SDS Grounding

[Mgraw]

I am doing a job in a city I have not worked in before. My 600 amp MDP (277/480) feeds two sub-panels. I have two 3/4 ground rods and a 20' CEE plus building steel and the water line are bonded. Each sub-panel feeds a transformer(120/208 secondary). The Electrical Inspector wants a seperate ground rod for each transformer. The EE says no. I agree with the EE.(250.30(A)4). Am I mising something. No other jurisdiction I have worked in required seperate rods. I would appreciate any comments.

 

[iwire]

Each transformer must have a GEC run where did you run yours too?

 

[Mgraw]

The 150 kva is 5 feet from the MDP. The GEC bonds to the grounding lug in the MDP. The 75 kva is 100 feet away and is bonded to building steel and the grounding lug in the subpanel

 

[Smart $]

The 150 kva is 5 feet from the MDP. The GEC bonds to the grounding lug in the MDP. The 75 kva is 100 feet away and is bonded to building steel and the grounding lug in the subpanel

It is a little unclear how the 150kVA xfmr is bonded. I believe you can use the same GES as the service. See 250.64(C). The part that is a bit vague is where it says busbar can be connected together to form the grounding electrode conductor, but it does not state a means of transition from wire type to busbar type. Many here believe you must meet 250.64(C)(1)'s requirements (i.e. irreversible connections) rather than connecting another system's GEC with a terminal lug to the grounding busbar of the service-supplied system where its GES is landed. I'm on the fence in this matter.

For the 75kVA xfmr, that is all that is required, provided the 'subpanel' contains the system disconnecting means in the form of a main circuit breaker.

 

[Mgraw]

Thanks for your help. Would it be better to bond the 150kva to one of the ground rods instead of the MDP? One is right next to the transformer.

 

[Volta]

The electrode is to be the building structure or the water line 250.30(A)(7).
If not available, then a rod might suffice, but assuming you have no larger than 600 kcm on the secondary of the 150 kva, you need a 1/0 to the GES. If the connection from the rod to the rest of the Grounding Electrode System is not at least that size that would not comply for size reasons.

 

[Mgraw]

I have parallel 350's on the secondary and 4/0 on the GES

 

[Volta]

I have parallel 350's on the secondary and 4/0 on the GES

Well for 700 kcm your GEC must at least be a 2/0, and brought to the water pipe electrode or building structure if available.

 

[Mgraw]

Thanks I appreciate your help.

 

[dereckbc]

By chance did you run the GEC in metallic conduit?

 

[Mgraw]

No PVC for the GEC.

 

[RUWired]

The Electrical Inspector wants a seperate ground rod for each transformer. The EE says no. I agree with the EE.(250.30(A)4). Am I mising something. No other jurisdiction I have worked in required seperate rods. I would appreciate any comments.

You should be quoting 250.30(A)7(1) and (2). Since building steel and water were used for the GES they are available and in the area.

The 150 kva is 5 feet from the MDP. The GEC bonds to the grounding lug in the MDP.

This would be allowed if the MDP had a bus bar with a minimum size of 1/4" x 2".(250.64(F)3 and is inter connecting all the GES and meets the requirements of 250.30(A)7. If not then building steel and water would be required. Again for 150 kva a rod is not needed since water and steel are available.
Rick

 

[augie47]

agree. ground rods would only be used if you did not have the water or steel available as shown in 250.26(7) and the 150 kva should have a GEC to the water or steel not to the bussbar.

 

[Mgraw]

I understand what you say but the Electrical Inspector says he wants a ground rod at each transformer. He says he requires all transformers to have their own ground rod. I have given up arguing with him and am letting the EE and the EI fight it out. Told both to let me know what they decide. Anything other than what has been done is an extra.

 

[Smart $]

agree. ground rods would only be used if you did not have the water or steel available as shown in 250.26(7) and the 150 kva should have a GEC to the water or steel not to the bussbar.

250.26(7)? I believe you meant 250.30(7).

Running an SDS GEC to the grounding bussbar of the MDP is effectively the same as running to all available electrodes present at the building or structure as required by 250.50, including water line and building steel.

 

[Smart $]

I understand what you say but the Electrical Inspector says he wants a ground rod at each transformer. He says he requires all transformers to have their own ground rod. I have given up arguing with him and am letting the EE and the EI fight it out. Told both to let me know what they decide. Anything other than what has been done is an extra.

Another case of an electrical inspector trying to instill unwritten code upon contractors (unless locally mandated... which I doubt).

 

[Mgraw]

Since 2007 we have a Statewide code, no local amendments allowed.

 

[augie47]

250.26(7)? I believe you meant 250.30(7).
.

correct. I should know not to trust memory.

Running an SDS GEC to the grounding bussbar of the MDP is effectively the same as running to all available electrodes present at the building or structure as required by 250.50, including water line and building steel.

Effectively the same, yes. But the way I read 250.30(A)(3) it says
"connect the grounded conductor of the SDS to the grounding electrode", not to an effective path.
I may fall in the category of those inspectors you dislike, but the book tells me "connected TO THE ELECTRODE" that's what I look for.

 

[RUWired]

He says he requires all transformers to have their own ground rod.

So if you have a high rise with transformers up on the floors, ask him where do all the rods get driven. I think a few show and tell articles will change his mind.

 

[Smart $]

...

Effectively the same, yes. But the way I read 250.30(A)(3) it says
"connect the grounded conductor of the SDS to the grounding electrode", not to an effective path.
I may fall in the category of those inspectors you dislike, but the book tells me "connected TO THE ELECTRODE" that's what I look for.

I understand and agree that is the letter of the code.

However I ask you to consider which connection method provides better grounding. The grounding electrode system for the service is required to connect all available electrodes in the building or structure, while the SDS requirement is but to a single elctrode. Either way, both are bonded together, one via the SDS GEC using the same electrode, the other via the SDS GEC to the GES. IMO, connecting the SDS GEC to the single electrode is bass ackwards if the service GES is closer. When the service GES is closer, running the SDS GEC to the specified grounding electrode more than doubles the effective resistance to the other electrodes.

If I remember, I'll put in a proposal to this effect for 2014.