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Grounding subpanel on outbuilding

Houghton123 | Posted in Construction Techniques on March 27, 2008 06:46am

We’re finally trenching the 40 or so feet from our house to the tankhouse to run proper electrical to my shop.  House electrical was done in the mid-1990s, with ground rod at main panel properly bonded and all.  This new subpanel will run on a subpanel at the back of the house, wires through buried conduit to the tankhouse.  Subpanel at tankhouse probably 40-60 amps, 220 volts (can’t remember the size of the conduit we ran under the foundation when we installed drain tile, so amperage will depend on what we can install within voltage drop limits for the distance).

A question: is the ground wire from the house panel to the tankhouse subpanel sufficient?  Should I be installing a ground rod at the tankhouse in addition to the ground wire from the house electrical, or does this create a floating ground problem?

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Replies

  1. davidmeiland | Mar 27, 2008 07:21am | #1

    In this jurisdiction we are required to install a ground rod or Ufer for a subpanel in a detached building such as you are describing.

    1. plumbbill | Mar 27, 2008 07:38am | #3

      Dave is the ground rod seperate local code?

      I could not find a grounding rod requirement in the NEC----- granted I don't have access to the latest & greatest update.

      “The world is a dangerous place, not because of those who do evil, but because of those who look on and do nothing.” —Albert Einstein

      1. davidmeiland | Mar 27, 2008 07:42am | #4

        I have done lots of jobs where we have installed a subpanel in a detached building (garage, carport, shop, etc.) fed from the main house panel. There is always a Ufer (preferred) or ground rods (plural) in that scenario.

        Now, possibly what the OP is describing is different, a subpanel fed from a subpanel. I don't know how that plays out.

        1. plumbbill | Mar 27, 2008 07:48am | #5

          I missed the sub to sub part of his post---- that is different.

          My question was more towards the grounding rod.

          We usually put them in, but the distance was substantial & not cost effective to rune that much wire for a ground.

          I couldn't find a ground rod being required in the NEC, nor did they show one in the illustrations, but whadda I know I was called a "hack" yesterday ;-)

          “The world is a dangerous place, not because of those who do evil, but because of those who look on and do nothing.” —Albert Einstein

        2. TJK | Mar 28, 2008 03:55am | #8

          "Now, possibly what the OP is describing is different, a subpanel fed from a subpanel. I don't know how that plays out."I put up a detached shed, and it's a subpanel fed from a subpanel. The feeder is a single, 15A circuit from the subpanel. I was advised to keep the feeder ground and neutral separate at the shed, and do not bond the feeder ground to the metal panel in the shed. The metal panel in the shed is bonded to a ground rod at the shed, but the circuit ground comes from the sub panel 150' away.

    2. RTSlater | Mar 28, 2008 04:16am | #9

      In this jurisdiction we are required to install a ground rod or Ufer What is a Ufer?
      RTSlater

      1. User avater
        BillHartmann | Mar 28, 2008 04:43am | #11

        Ufer - concrete encased electrode.The rebar or a separate copper wire is put in the footing and used for the ground electrode. It needs to be at least 20 ft long..
        .
        A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.

  2. plumbbill | Mar 27, 2008 07:37am | #2

    A question: is the ground wire from the house panel to the tankhouse subpanel sufficient?

    Yes, if you can run a big enough wire #8 IIRC.

    Should I be installing a ground rod at the tankhouse in addition to the ground wire from the house electrical

    Not required, but if you do decide to install one make sure that it is bonded to the ground that is comming from the main service.

    NOTE***************

    Do NOT bond the neutral & grounds together in the sub panel!!!!!!!!!!!!!

    “The world is a dangerous place, not because of those who do evil, but because of those who look on and do nothing.” —Albert Einstein

    1. renosteinke | Mar 27, 2008 04:39pm | #6

      A detached structure, with more than one circuit, is required to have a ground rod (or similar electrode). This is in addition to the ground wire that comes along with the feeders. Since he's installing a panel, there's no doubt these are 'feeders,' and not a single branch circuit. And, yes, the correct practice is to keep tha grounds and neutrals completely separate.

      1. plumbbill | Mar 28, 2008 03:03am | #7

        Yeah i wasn't sure on the rod part, the NEC I have didn't state that it was required, but like I said it's not the latest & greatest.

        We always had ground rods, but the structure was always so far away it wasn't cost effective to run a ground from the panel.

        Did you notice in his post that he is feeding this structure from a sub panel & not the main?

        “The world is a dangerous place, not because of those who do evil, but because of those who look on and do nothing.” —Albert Einstein

        1. renosteinke | Mar 28, 2008 05:14am | #12

          I did ... it's really not an issue. He would only connect the ground wire to the separate ground buss in the sub panel, rather than at the common buss as he would at a main panel.

          1. Houghton123 | Mar 28, 2008 06:47am | #13

            I had expected to keep the ground and neutral separate - it's always been my understanding that they're connected only at the main, and the subpanel has separate ground and neutral busses.

            So the consensus seems to be that I'm best off to run four wires - the two 220 hots, a neutral, and a ground - AND install a driven ground rod.  This makes sense to me - seems like it never hurts to stay grounded (sorry - West Coast joke).

            Thanks for all the replies - off to the supply house tomorrow, I think.  Now, if only I could remember what size the conduit is...

          2. Houghton123 | Mar 28, 2008 06:49am | #14

            Oh, and incidentally - a local card-carrying journeyman electrician who seems to know his stuff says that running a subpanel from a subpanel is perfectly legit, as long as the sub-subpanel's main breaker is equal to or smaller than the subpanel's main breaker.  I'm planning to go down from the subpanel's capacity, so it shouldn't be a problem.

  3. User avater
    BillHartmann | Mar 28, 2008 04:41am | #10

    Each structure needs a ground electrode system.

    Under the 2005 and earlier code you have the option of running 3 wires (neutral and 2 hots) IF there is no other metalic pathes from this structure to the sturcture with the source. In that case the ground and neutral are bonded at the sub-panel just as in the main panel.

    If there is a metalic path (metal water pipe or cable or phone) then you must run a separate EGC from the main panel to the sub-panel and isolate the neutral bus in the sub-panel.

    I understand that some locals don't allow the 3 wire system. And that the 2008 does not allow it, but I have not seen a copy and many areas have not yet adopted it.

    Many people recommend the 4 wire system as being the best option and allows for future metallic paths.

    .
    .
    A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.

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