Step-down Transformer Ground Connection Point

[ElecEngbw]

Hello,
We are working on a renovation project where we have a step-down transformer on the 3rd floor of a building. That transformer steps down 480V to 208V and serves a new equipment panel. We called for the secondary neutral to be bonded to the transformer case and the nearest cold water piping. The building code plans reviewer has called for that bond to take place within 5' of where the domestic water pipe enters the building and is quoting 2008 NEC 250.30 (A) (4) (7) (1). Knowing that the reference doesn't exist, I think they are referring to 250.30 (A) (7) (1), which references in itself 250.52 (A) (1) which deals with grounding electrodes. The bonding of the transformer secondary neutral to the nearest cold water pipe was not intended to serve as an electrode, but rather was to satisfy 250.104 (D) (1). The grounding conductor that is ran with the primary conductors to the step-down transformer, which is tied to the transformer case and secondary neutral, would serve as the grounding electrode conductor back to the main grounding electrode.

Should the step-down transformer be bonded to the water pipe within 5' of the building entrance or can it be bonded in the area being served by the transformer, as noted in 250.104 (D) (1)? Thanks for the help!

 

[GoldDigger]

The important bond is to the rest of the building GES for feeding EGCs unless the wiring off the secondary is going to be totally isolated from other bonded grounds....

 

[david luchini]

I think the plan reviewer is correct. The ground electrode for the SDS shall be the nearest of either metal water pipe grounding electrode or structural metal grounding electrode. If neither of those electrodes exist, then you can provide other electrodes. The equipment grounding conductor with the feeder cannot serve as the ground electrode conductor.

 

[augie47]

...............................which references in itself 250.52 (A) (1) which deals with grounding electrodes. The bonding of the transformer secondary neutral to the nearest cold water pipe was not intended to serve as an electrode, but rather was to satisfy 250.104 (D) (1). T.!

That used to be my thinking but I can find nothing in the Code to substantiate that.
Looks like it 250.52(A)(1).
Is it possible the exception in 250.52(A(1) will apply or that you might have the situation described in 250.30(A)(4)?

 

[iwire]

The bonding of the transformer secondary neutral to the nearest cold water pipe was not intended to serve as an electrode, but rather was to satisfy 250.104 (D) (1).

In that case it does not have to be run back to within 5'

However the transformer must also have a connection to a grounding electrode.

The grounding conductor that is ran with the primary conductors to the step-down transformer, which is tied to the transformer case and secondary neutral, would serve as the grounding electrode conductor back to the main grounding electrode.

Can't do that, and even if you could it would be difficult as a GEC has to be continuous and bonded to each end of each raceway it enters.

You will have to run a GEC directly from XO all the way, unbroken to any point on the buildings grounding electrode system.

 

[ElecEngbw]

The exception in 250.52(A) (1) won't be allowed per building plans reviewer for this building.

Pardon my confusion with this, but this is the first time in 8 years of engineering that I have seen this commented on. So each SDS (i.e. step-down transformer) should have a separate grounding electrode conductor back to the main service entrance grounding electrode connection point (i.e. within 5' of building entrance of water pipe or ground rods outside building)?

 

[infinity]

There is no building steel in this structure?

 

[texie]

The exception in 250.52(A) (1) won't be allowed per building plans reviewer for this building.

Pardon my confusion with this, but this is the first time in 8 years of engineering that I have seen this commented on. So each SDS (i.e. step-down transformer) should have a separate grounding electrode conductor back to the main service entrance grounding electrode connection point (i.e. within 5' of building entrance of water pipe or ground rods outside building)?

Things will depend on the construction type. If wood or concrete then you would have to go back to the GES for the building. But if you have a steel frame building you can use the steel as a GEC. See 250.68(C)(2). I do think the plan reviewer is wrong that if only bonding the water pipe it does not have to be within 5'.

 

[iwire]

The exception in 250.52(A) (1) won't be allowed per building plans reviewer for this building.

Pardon my confusion with this, but this is the first time in 8 years of engineering that I have seen this commented on. So each SDS (i.e. step-down transformer) should have a separate grounding electrode conductor back to the main service entrance grounding electrode connection point (i.e. within 5' of building entrance of water pipe or ground rods outside building)?

Every SDS needs a grounding electrode connection, in most buildings that is accomplished with a connection to the buildings structural steel frame. On buildings with out that steel you need to provide another way.

On buildings without structural steel and multiple SDS there is 250.30(A)(4) [2008 NEC] that allows a single 3/0 run around the building with taps run to each SDS.

 

[ElecEngbw]

Great! Thank you all for clearing this up. This building has concrete structure. I will have to provide another route back to GES for the building. Thanks

 

[augie47]

Seems a bit unusual to have a multi-story building and this lone transformer.
Has the requirement been ignored for other transformers or is it possible there is a common grounding electrode conductor.

 

[ElecEngbw]

Seems a bit unusual to have a multi-story building and this lone transformer.
Has the requirement been ignored for other transformers or is it possible there is a common grounding electrode conductor.

There is an existing step-down in same room as new transformer being installed. It is currently connected to water pipe in adjacent room, where we had the new one shown to connect to. No other grounding conductors appear to leave the existing transformer.

 

[augie47]

There is an existing step-down in same room as new transformer being installed. It is currently connected to water pipe in adjacent room, where we had the new one shown to connect to. No other grounding conductors appear to leave the existing transformer.

must have been a different building plans reviewer or inspector for that one

 

[texie]

There is an existing step-down in same room as new transformer being installed. It is currently connected to water pipe in adjacent room, where we had the new one shown to connect to. No other grounding conductors appear to leave the existing transformer.

It was allowed before the 2011 edition if in compliance with 250.68(B).

 

[iwire]

It was allowed before the 2011 edition if in compliance with 250.68(B).

I am not seeing that.

250.68 Grounding Electrode Conductor and Bonding
Jumper Connection to Grounding Electrodes. The connection
of a grounding electrode conductor at the service, at
each building or structure where supplied by a feeder(s) or
branch circuit(s), or at a separately derived system and associated
bonding jumper(s) shall be made as specified
250.68(A) and (B).

(B) Effective Grounding Path. The connection of a
grounding electrode conductor or bonding jumper to a
grounding electrode shall be made in a manner that will
ensure an effective grounding path. Where necessary to

The water line line is not part of the GES unless we connect to it less than 5' from the entrance.

 

[texie]

I am not seeing that.



The water line line is not part of the GES unless we connect to it less than 5' from the entrance.

Prior to 2011 the 5' rule was not there. You could use the piping as a GEC.

 

[augie47]

I'm a bit lost. I understand that they added "C" to 250.68 in '11 but it seems it was more for clarity. 250.68 connections between electrodes and 250.52(A)(1) has always (or for quite a while) stated that beyond the 5 ft point the water pipe is not considered an electrode.

 

[texie]

I'm a bit lost. I understand that they added "C" to 250.68 in '11 but it seems it was more for clarity. 250.68 connections between electrodes and 250.52(A)(1) has always (or for quite a while) stated that beyond the 5 ft point the water pipe is not considered an electrode.

I'm not saying it is a GE. And yes, the 5' rule has been there for sometime if you are using it as a GE. I'm saying you could use the water pipe as a GEC prior to 2011.
Hope I'm making sense.

 

[augie47]

I'm not saying it is a GE. And yes, the 5' rule has been there for sometime if you are using it as a GE. I'm saying you could use the water pipe as a GEC prior to 2011.
Hope I'm making sense.

But in '08 (I have not had time to review earlier) 250.52(A)(1) states "located more than 5 ft ..shall not be used as part of the grounding electrode system or as an conductor to interconnect electrodes that are part of the system."

 

[texie]

But in '08 (I have not had time to review earlier) 250.52(A)(1) states "located more than 5 ft ..shall not be used as part of the grounding electrode system or as an conductor to interconnect electrodes that are part of the system."

You're right. I lost track of that as there were so many Art. 250 changes for 2011. It just moved and I remembered it as not being there. Foot in mouth.
Thats what I get for relying on memory. I looked it up in '05 and '08 and you are correct.