Shouldn't Light Fixtures Have Bare Grounding Wire Connected?

Discussion in 'Electrical Forum discussion & Blog' started by DavidSeon, Dec 11, 2012.

  1. jwelectric

    jwelectric Electrical Contractor/Instructor

    Messages:
    2,534
    Location:
    North Carolina
    You don’t have to run a wire all the way out to the ground rod to take a measurement. All you need to do if read the continuity of the green or bare and the white in the fixture.

    In the service equipment the neutral (white), the equipment grounding (green or bare), the metal enclosure, and the ground rod will bond together so there is no need for the zip cord all the way back to the rod.
    The path that you were reading was from the meter through the zip cord to the rod up the grounding electrode conductor to the neutral in the service back down the neutral to the meter.

    All the rod does is the following;
    250.4(A) Grounded Systems.
    (1) Electrical System Grounding. Electrical systems that are grounded shall be connected to earth in a manner that will limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines and that will stabilize the voltage to earth during normal operation.


    As you can see the only purpose of the rod is incase lightning strikes or should there be a surge of voltage due to the switching of grids, should the conductors on top of the pole break due to ice or wind and fall down on the ones underneath and to keep the voltage of earth’s magnetic field and the voltage on the neutral conductors the same (stabilize).
  2. DavidSeon

    DavidSeon New Member

    Messages:
    47
    Location:
    MS Gulf Coast
    Yeah, I kind of figured that. I did first verify continuity between the bare grounding and neutral conductors at the fixture as DonL suggested. I think I just got paranoid after finding so many fixture grounding wires hanging loose that I was afraid maybe they could have left things hanging loose in the main panel. In my simple mind I thought I was just making sure everything was properly bonded to the rod and really at earth ground.

    I just can't get my brain around the principles of ground vs neutral in building wiring. I don't understand, if they both have the exact same continuous path back to each other and to earth ground, and apparently even back to the power company, what purpose the additional bare conductor has. Why do we care which of the two "ground" conductors the electrons use; I know this is wrong but to me, it seems like you could just use the neutral for grounding fixture housings and do away with the grounding conductor.

    I guess that's just one of the reasons you're a pro and I'm NOT :)

    Thanks again to everyone who replied.
    Dave
  3. big2bird

    big2bird IBEW Electrician

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    Location:
    Anaheim, Ca.
    Oversimplified, a nuetral is a current carrying conductor that "happens" to be at ground potential.
    A ground conductors sole purpose is to bond all the metal parts, and to carry any fault current.
    Even though they are at the same potential, they have differnent distinct "chores."
    IF the nuetral was NOT at ground potential, and you looked at it as a hot wire, you would not be confused.
  4. jimbo

    jimbo Plumber

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    Location:
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    Once a conductor is carrying current, then it becomes a load...meaning it has resistance and Ohm's law kicks in. The voltage at the source of the current will be I x R. A point in the neutral system can conceivably have several volts potential, with respect to earth ground. IF that point were tied to the bare ground wire, then the metal cabinet of your refrigrator, microwave, maybe even your shower head...whatever, would be at that potential...which is potentially dangerous.
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Location:
    New England
    All of the current that goes out on the hot must return through the neutral. The ground wire should never be designed to carry current - it is there for protection as a safety backup and, if there were a short, to cause the breaker or fuse to trip. Ground would normally only carry current when there is some soft of fault.
  6. DavidSeon

    DavidSeon New Member

    Messages:
    47
    Location:
    MS Gulf Coast
    I think that's the part I was missing. With electronic equipment I slipped into the habit of thinking of paths to chassis ground as dead shorts because, except with RF, the very small resistance/impedance can be ignored. So I was thinking of the neutral the same way; ie, if one end of the neutral is at 0 volts (relative to earth) at the panel, then the other end of that big copper wire was at 0 volts, too. Obviously, since the neutral carries current, that's not possible. It's not nice to forget Ohm's Law.

    Thanks to everyone for your patience.
    Dave
  7. DonL

    DonL Jack of all trades Master of one

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    "The voltage at the source of the current will be I x R."

    If there is much R at the Source then something is wrong.

    R should only come into play on the wire feeding the circuit, At its end destination point.


    Jimbo is correct , but the wording could be a bit confusing. I get confused easy.
    Last edited: Dec 18, 2012
  8. DavidSeon

    DavidSeon New Member

    Messages:
    47
    Location:
    MS Gulf Coast
    Yes, what I was missing was a possible 15A across only 0.3 ohms at 4.5 volts is still 15A :eek:
  9. DonL

    DonL Jack of all trades Master of one

    Messages:
    4,127
    Location:
    Houston, TX

    That is not to bad of a voltage drop, depending on the wire length.

    You just need to make sure the wire does not over heat.

    When the wire gets hot, or over rated then it is best to use a bigger conductor. Voltage drop makes heat, somewhere.


    If the source Voltage is low then a bigger wire will not help.

    A better feed from the source current could help, but only if the power is available at its input.
    Last edited: Dec 18, 2012
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