NEC 250.104(D) Separately Derived Systems

[Anderman123]

Suppose we have a large building with multiple 480-208V transformers in multiple electrical rooms. Each electrical room has a ground bar with a ground wire going back to the grounding system in the main electrical room. The transformers are bonded to the ground bar. The incoming water pipe and the building steel is bonded at the main electrical room.

Does 250.104(D)2 mean that the ground bar at the transformers need to bond to the building steel in the room? Or is the building steel bond in the main electrical room compliant?

Does 250.104(D)1 mean that the ground bar at the transformers need to bond to the water pipes in the area? Or is the water pipe bond in the main electrical room compliant?

 

[jaggedben]

The main electrical room is not compliant if it is not 'in the area'.

 

[Anderman123]

I don't believe have ever seen local water pipes bonded to a transformer other than at the main service.

 

[wwhitney]

Suppose we have a large building with multiple 480-208V transformers in multiple electrical rooms. Each electrical room has a ground bar with a ground wire going back to the grounding system in the main electrical room. The transformers are bonded to the ground bar. The incoming water pipe and the building steel is bonded at the main electrical room.

Then it sounds like Exception 1 applies for each of 250.104(D)(1) and (2).

Cheers, Wayne

 

[jaggedben]

While lack of enforcement doesn't particularly surprise me, it is nonetheless what the quoted section requires. The present language goes back to the 2005 NEC, but some similar requirement goes back farther.

Note that if the transformer GECs are bonded to the piping and/or building steel, the exceptions can be met.

 

[jaggedben]

Then it sounds like Exception 1 applies for each of 250.104(D)(1) and (2).

Cheers, Wayne

How do you figure that without making assumptions?

 

[infinity]

I had a similar question 6 months ago:

Transformer GEC-2008 NEC

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...

forums.mikeholt.com

 

[wwhitney]

How do you figure that without making assumptions?

Well, the OP describes each transformer as being bonded to the "water pipe" and "building steel" via the ground bar and "ground wire" going back to the main electrical room. So I took that to mean that the "metal water piping system" and the "metal frame of a building or structure" are being used as grounding electrodes, in which case the Exceptions would apply.

Are you suggesting that the details of that connection via the ground bar and "ground wire" going back to the grounding electrodes via the main electrical room may not meet all the requirements of a GEC? If so, then I agree, I was assuming they do. Or is there something else I'm overlooking?

Cheers, Wayne

 

[Anderman123]

Well, the OP describes each transformer as being bonded to the "water pipe" and "building steel" via the ground bar and "ground wire" going back to the main electrical room. So I took that to mean that the "metal water piping system" and the "metal frame of a building or structure" are being used as grounding electrodes, in which case the Exceptions would apply.

Are you suggesting that the details of that connection via the ground bar and "ground wire" going back to the grounding electrodes via the main electrical room may not meet all the requirements of a GEC? If so, then I agree, I was assuming they do. Or is there something else I'm overlooking?

Cheers, Wayne

I'm assuming that water pipe and building steel are used as grounding electrodes.

I guess my hang-up is the "in the area served". If area means in the building, yes. If it means the area served specifically by the transformer, probably not.

Has anyone seen water pipes bonded to the ground bus on like the 5th floor electrical room containing transformers?

 

[wwhitney]

Ah. I take the language "in the area served" to cover the case that the building has two or more disjoint metal water piping systems (or metal frames). Then you only get to skip bonding (per the exception) if the closer system is the one being used as a grounding electrode.

So that's another assumption I was making, that the building has only one continuous metal water piping system and only one continuous metal frame.

Cheers, Wayne

 

[jaggedben]

Can't say I've seen it, but it's not my normal work.

I have always read the intent to be that if you have a transformer on, say, the 40th floor of a skyscraper, you bond it to the steel there.

Note that in the 2014 NEC and earlier a separately derived system was required to be grounded to the nearest of the water pipe or structural steel if available. That is, grounding it only back to the main service like you describe would have been illegal. In the 2017 NEC they completely switched that up. At any rate, whether you call the wire a GEC or a bonding jumper, it seems like bonding to nearby metal has always been in the code.

As far as 'in the area served', it's clearly somewhat open to interpretation. But for example, if one transformer serves the 2nd through 4th floors of a building, and the next serves the 3rd through 6th, then I think your areas served, and therefore your acceptable bonding locations, are pretty well defined.

 

[Anderman123]

Can't say I've seen it, but it's not my normal work.

I have always read the intent to be that if you have a transformer on, say, the 40th floor of a skyscraper, you bond it to the steel there.

Note that in the 2014 NEC and earlier a separately derived system was required to be grounded to the nearest of the water pipe or structural steel if available. That is, grounding it only back to the main service like you describe would have been illegal. In the 2017 NEC they completely switched that up. At any rate, whether you call the wire a GEC or a bonding jumper, it seems like bonding to nearby metal has always been in the code.

As far as 'in the area served', it's clearly somewhat open to interpretation. But for example, if one transformer serves the 2nd through 4th floors of a building, and the next serves the 3rd through 6th, then I think your areas served, and therefore your acceptable bonding locations, are pretty well defined.

So I read further (as always) to (3) Common Grounding electrode conductor. The exception states, "A separate bonding jumper for each derived system to metal water piping and to structural metal members shall not be required if the metal water piping and structural metal members in the are served by the separately derived system are bonded to the common grounding system. "

Seems like if I have a common grounding system, with the pipes and steel grounded in the main electrical room, the bond at the jumper just needs to go to the ground bar, which goes by a wire to the common grounding system in the main electrical room.

Thoughts?

 

[jaggedben]

That exception finishes with 'grounding electrode conductor' not 'system.

In other words, if you bond the transformer GEC to the steel and piping before sending it down to the main electrical room, you're done.

If steel and metal piping are present in your transformer rooms, you could bond them to the ground bars rather than to the transformer itself. Do you ever see that?

(Btw do I think this is all overkill? Probably, to some extent. But you've asked what code requires. )

 

[Anderman123]

That exception finishes with 'grounding electrode conductor' not 'system.

In other words, if you bond the transformer GEC to the steel and piping before sending it down to the main electrical room, you're done.

If steel and metal piping are present in your transformer rooms, you could bond them to the ground bars rather than to the transformer itself. Do you ever see that?

(Btw do I think this is all overkill? Probably, to some extent. But you've asked what code requires. )

Sorry, it does say "conductor"

I do see what you are describing. I'm just looking for what is required. It seems that if building steel or metal pipes are "in the area", then it wouldn't need to be bonded because it is already connected to the common grounding electrode.

And yes, all of this is probably overkill and overthinking.

 

[david]

I always found the design more simplified to do the bonding in the panel not at the transformer. nothing wrong with doing the bonding at the transformer though

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