Page 2 of 2 FirstFirst 12
Results 16 to 20 of 20

Thread: Electrical service question

  1. #16
    FWIW, imagine what would happen if you ever lost the neutral in the sub-panel and it happened to be bonded to ground in the sub-panel. Your ground wire would suddenly become a current carrying conductor with a subsequent voltage drop all the way back to the service panel, and you would have stray currents energizing every ground connection on that panel, and you would never know it until it caused problems.

    Even worse, if that cable were ever damaged, say by accidentally drilling through it...or a fire....or an earthquake....or anything like that, and you happened to loose the neutral AND the ground, you would then have a situation where the return path because the metal chassis of equipment. Every piece of metal in the shop suddenly becomes deadly, including the sub-panel itself. Keeping the ground and neutral separate insures this can never happen.
    Last edited by John Coloccia; 06-25-2013 at 11:44 AM.

  2. #17
    Join Date
    Apr 2012
    Location
    Great Falls, VA
    Posts
    813
    Some of the discussion in this thread does not reference a distinction between a sub-panel in the same structure as the service panel, and one in a separate outbuilding, such as a detached garage, shop or barn. Just want to be sure my understanding is correct: That is, in both cases, the sub-panel should be bonded back to ground at the service panel through the ground conductor in the feeder cable, and, if in a separate structure, it must also be bonded to its own ground rod (or other approved electrode) at that structure. Also, at least if residential, in both cases the neutral should be bonded to ground at both the sub-panel and service panel (through bonding of the neutral and ground busses).

    Is that right? Not an electrician, but I take this stuff very seriously.

    David

  3. #18
    Join Date
    Dec 2011
    Location
    North-central Minnesota
    Posts
    318
    Quote Originally Posted by David C. Roseman View Post
    Some of the discussion in this thread does not reference a distinction between a sub-panel in the same structure as the service panel, and one in a separate outbuilding, such as a detached garage, shop or barn. Just want to be sure my understanding is correct: That is, in both cases, the sub-panel should be bonded back to ground at the service panel through the ground conductor in the feeder cable, and, if in a separate structure, it must also be bonded to its own ground rod (or other approved electrode) at that structure. Also, at least if residential, in both cases the neutral should be bonded to ground at both the sub-panel and service panel (through bonding of the neutral and ground busses).

    Is that right? Not an electrician, but I take this stuff very seriously. David

    David,


    The only time you bond the neutrals and grounds together is where the panel in question is fed with a 3-wire feeder. The neutral bus in a sub-panel fed with a 4-wire feeder must be isolated from the grounding system, whether it is in the same structure, or a seperate structure. A panel in a seperate structure, whether it is fed with a 3-wire or 4-wire feeder, must have it's own seperate grounding electrode system. The only exeption to this rule, is when the seperate structure is fed by a single branch circuit. A multi-wire branch circuit is still considered a single branch circuit.

  4. #19
    Join Date
    Apr 2012
    Location
    Great Falls, VA
    Posts
    813
    Quote Originally Posted by Matt Marsh View Post
    David,


    The only time you bond the neutrals and grounds together is where the panel in question is fed with a 3-wire feeder. The neutral bus in a sub-panel fed with a 4-wire feeder must be isolated from the grounding system, whether it is in the same structure, or a seperate structure. A panel in a seperate structure, whether it is fed with a 3-wire or 4-wire feeder, must have it's own seperate grounding electrode system. The only exeption to this rule, is when the seperate structure is fed by a single branch circuit. A multi-wire branch circuit is still considered a single branch circuit.
    Thanks, Matt! That explanation is very clear, and makes complete sense.

    David

  5. #20
    Join Date
    Oct 2007
    Location
    New Hill, NC
    Posts
    2,568
    Quote Originally Posted by Julie Moriarty View Post
    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.
    Julie, you brought back some memories! I spent several years with Nextel in Chicago, the first year and a half was on the site development side. I'll be you know what "Motorla R56" is....

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •