I live in Mason County, in Washington State. I am installing a 100amp RV pedestal (ground mounted). I am running 4/0,4/0,20 triplex direct bury cable (not sure if going in conduit or not yet)about 160ft from the power companies transformer box that is on my property. I understand I will need a 8ft long grounding rod, but do I need to have 2 grounding rods for 100amp service? I am not sure if that only applies to 200amp service. And do I need 5/8" or 3/4" dia. Thanks in advances Dean

  • Why 4/0 for a 100A? Apr 16, 2021 at 3:05
  • Having 4/0 in the ground would certainly make a heavy-up easier. Apr 16, 2021 at 3:53
  • Keep in mind that if Code requires you have a disconnect out at the transformer box, then you must run 4-wire, not triplex. The first disconnect past the meter is the service point. You run 3-wire from the weatherhead through the meter to the first disconnect, master ground is there, then 4 wire past the disconnect. Apr 16, 2021 at 4:02
  • My pedestal is my meter. It's all in one. Meter on top, 3 breakers in panel below and 50amp/30amp & 20amp outlets. I was told by the PUD3 engineer that I needed to run triplex. I am running 4/0 for possible upgrade to 200 amp at later date. What does “4/0 ground would make a heavy up easier” mean
    – Dean
    Apr 17, 2021 at 0:08
  • Harper, additional question about your comment on needing a disconnect out of the transformer. With my pedestal being meter, breakers and outlets does that have me covered going directly from my transformer and connecting to the pedestal with my triplex and not needing any other disconnect? From what I have read I should be ok.
    – Dean
    Apr 17, 2021 at 0:31

1 Answer 1


The NEC and your state codes do not reference size of service.

Washington Administrative Code 296-46B-250-053 (A)(2) Resistance of rod, pipe, and plate electrodes. (3) For rod, pipe, and plate electrodes other than those installed in accordance with the exception in subsection (2) of this section, if a ground resistance test is not performed to ensure a resistance to ground of 25 ohms or less, two or more electrodes as specified in NEC 250.52 must be installed a minimum of 6 feet apart. A temporary construction service is not required to have more than one made electrode.

NEC 250.52 says "at least 15.87 mm (5/8 in.) in diameter, unless listed."

  • I think in most cases you would just use dos ground rods to avoid the necessity of measuring, and also because adding one later is a real pain, especially if you already have the ground rod pounder out.
    – K H
    Apr 17, 2021 at 4:31

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