Hello to All

Need your feed back on a transformer located in a tenant buildout electrical room (existing building)there is no steel present in the building and plans show 1 driven rod to Ground for the transformer; the main electrical room is about 200ft away. Do you agree that a 2nd ground rod is needed to supplement at the electrical room tenant buildout?

I don't yet agree that a ground rod can be used at all. Only if building steel and metal water pipe are not available, then other electrodes may be used based on 250.50 and 250.52. Also check for some type of primary ground.

How is the service grounded???

I agree with Nick, Find out how the service is grounded first.

Yes. A grounding electrode is required for a separately derived system along with a system bonding jumper per 250.30. As Nick stated, you must use the building steel or metal water piping as near as possible to the transformer. 250.30(a)(7). If they are not availabe than a ground rod installed near the transformer is ok. 250.30(a)(7) exc.1.

Back to the original question, if building steel and or metal water pipe is unavailable, and a ground rod is used instead of running back to the service grounding electrode system, a second ground rod would be required only if the grounding electrode system had a resistance greater than 25 ohms. You could ask for a 3rd party mig test if you suspect the GES resistance is high. If the contractor decides to get the mig test and the system does have a resistance higher than 25 ohms, then all they are required to do is add one additional ground rod and no additional testing is required by the code. Most contractors will just install the 2nd rod and save the $1,200 go get an engineer with calibrated equipment to perform a mig test.

IMHO
Neal Burdick
City of Tampa Construction Insprctor II

FYI, the whole 250.56 - "25 ohm" section has been deleted in the 2011 NEC. It will still show up as a FPN, but I can live with that...

Thank's for that info Bryan. So that means that we can't require a 2nd rod with the 2011?

Wouldn't you still need to bond this ground rod to the building GES?

Of course...an isolated electrode can not be used. The GES is a system after all and all separately derived systems and all utility services of 1ph. and 3ph. are required to be interconnected to the same system; otherwise we have voltage (potentials) between adjacent equipment fed from different systems...Connection to the GES (earth) draws all grounded conductors to earth potential...(hopefully 0 volts)...similar to bonding which interconnects equipment enclosures to eliminate potential between them...