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Thread: Garage grounding and bonding questions.

  1. #1
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    Garage grounding and bonding questions.

    Hello I have a 3 wire overhead line coming from the house which comes from the service on its own 70 amp disconnect.

    The garage had fuses and now has a breaker panel, so I have the two hots and the neutral/ground coming into the box from the service disconnect.

    The neutral bus has the ground from the service, and there is an optional lug to bond the neutral bus and ground together.
    I assume I use the lug when I have an old 3 wire service coming in but I was not sure.

    If the housing wiring gets redone and changed to a 4 wire garage service do I separate the two and run a wire (green) to the ground bus and a wire (white) to the neutral bus?


    Part 2

    The garage did not have any sort of grounding actually running to the ground so I have bought two 8 foot grounding rods and #6 bare wire to run from the rods to the ground bus in the panel.

    Are there spacing requirements for separating the two grounding rods from each other?

    Do they have to be driven into the ground vertically or can I dig two trenches and lay them horizontally next to the slab?

  2. #2
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    Read 250.53(A) and (G) for information on your rod questions.

  3. #3
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    You are allowed to reground the neutral bus for existing installations only. If you replace the wiring from the house to the garage you must run a 4-wire circuit with a ground sized according to Table 250.122.

    The ground rods must be spaced 8 feet apart at a minimum but the further apart you place them the better.

    If there are any common metal systems between the garage and the house such as metal water pipes it is absolutely imperative that the neutral be isolated from the equipment ground and a new feeder run from the house with a dedicated equipment ground. If there are parallel paths for the neutral current to return to the service then neutral current will be present on the equipment ground.

    The ground rods are allowed to be buried in a trench horizontally only if it is impossible to drive them in vertically. You will get suboptimal results by laying them in a trench rather than driving them in.
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  4. #4
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    Quote Originally Posted by DrSparks View Post
    ....The ground rods are allowed to be buried in a trench horizontally only if it is impossible to drive them in vertically. You will get suboptimal results by laying them in a trench rather than driving them in.
    Is that because the ground rods are laying down on the job?
    If you go and decide to dance with a gorilla the dance ain't over till the gorilla decides it's over.

  5. #5
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    Quote Originally Posted by ActionDave View Post
    Is that because the ground rods are laying down on the job?
    If only there were a roll your eyes emoticon.

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  6. #6
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    Quote Originally Posted by DrSparks View Post
    You are allowed to reground the neutral bus for existing installations only. If you replace the wiring from the house to the garage you must run a 4-wire circuit with a ground sized according to Table 250.122.

    The ground rods must be spaced 8 feet apart at a minimum but the further apart you place them the better.

    If there are any common metal systems between the garage and the house such as metal water pipes it is absolutely imperative that the neutral be isolated from the equipment ground and a new feeder run from the house with a dedicated equipment ground. If there are parallel paths for the neutral current to return to the service then neutral current will be present on the equipment ground.

    The ground rods are allowed to be buried in a trench horizontally only if it is impossible to drive them in vertically. You will get suboptimal results by laying them in a trench rather than driving them in.
    6' min on the rods (mis spoke?) But are you saying if existing rigid conduit with 3 conductors you can't reuse it?

  7. #7
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    Quote Originally Posted by DrSparks View Post
    If only there were a roll your eyes emoticon.

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    You mean this one

    I also believe the NEC only requires 6 feet minimum between the driven rods. 8 feet might be better design in many cases though.

  8. #8
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    Quote Originally Posted by GerryB View Post
    6' min on the rods (mis spoke?) But are you saying if existing rigid conduit with 3 conductors you can't reuse it?
    you could use rigid conduit as the equipment grounding conductor.
    Bob

  9. #9
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    Quote Originally Posted by petersonra View Post
    you could use rigid conduit as the equipment grounding conductor.
    I apologize. The code minimum is 6 feet. I always use the length of a ground rod as a meter to space my ground rods apart. But it still stands that the further apart do you place them the lower the impedance to ground you will have.

    If you have buried ferrous metal conduit then there's a good chance that the conduit has long rotted away. I would not rely on the Conduit as an equipment ground. I would be amazed if you were able to pull new wire through the existing conduit and if you were able to I'm sure it would be caked with mud and rust.

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  10. #10
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    Quote Originally Posted by DrSparks View Post
    I apologize. The code minimum is 6 feet. I always use the length of a ground rod as a meter to space my ground rods apart. But it still stands that the further apart do you place them the lower the impedance to ground you will have.

    If you have buried ferrous metal conduit then there's a good chance that the conduit has long rotted away. I would not rely on the Conduit as an equipment ground. I would be amazed if you were able to pull new wire through the existing conduit and if you were able to I'm sure it would be caked with mud and rust.

    Sent from my A574BL using Tapatalk
    Also it is also common practice to run PVC underground and then transition to RMC as it emerges from grade. This would obviously interrupt ground continuity of the pipe.

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