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Electrical Question

Discussion in 'Electrical Engineering' started by obanion, Oct 18, 2003.

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  1. obanion

    obanion Guest

    I know the basics on residential wiring, and have done various indoor wiring
    projects, but I have a question for you regarding an outdoor project. I
    want to install a 30 amp 110V receptacle for an RV. I purchased a
    receptacle that is already mounted inside of a metal box. I am planning on
    mounting it to a metal shed. The receptacle will be on its own 30 amp
    circuit on 10# wire. From the factory, the receptacle is grounded to the
    metal box by a wire from the grounding terminal on the receptacle to a
    terminal mounted on the box. If the receptacle is grounded to the metal
    box, how should I ground the box? If I were to remove this wire and connect
    the receptacle to the grounding wire going back to my service panel, the box
    would not be grounded. Would this be safe? Any advice that anyone could
    give me would be much appreciated.
     
  2. Zathera

    Zathera Guest


    Be sure to call your trenching call center before you start. I just replaced
    my pool circuit, 20 years old and it rusted apart under the kool deck. I
    already knew where everything was but if you hit something your going to pay
    for it unless you call first. Once that is done you could just buy some UF
    cable, as long as it is protected by conduit once exposed. Uf is designed
    for underground use. It is a bear to strip and terminate but quite useable.

    http://www.electrician.com/vd_calculator.html Will allow you to figure out
    the voltage drop based on the distance. Based on a 30 amp load, starting
    loads etc, your run should be less than 80 feet or your going to need larger
    wire.

    Connect the ground conductor to the box, using a screw or lug. There should
    be something in the box already. Driving a ground rod is a NEC code
    violation. This outlet is not a service nor is it a permanent structure. If
    you install a ground rod then you will be creating a ground loop. Example A
    fault or short will travel to the 30 amp outlet box see the ground rod, then
    travel through the ground to the electrical service and finally trip the
    breaker, maybe. Depends on the soil. Using a ground conductor it will travel
    on it and trip the breaker guaranteed.

    Be safe out there.
     
  3. Ben Miller

    Ben Miller Guest

    Cite NEC reference(s) please.
    Driving a rod does not violate anything. After all, we don't isolate
    everything from metal building structures (as in this case, mounted to a
    metal shed), we routinely bury metal conduit, we bond to buried water pipes,
    etc. There are usually many connections to earth in a system. Although there
    may be no reason for it, there is nothing wrong with grounding the box to
    the earth, including adding a ground rod, as long as there is an egc
    conductor, metal conduit, etc. that completes the path back to the service
    point. You can not rely on the earth as the sole fault current path, but it
    can be, and almost always is, part of it. Most of the current will flow
    through the metal path.

    What is important is that the neutral is not bonded to the ground anywhere
    except the service entrance, other than the cases of a permanent structure
    with multiple receptacles and no egc, which is being covered in other
    threads.

    Ben Miller
     
  4. Zathera

    Zathera Guest


    all from the 96 NEC

    250-91 a b c

    250-53

    250-23 a last sentence
    "a grounding connection shall not be made to any grounded circuit on the
    load side of side of the service disconnect means."

    250-24 does not apply as this is a outlet not a building
     
  5. Ben Miller

    Ben Miller Guest

    These discuss the grounding elecrode conductors at a service, and equipment
    grounding conductors. Nowhere does it say that the egc anywhere in the
    system can not be grounded to earth. As I pointed out previously, it will be
    in many cases.
    The correct quote is"... grounded CIRCUIT CONDUCTOR..."

    This refers to not grounding the neutral downstream of the service panel,
    which I believe everyone here agrees with. I stated that in my post as well.
    It says nothing about not connecting a grounding electrode to the enclosure,
    which is not a circuit conductor.

    Ben Miller
     
  6. MainTech

    MainTech Guest

    This wire that goes from the receptacle to the box is called a bonding
    jumper. I would assume that the receptacle is actually mounted to the cover
    of the box instead of the box itself. Am I right? IAW NEC article 250.146,
    this bonding jumper is required to insure a low resistance to ground in case
    of a fault. My recommendation is that you leave this wire intact and run
    your ground to either point, the receptacle or the box since, with this
    jumper, both will be grounded no matter what, i.e. loose cover screws.
     
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