grounding SDS NFPA Informal Interp. or When Inspectors Collide

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sandsnow

sandsnow

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Submitted for your consideration

We came to an impass at work over this subject. It concerns an electrode installed for an SDS and does it have to be bonded back to the GES created at the service. We agreed to let an NFPA informal interpretation be the tie breaker. Can't say as I agree but it is what it is.

My position is that the electrode is bonded back to the GES via the EGC.

HERE'S THE QUESTIONS I POSED AND THE RESPONSE:

I have a request regarding NFPA 70 - 2002.

A transformer is installed in a large wood frame warehouse. There is not any water piping or building steel available as in 250.30(4). The transformer is required to be grounded. The transformer is grounded to a rod installed to meet the requirements of 250.53(G) and 250.56.

Question:

1. Is it the intent of the Code that this rod be bonded back to the grounding electrode system described in 250.50?
2. If Q1 is yes, is it the intent of the Code that this conductor be a minimum #6 copper and installed in addition to the Equipment Grounding Conductor installed with the primary conductors?
3. IF Q1 is yes and Q2 is no, is it the intent of the Code that any properly sized Equipment Grounding Conductor with the primary conductors serve as the bonding means described in question 1?
4. If Q3 is yes, does the Equipment Grounding Conductor have to meet the requirements in 250.64(E)?
5. If Q1 is yes; Q2 is no and Q3 is no, is it the intent of the Code that the Equipment Grounding Conductor with the primary conductors be equal to or greater than the size required by Code for the Grounding Electrode Conductor grounding the secondary side of the transformer to the ground rod?

NFPA Response:

Question # 1 Yes. In accordance with 250.50 all grounding electrodes present at a building or structure described in 250.52(A)(1) through (A)(6) shall be bonded together including grounding electrodes that are installed in accordance with 250.30(A)(7) Exception No. 1. In addition, 250.58 requires the use of a common grounding electrode for grounding multiple ac systems (could be supplied by utility service(s), separately derived system(s) or a combination of both) that supply a building or structure.



Question # 2: Yes. A bonding jumper installed in accordance with 250.53 shall be used to bond the multiple electrodes together. The equipment grounding conductor of the feeder circuit supplying the transformer cannot be used as the means to accomplish the 250.50 requirement to bond together all grounding electrodes that are present at a building or structure. The bonding jumper(s) used to connect the multiple electrodes together shall be sized in accordance with 250.66 based specific type(s) of grounding electrode(s) that the bonding jumper(s) is being connected to.



Question # 3: No. The equipment grounding conductor is required to be run with the primary circuit conductors to the transformer and connected to the equipment grounding terminals at the point of supply and at the transformer. Bonding jumpers used to connect grounding electrodes together are required to be connected to the electrode in accordance with 250.70 and it is intended that this be a direct connection between the bonding jumper and the grounding electrode and not through the equipment grounding terminal of electrical equipment.



Question # 4: No. See answers above regarding the use of the equipment grounding conductor as a bonding jumper for connecting multiple electrodes together.



Question # 5: No. See answers above regarding the use of the equipment grounding conductor as a bonding jumper for connecting multiple electrodes together.
 
Larry Thanks for sharing that.

How long did it take to get your reply?

If I recall correctly Don has put in proposals asking that the EGC be allowed to be used as the GEC but they have been rejected.

We work in a lot of old brick and wood mill buildings that have been updated with 480Y/277 services.

We end up having to run a GEC from each SDS back to the service using the rules in 250.30(A)(3) GEC taps.

Sometimes the distance may be as much as six floors vertical and several hundred feet horizontally, it is a major pain when there is already an EGC doing the same thing.
 
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Bob,
My proposal was to eliminate the requirement for the local grounding electrode and use the primary EGC for both functions. They rejected the idea of eliminating the local grounding electrode. I see nothing in the code that says that the primary EGC cannot also serve as the bonding conductor beween the main and SDS grounding electrodes, assuming that it is of the correct size and that the primary circuit originates in the service equipment. As far as question #3, it could be a direct connection if the SDS system bonding jumper is in the transformer.
Note that these informal interpretations are not binding because the answers were not processed by the code making panel.
Don
 
Bob,
It took a couple weeks. Less complicated questions get a quicker reply.

Don,
You are correct in that it is not binding. I was fully aware of that. Short of boxing gloves, it was the most official way to solve the impass.

Seems to be an awful lot of work for no benefit. I'd be more than willing to listen if someone could show by math or incident the danger of not bonding the electrodes together directly.

It does not sit well with me. It's on my list to get a formal interpretation. I would think that now would be a good time since the panels are meeting for ROC soon. Or have they already met?
 
If you go to a Mike Holt Grounding vs Bonding Seminar, he makes the exact same point, that the transformer secondary is already bonded to the primary GES.
He states we are not really sure why a GEC is required for a transformer, except its the way we've always done it.
 
tom baker said:
He states we are not really sure why a GEC is required for a transformer, except its the way we've always done it.
Click to expand...

There must have been a reason way, way,way back when it was first put in the Code. Maybe a long forgotten reason.
 
Larry,
Oh, we all know why: in case lightning strikes the secondary conductors.
Click to expand...
I tried to cover that in my propsal by saying that a local grounding electrode is not needed for a SDS that is within the same building as the primary OCPD, but the proposal was still rejected.
Don
 
sandsnow said:
Seems to be an awful lot of work for no benefit. I'd be more than willing to listen if someone could show by math or incident the danger of not bonding the electrodes together directly.
Click to expand...

I don't know enough to show you the math, but my _hunch_ is that bonding the various grounding electrodes together directly is important because _indirect_ lightning strikes can travel into the various grounding electrode conductors via one grounding electrode, and then out via another.

This would be the same reason that all grounding electrodes must be bonded together in the first place.

Of course, this does not explain why 'supplemental' grounding electrodes can be connected to the grounding electrode system via an EGC, and more importantly this does not explain why this SDS must have its own grounding electrode in the first place. IMHO Don's proposal is quite reasonable and would save wasted effort.

-Jon
 
Larry: The reason Mike suspects we install a GE and GEC for an indoor transformer is when we needed more power in buildings (as they became larger) higher voltage was brought in and stepped down. Since transformers outside had GEs, then we need the same for an indoor transformer.
And he says thats the way we've always done it....
 
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