Transformer (seperately derived) Bonding

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mwh1023

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I have a 225 kVA 480:120/208 3-phase transformer located 100' from the Main Distribution Panel. The engineered plans have the transformer being fed from a 350A 3-pole 480V breaker with a 3-1/2" conduit with (3) 500 kcmil conductors and a #3 copper equipment ground. The transformer feeds a 600 amp 120/208V 3-phase distribution panel. The drawings call for a parallel set of 500 kcmil conductors from the secondary of the transformer to the distribution panel 600A main breaker. Based on these conductors, the drawings also call for a 2/0 copper to the water pipe, building steel and ground rod.

Here is my question, why can't I substitute the #3 copper equipment ground in the feeder conduit to the transformer with a 2/0 copper conductor? This would bond the transformer case and the XO terminal (grounded terminal) back to all of the building electrodes that have already been established at the MDP? the 2/0 is sized off of the secondary ungrounded service conductors of the transformer secondary and would provide an easy way to bond the seperately derived service. The copper water piping in the vicinity of the transformer are the same copper lines near the service as the office building isnt that large.

I appreciate any thoughts on this matter.

Thanks,

Mike
 
augie47 said:
Sounds reasonable, however, 250.30(A)(7) does not allow it.
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Which refers you to 250.64 A, B, C & E, which part wouldn't allow it? If I keep the #3 equipment ground in my feeder conduit, then it's ok to run the 2/0 back to the MDP in a seperate conduit assuming I use all required bonding methods? Seems odd that I have to bond the steel, water and ground rod not far from where I have already established these methods?
 
I think the part that causes problems is 250.30(A)(7) that states "grounding electrode shall be the nearest one of...."

add to that the fact that exception 2 provides the condition where you can do as you suggest, but your situation does not fit those parameters.
 
That would be using the primary EGC as the secondary GEC and is a violation of a new rule in the 2011 code. See 250.121.
 
don_resqcapt19 said:
That would be using the primary EGC as the secondary GEC and is a violation of a new rule in the 2011 code. See 250.121.
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Not necessarily. If the conduit or raceway qualifies as an EGC, his proposal is sound.

Also, I'm wondering, let's say the raceway does not qualify as an EGC. For example, say the MDP and xfmr are close to each other, and the raceway is FMC and cannot serve as an EGC. The perplexing issue is that no EGC is required. The GEC/EGC connections (perhaps SBJ, too) in the transformer take care of the xfmr enclosure being connected to an EGC.

Look at the conditions which require an EGC, in 250.110, which says only that the NCCMP's must be connected to an EGC. It does not say from where the EGC must originate. With both the MDP and xfmr enclosures being connected to anEGC. The only thing left is that the FMC must be connected to an EGC... not that it be an EGC, nor that a separate EGC must be run from MDP to xfmr.

Perplexing, huh??? Well if there is any acceptance on that interpretation, then the next question is, what if the primary conductors are run in PVC? :huh::cry:

PS: worked backshift and haven't been to sleep yet. no apologies, but it may explain the divergence... :sleep:
 
augie47 said:
I think the part that causes problems is 250.30(A)(7) that states "grounding electrode shall be the nearest one of...."

...
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Yeh, it does say the nearest electrode... but it don't say how the GEC gets routed, or in the case of structural steel being the closest electrode, at what point the GEC is to be bonded.
 
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In my opinion if the conductor is in the raceway and connected to the transformer enclosure is it an equipment grounding conductor for the primary, even if it is not required.
I don't really see any technical issue or safety issue with using a common conductor for both purposes, just a code issue.
 
don_resqcapt19 said:
In my opinion if the conductor is in the raceway and connected to the transformer enclosure is it an equipment grounding conductor for the primary, even if it is not required.
I don't really see any technical issue or safety issue with using a common conductor for both purposes, just a code issue.
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Just what urges you to say it is an equipment grounding conductor? It can easily be argued there is more specific text requiring a GEC to be run to the transformer than there is an EGC.

Consider for a brief moment (or substantially longer, if you wish :p) that the new section, in the part about EG/EGC nonetheless, states you cannot use an EGC as a GEC. Nowhere, including the part about GES/GEC, does it say you cannot use a GEC as an EGC, or rather as a means of equipment grounding.

I'll let y'all argue the semantics... :D
 
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