted that has a SSBJ and a SBJ at the tranny, not an install using the exception you posted.

Gotcha...I thought you were responding to the OP's question...

Is this correct?

 

[Sunny_92]

Thanks for the responses, it looks like we're in agreement with regard to the 2014 & 2017 code.

How about here in PA where we're under the 2008 code? It looks like a "supply-side bonding jumper" was formerly referred to as an "equipment bonding jumper," and 250.30(A)(2) doesn't seem to require it. Is a supply side equipment bonding jumper required by 250.102?

 

[ron]

The OP makes reference to "underground service conductors", I don't think that the secondary of a customer owned MV - LV transformer results in "underground service conductors".


The secondary is just a feeder that needs to be in compliance with 240.21(C).

Just like any feeders from the secondary of a transformer that is a SDS, it needs to have a a SSBJ run with it if the N-G bond is at the XFMR.

 

[publicgood]

The specific reference is:

250.32 Buildings or Structures Supplied by a Feeder(s) or Branch Circuit(s). (B) Grounded Systems. (1) and (2).

No separate equipment grounding conductor, just the SSBJ.

 

[david luchini]

it needs to have a a SSBJ run with it if the N-G bond is at the XFMR.

Unless a system bonding jumper is installed at both the XFMR and the disconnecting means per 250.30(A)(1) Ex. No. 2. Then a SSBJ is not required.

 

[publicgood]

Unless a system bonding jumper is installed at both the XFMR and the disconnecting means per 250.30(A)(1) Ex. No. 2. Then a SSBJ is not required.

Agree.

I'll add that: If the neutral is to be used, it may already be sized to match the ungrounded conductors.

 

[ron]

Unless a system bonding jumper is installed at both the XFMR and the disconnecting means per 250.30(A)(1) Ex. No. 2. Then a SSBJ is not required.

Then a EGC is needed per Table?

 

[augie47]

Agree.

I'll add that: If the neutral is to be used, it may already be sized to match the ungrounded conductors.

Code reference, please

 

[david luchini]

Then a EGC is needed per Table?

No. The egc would be needed downstream from the OCPD.

 

[publicgood]

Code reference, please

No reference, just typical 100% sized neutral because "it's just money." Although a reduced neutral may be a design consideration.

 

[Sunny_92]

So to summarize, in this situation, the requirements for a "supply-side bonding jumper" are the same in the 2008 NEC as they are in the 2014 & 2017 NEC, except the wording/terminology is much clearer in the later editions.

Does anyone disagree?