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Thread: Electrical service question

  1. #1

    Electrical service question

    I know that the electrical code for sub panels calls for 4 wires to be run from the main panel to the 220 volt sub panel (2 hots, neutral & ground), isolating the neutrals & grounds in the sub panel and no ground rod at the sub panel.

    What is the reason for using the ground at the main panel & no ground rod at the sub panel?

  2. #2
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    I do not believe it is a violation of the NEC to use a local ground rod at the sub panel. I certainly did because it is safer than counting on a ground reference that is 150 feet away (in my case). The important thing is to keep this ground separate from the neutral, because it is current carrying and the ground is not.

  3. #3
    Agreed.
    no restriction against using a local ground rod. Depedning on the mood of your local inspector, he may either demand that you install a local ground rod or remove one that you already have installed. Only reliable adviec that I can offer you is that however you ground it, the inspector will likely want it the other way.

  4. #4
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    You don't state it, but my experience is for a local ground rod if the sub-panel is in a separate structure and using the existing grounding if it is in the same structure as the main panel.

  5. #5
    From 2011 NEC:

    250.24 Grounding Service-Supplied Alternating-Current Systems.

    (A) System Grounding Connections. A premises wiring system supplied by a grounded ac service shall have a grounding electrode conductor connected to the grounded service conductor, at each service, in accordance with 250.24(A)(1) through (A)(5).

    (1) General. The grounding electrode conductor connection shall be made at any accessible point from the load end of the service drop or service lateral to and including the terminal or bus to which the grounded service conductor is connected at the service disconnecting means.

    Informational Note: See definitions of Service Drop and Service Lateral in Article 100.

    (2)
    Outdoor Transformer. Where the transformer supplying the service is located outside the building, at least one additional grounding connection shall be made from the grounded service conductor to a grounding electrode, either at the transformer or elsewhere outside the building.

    Exception: The additional grounding electrode conductor connection shall not be made on high-impedance grounded neutral systems. The system shall meet the requirements of 250.36.


    (3) Dual-Fed Services. For services that are dual fed (double ended) in a common enclosure or grouped together in separate enclosures and employing a secondary tie, a single grounding electrode conductor connection to the tie point of the grounded conductor(s) from each power source shall be permitted.

    (4) Main Bonding Jumper as Wire or Busbar. Where the main bonding jumper specified in 250.28 is a wire or busbar and is installed from the grounded conductor terminal bar or bus to the equipment grounding terminal bar or bus in the service equipment, the grounding electrode conductor shall be permitted to be connected to the equipment grounding terminal, bar, or bus to which the main bonding jumper is connected.

    (5) Load-Side Grounding Connections. A grounded conductor shall not be connected to normally non–current carrying metal parts of equipment, to equipment grounding conductor(s), or be reconnected to ground on the load side of the service disconnecting means except as otherwise permitted in this article.


    Take from that what you want, but every sub-panel I have ever installed (and I've installed a lot) is grounded back to the main panel. That may be through metallic conduit or through a grounding conductor. Some inspectors have required a grounding rod in addition to the grounding conductor. There's some controversy regarding potential differences created by two separate grounding systems on the same feeder but I've never seen that issue resolved. Grounding is the grey area of electrical systems.

    The easiest way to know what you need to do in your area is to call the inspector.

  6. #6
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    This most certainly will not apply to your situation, but multiple paths to ground is an absolute nuisance when dealing with professional audio. The potential difference between the paths to ground creates a ground loop and shows itself as a terrible 60Hz hum with a buzz several harmonics above the fundamental.

    For that, I would personally prefer to have one path to ground. But I'm also an audio engineer in addition to being a woodworker.
    Thanx,

    shotgunn

    -----------------

    More is DEFINITELY more!!!

  7. #7
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    Quote Originally Posted by Chris Rosenberger View Post

    What is the reason for using the ground at the main panel & no ground rod at the sub panel?
    Short answer: It makes it less likely that lightning ground currents will take a shortcut through your house and fry all of your line powered electronics.

    Google "lightning ground potential rise" for some really scary numbers.

  8. #8
    Thank you for the replies. I will check with the inspector.

  9. #9
    The ground rods are not needed for clearing faults, as electricity is not "trying to get to the ground" as as most folks think, it is trying to return to it's source & will take all avail. paths including the earth. If a panel is in a separate structure then a rod or other allowed grounding electrode is required, & if a rod is used if the installer cannot prove 25 Ohms of resistance or less a 2nd one has to be driven at least 6 feet from the other per NEC art. 250.56 & it's cheaper to just drive another rod because the testing equipment is expensive & the user needs to be qualified to use it. There is no need to use a rod for a subpanel in the same structure it is fed from & it will not be any safer if one is used, if done improperly it can less safe.

  10. #10
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    Quote Originally Posted by Rollie Meyers View Post
    The ground rods are not needed for clearing faults, as electricity is not "trying to get to the ground" as as most folks think, it is trying to return to it's source & will take all avail. paths including the earth. If a panel is in a separate structure then a rod or other allowed grounding electrode is required, & if a rod is used if the installer cannot prove 25 Ohms of resistance or less a 2nd one has to be driven at least 6 feet from the other per NEC art. 250.56 & it's cheaper to just drive another rod because the testing equipment is expensive & the user needs to be qualified to use it. There is no need to use a rod for a subpanel in the same structure it is fed from & it will not be any safer if one is used, if done improperly it can less safe.
    Chris, I think Rollie has stated this correctly. Would be interested to hear what you learn from talking to the inspector.

    David

  11. #11
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    Quote Originally Posted by Rollie Meyers View Post
    The ground rods are not needed for clearing faults, as electricity is not "trying to get to the ground" as as most folks think, it is trying to return to it's source & will take all avail. paths including the earth. If a panel is in a separate structure then a rod or other allowed grounding electrode is required, & if a rod is used if the installer cannot prove 25 Ohms of resistance or less a 2nd one has to be driven at least 6 feet from the other per NEC art. 250.56 & it's cheaper to just drive another rod because the testing equipment is expensive & the user needs to be qualified to use it. There is no need to use a rod for a subpanel in the same structure it is fed from & it will not be any safer if one is used, if done improperly it can less safe.
    Rollie,


    Your answer is correct however I want to expound slightly less someone miss-read and thereby misunderstand the full intent. Please understand that this is intended in no way to reflect in any way negatively on you; I have no doubt that you fully understand the intent of the code. This is from the 2011 code, where 250.56 is deleted, being covered by 250.53 (3)B:

    (B) Electrode Spacing. Where more than one of the electrodes
    of the type specified in 250.52(A)(5) or (A)(7) are
    used, each electrode of one grounding system (including
    that used for air terminals) shall not be less than 1.83 m
    (6 ft) from any other electrode of another grounding system.
    Two or more grounding electrodes that are bonded
    together shall be considered a single grounding electrode
    system.

    Section 250.58 goes on to say:

    250.58 Common Grounding Electrode. Where an ac system
    is connected to a grounding electrode in or at a building
    or structure, the same electrode shall be used to ground
    conductor enclosures and equipment in or on that building
    or structure. Where separate services, feeders, or branch
    circuits supply a building and are required to be connected
    to a grounding electrode(s), the same grounding electrode(
    s) shall be used.
    Two or more grounding electrodes that are bonded together
    shall be considered as a single grounding electrode
    system in this sense.

    Just my $0.02… YMMV.

    Jim in Alaska
    One can never have too many planes and chisels... or so I'm learning!!

  12. #12
    When the cell phone carriers were first coming into Chicago to set up their systems I was involved in the electrical end of the installations. We had to install a ground grid around both the equipment cabinets and the tower. We'd drive 10' x 5/8" copper ground rods spaced 6' apart. They would all be bonded by cad-welding a #2 tinned bare copper wire from one ground rod to the next so as to create a loop around the equipment and the tower. We'd then run tails off the ground loop to various points on the equipment, platform and tower. If there was a fence around it, we'd bond the fence too. It was the most extensive grounding I have seen in my 34 years as an electrician.

    So you can certainly have two or more ground rods in the same system but if they are connected electrically through panels or bus bars, there might be an issue. Grounding-wise, you can't isolate the ground in the main panel from the ground in the sub panel unless you isolate the neutral bus from ground. I've never seen that done in a residential application. So if the main panel and sub panel are not bonded either through the case or through a grounding conductor, the neutral will act as a ground path between the two panels. And that's not good.

  13. #13
    Bingo...I am a state listened master electrician and this is indeed the correct answer.

  14. #14
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    in which state?

  15. #15
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    The information that has been offered in the most recent posts, which Chris Hachet endorsed, is based on the National Electrical Code. State codes are additions to the NEC.

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