Page 2 of 4 FirstFirst 1234 LastLast
Results 11 to 20 of 33

Thread: 320/400 Dual Panel Service Bonded at the Panels - GEC Current

  1. #11
    Join Date
    Jan 2009
    Location
    3 Hr 2 Min from Winged Horses
    Posts
    15,030
    Yeah, I thought you were referring to the above.
    "Electricity is really just organized lightning." George Carlin


    Derek

  2. #12
    Join Date
    Aug 2018
    Location
    Virginia
    Posts
    13
    OK, I've taken the figure and added some imbalance current. Don't get too caught up on the chosen numbers. It's the concept I'm illustrating. I'm assuming the GEC path between neutrals is equal in impedance to the path from each neutral to gutter. In this example, I've illustrated 90A of imbalance at panel 2, and 60A of imbalance at panel 1. The imbalance at each panel are split phase wrt to each other. They cancel at the gutter, leaving 30A net back to transformer. Look at the GEC current. It's the most of all. Yet, the panel 1 tap is the smallest wire of them all. The 8 AWG is based on the panel 1 feeder. But in this configuration, panel 2, a much larger feeder, can tax that wire.

    Name:  GECtaps - Current.JPG
Views: 29
Size:  57.5 KB

  3. #13
    Join Date
    Feb 2003
    Location
    Fl
    Posts
    17,230
    I may not be completely understanding the OP's installation but, the illustration below is the way I envision it and even though it is legal it is a bad design.




    Roger
    Moderator

  4. #14
    Join Date
    Jan 2009
    Location
    3 Hr 2 Min from Winged Horses
    Posts
    15,030
    Quote Originally Posted by roger View Post
    I may not be completely understanding the OP's installation but, the illustration below is the way I envision it and even though it is legal it is a bad design.




    Roger
    Either set up illustrates the problem with our services and noodle current, we say no “objectionable current” and then waffle and say well, if it is the service, okay...

    Kinda why I said we pretend and ignore this particular problem, what else can we do?
    "Electricity is really just organized lightning." George Carlin


    Derek

  5. #15
    Join Date
    May 2005
    Location
    Richmond, Virginia
    Posts
    23,200
    In the "olden days" before plastic water piping, it was not uncommon for a poor service neutral to remain undiscovered . . . until a water meter was removed.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

  6. #16
    Join Date
    May 2011
    Posts
    4,428
    Quote Originally Posted by JPinVA View Post
    ... We have 2/0 neutrals running back to the meter, but #2 at the taps. As both paths are low impedance, I have to wonder if 2/0 is needed to handle the phase imbalance along the neutral to the meter why is #2 good enough for current on the tap?
    Since there are two paths the #2 would only have to handle half of the maximum current possible. I kinda see your point if I study the ampacity tables, but they don't have to both be sized for the maximum load. Also it doesn't matter too much if any insulation on the GECs melts. The copper isn't going to melt. A bare GEC probably has no issue carrying the current, although I suppose it might get hot to the touch in an extreme situation.

    Also keep in mind that the metal parts between the two disconnects are yet another pathway that can take some burden off the neutral and GEC.

    Quote Originally Posted by JPinVA View Post
    Of particular interest is the sizing of the taps are a couple of notches smaller than the neutral feed, but will be carrying similar levels of current. In the example above, it is compounded even more by the different feeders to each panel. We have an 8 AWG tap on the first panel (sized based on the neutral feeder to that panel), but it may be carrying current imbalance off of the neutral from panel 2. My initial query is whether anyone else sees this as an issue. I know it's code compliant. So I don't have an issue in terms of code. I presume when the code was written, this situation was assessed and deemed "OK". I'm just looking to see some of the logic behind the decision.
    I see the point you are raising about different sizes and I think it's in an interesting issue. I'm doubtful of it being a real world issue unless the two different sizes are seriously out of whack, say the max 3/0 on one tap and 8awg on the other. Maybe the reason the code hasn't been changed is that tolerances are high enough that in the real world this has never been known to start a fire. Or at least, not often enough to catch the attention of anyone who writes the code.

    ...Edit: One other observation. I believe the phase delta add/subtract will be opposite along the GEC link. That is, same phase current (arriving at each neutral) will have a subtracting effect on the current observed along the GEC link. Opposite phase current will have an additive effect. It all comes out in the wash at the meter as the current heads for home. But one thing to note is you can have zero from meter to transformer and have substantial current along the GEC between the panel neutrals.
    Quote Originally Posted by JPinVA View Post
    OK, I've taken the figure and added some imbalance current. ...
    The last picture is sort of useless without a red wire shown, but I get what you are trying to say. However, I'd point out the worst case scenario is simply maximum unbalanced current on one wire of one meter (the larger if they are different) and zero current on the other meter. The balancing of current on different phases between the two different meters can't make it any worse than that.

  7. #17
    Join Date
    May 2018
    Location
    Jamaica and london
    Posts
    347
    Quote Originally Posted by roger View Post
    I may not be completely understanding the OP's installation but, the illustration below is the way I envision it and even though it is legal it is a bad design.




    Roger
    Wouldn’t this be better if you ran it through a trough, running a grounding bar in the trough, then feeding both panels with neutral and grounds separated? Or even a 320 breaker or fused disconnect to make ground spot before splitting to the 200 amp panels? Remember, before yelling, am a student, not experienced in these big loads

  8. #18
    Join Date
    Dec 2007
    Location
    NE Nebraska
    Posts
    37,564
    Quote Originally Posted by Adamjamma View Post
    Wouldn’t this be better if you ran it through a trough, running a grounding bar in the trough, then feeding both panels with neutral and grounds separated? Or even a 320 breaker or fused disconnect to make ground spot before splitting to the 200 amp panels? Remember, before yelling, am a student, not experienced in these big loads
    250.24(B) still would require bonding the grounded conductor within the enclosure for each service disconnecting means, which would still create the parallel paths you are trying to avoid.
    I live for today, I'm just a day behind.

  9. #19
    Join Date
    Aug 2018
    Location
    Virginia
    Posts
    13
    Quote Originally Posted by kwired View Post
    250.24(B) still would require bonding the grounded conductor within the enclosure for each service disconnecting means, which would still create the parallel paths you are trying to avoid.
    Not necessarily. The neutral/ground of each enclosure will not "itself" create a low impedance parallel path between enclosures. If there is no other low impedance path between enclosures, and the GEC taps are removed because the GEC is now connected in the trough, the only low impedance path is the neutral feeder between each panel and the trough. The trough becomes a "true" single point bond, trough connection notwithstanding.

  10. #20
    Join Date
    Jan 2009
    Location
    3 Hr 2 Min from Winged Horses
    Posts
    15,030
    Quote Originally Posted by JPinVA View Post
    Not necessarily. The neutral/ground of each enclosure will not "itself" create a low impedance parallel path between enclosures. If there is no other low impedance path between enclosures, and the GEC taps are removed because the GEC is now connected in the trough, the only low impedance path is the neutral feeder between each panel and the trough. The trough becomes a "true" single point bond, trough connection notwithstanding.
    Are you referring to the pic you posted?

    If GEC was landed in the trough, taps are gone.

    Assuming metal trough and metal nipples, parallel neutral path. Use PVC, no parallel path.
    "Electricity is really just organized lightning." George Carlin


    Derek

Posting Permissions

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