National Electrical Installation Standards

Standards as High as Your Own

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Monday, February 11, 2019

Question:

I read on a webpage that you consider a RV pedestal as a “separate structure” and must contain a ground rod to be driven at each location. However, my reading of what follows contradicts that. Can you let me know if I’m wrong?

The only equipment requiring grounding electrodes is the electrical service equipment.  250.54 contains permissive, but not required, language that allows for auxiliary grounding electrodes to be installed and connected to the equipment grounding conductor. Keep in mind that the pedestal is equipment, and NOT A STRUCTURE, so there are no NEC code requirements for having grounding electrodes on equipment that does not require them.  Only equipment for separately derived systems and buildings or structures supplied by feeders or multiple branch circuits (other than multiwire) are required to have grounding electrodes.

Auxiliary electrodes serve no real purpose, you will have to have an equipment grounding conductor installed with your feeder conductors to each pedestal.  The EGC will bond all of the non-current carrying metal parts back to source of electricity, the electrical service.   Some people will argue that extra ground rods are good for lightning protection, but that's not how grounding works.  The earth is not an effective fault return path, but your EGC is.  If lightning stuck a pedestal with rods attached, the energy will dissipate through the earth. As the potential dissipates, it spreads out from the location of the ground strike.  As soon as this energy comes across other pedestals with ground rods, there will be a difference of potential between the pedestals and ground rods and the current will now be induced onto the equipment grounding conductor and will pass through every piece of equipment connected to the system.

I have seen this many times with nonseparately derived system generators where installers connect an unnecessary ground rod to the generator frame because UL requires a terminal for such a connection.  They see the terminal and assume it's required. The transfer equipment, not the generator determine if grounding electrodes are required. As soon as there is a lightning storm, the controller in the generator gets hit with a power surge and shorts out along with all of the electronics connected to the system. Unfortunately, this also happens where there are multiple building on structures connected to a common electrical service and requires the installation of multiple surge protective devices.

Mike Lamberton

A

Answer:

Hey Mike thanks for your question. There were some changes in the 2017 NEC that were intended to clear up some confusion. Grounding electrodes are not required for power outlets or recreational vehicle site supply equipment unless they are used as service equipment as stated in 551.75(B). The words "other than equipment" were added to the definition of Structure in Article 100 to help in this effort also.

I expect that not everyone will agree with you about auxiliary grounding electrodes, as allowed by 250.54, not serving a real purpose.

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