Grounding a transformer

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Jeff Greef

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When a transformer is a separately derived system, it has to have a GEC to the electrode, or at least to the service ground bar, correct? And, the GEC has to be sized by 250.66 based upon the secondary conductors, right? So, when they pull feeders for the transformer, they have to include the GEC sized to the secondaries. As well, that conductor can function as a EGC for the transformer box and conduit. Do the special bonding rules for conduit into the transformer apply?

Thanks

Jeff G.
Bldg Insp.
Cupertino CA
 
Except as covered by the rare exceptions in 250.30, the equipment grounding conductor installed with the supply to the transformer is dis-associated with the grounding electrode conductor for the secondary.
As a general rule, the grounding electrode conductor will connect to it's own grounding electrode system as specified in 250.30(A)(7) and there will be no intended connection between it and the equipment grounding conductor of the supply.
 
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Thanks Augie-

So, if someone wants to put a transformer on an existing set of feeders somewhere in the building away from the services, they'll need to run a new GEC from the transformer to the electrode or service ground bar- is that correct?

Jeff G.
 
Jeff Greef said:
Thanks Augie-

So, if someone wants to put a transformer on an existing set of feeders somewhere in the building away from the services, they'll need to run a new GEC from the transformer to the electrode or service ground bar- is that correct?

Jeff G.
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250.30(A)(7) (2008 NEC) requires the secondary of the transformer to be connected to the nearest available metal water pipe or building steel grounding electrode. If neither of these electrodes are available then you can connect to another electrode as specified in 250.52(A).

Also a new section in the 2011 NEC section 250.121 prohibits an equipment grounding conductor from also being used as a grounding electrode conductor.

Chris
 
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