Transformer GEC-2008 NEC

[infinity]

I'm working in a high rise building where all of the transformer GEC's are connected to a ground bus. This ground bus is connected to a common grounding electrode conductor that runs from the service room to top floor of the building. The building is structural steel construction and the connection of the transformer GEC is only to the common grounding electrode conductor. IMO since the building steel is available adjacent to the transformer the transformer GEC is required to connect to the nearest building steel as in 250.32(A)(7)(2). Does the installation of a common grounding electrode conductor negate the nearest connection requirement?

2008 NEC:

250.32(A)(7) Grounding Electrode. The grounding electrode shall be as near as practicable to and preferably in the same area as the grounding electrode conductor connection to the system. The grounding electrode shall be the nearest one of the following:
(1) Metal water pipe grounding electrode as specified in 250.52(A)(1)
(2) Structural metal grounding electrode as specified in 250.52(A)(2)

 

[augie47]

I don't see any clear answer but the wording in 250.30(A) (4) "it shall be permissible" seems to indicate that the "common electrode conductor" connected to a grounding electrode would be in lieu of the individual conductors connected at each source.
250.104 (D)(3) seems to clarify a common connection is all that is required.

 

[infinity]

I don't see any clear answer but the wording in 250.30(A) (4) "it shall be permissible" seems to indicate that the "common electrode conductor" connected to a grounding electrode would be in lieu of the individual conductors connected at each source.
250.104 (D)(3) seems to clarify a common connection is all that is required.

Here's what is hanging me up. 250.30 says that it shall comply with (A)(1) through (A)(8) so I don't see how if the water pipe or structural steel in (A)(7) is near how you cannot not connect to it. So even though in this case the common grounding electrode riser was not required it cannot be the sole GEC connection when the structural steel is right next to the transformer.

250.30 Grounding Separately Derived Alternating Current Systems.
(A) Grounded Systems. A separately derived ac system that is grounded shall comply with 250.30(A)(1) through (A)(8).

250.32(A)(7) Grounding Electrode. The grounding electrode shall be as near as practicable to and preferably in the same area as the grounding electrode conductor connection to the system. The grounding electrode shall be the nearest one of the following:
(1) Metal water pipe grounding electrode as specified in 250.52(A)(1)
(2) Structural metal grounding electrode as specified in 250.52(A)(2)

 

[augie47]

Your point is well taken and I don't find any specific detail that would exclude that requirement but, to me, the wording of (A)(4) combined with 250.104(D) makes the common conductor and connection an alternative ("shall be permissible"),
I am anxious to see what others may say.

 

[infinity]

Your point is well taken and I don't find any specific detail that would exclude that requirement but, to me, the wording of (A)(4) combined with 250.104(D) makes the common conductor and connection an alternative ("shall be permissible"),
I am anxious to see what others may say.

I see the common grounding electrode (CGE) as an alternative to when the structural steel doesn't exist. We install the common grounding electrodes in poured concrete buildings where there is no steel for connection of a GEC to the transformer.

Regarding 250.104(D)(3) if the common grounding electrode is used then it would need to be bonded to the steel. Going back to my original question I should have said that if the common grounding electrode riser is used then the transformer GEC can connect solely to the CGE provided that the CGE is bonded to the structural steel.

That leads to my next question, where is that bond required to be located? (D)(3) mentions that if the building frame exists in the area served by the SDS so should it be bonded there or is a bonding connection of all of the electrodes including the CGE permitted somewhere else say at the service?

250.104(D)(3) Common Grounding Electrode Conductor. Where a common grounding electrode conductor is installed for multiple separately derived systems as permitted by 250.30(A)(4), and exposed structural metal that is interconnected to form the building frame or interior metal piping exists in the area served by the separately derived system, the metal piping and the structural metal member shall be bonded to the common grounding electrode conductor.
Exception: A separate bonding jumper from each derived system to metal water piping and to structural metal members shall not be required where the metal water piping and the structural metal members in the area served by the separately derived system are bonded to the common grounding electrode conductor.

 

[electrofelon]

Whenever Rob posts a question I know its going to be tough and I'm not going to have an answer off the top of my head or even after a quick glance at the code book

 

[infinity]

Whenever Rob posts a question I know its going to be tough and I'm not going to have an answer off the top of my head or even after a quick glance at the code book

I often stump myself with these questions so I post them here where the people are much smarter than me.

 

[Tulsa Electrician]

Could this be an example why the change to 250.52(A) (2) was made for 2011. I'm sure ( just guessing) the steel is in direct contact with the earth to qualify. If not I can see where the common was used. Maybe at time of drawing there was a reason. Since we can not see now what was done. Other wise were just bonding and the steel can not be used as an electrode.

Let me just throw out an example for thought. If a building has seismic isolation or stabilizer pads.
Maybe design was later than 08 however code for the time was 08. The drawing and specs called for latest code. Then the change in 11 may apply.

 

[infinity]

The steel is an electrode and is connected to a ground bus in the service room with a GEC. The CGE riser is also connected to the same bus.

 

[augie47]

I often stump myself with these questions so I post them here where the people are much smarter than me.

That certainly leave me out

 

[electrofelon]

That certainly leave me out

New ominous avatar for 2023?

 

[augie47]

New ominous avatar for 2023?

yep