208v Disconnect Grounding

Discussion in 'Electrical Forum discussion & Blog' started by jdh, Jun 26, 2008.

  1. jdh

    jdh New Member

    Messages:
    2
    Location:
    AZ
    My question is regard to the need to ground disconnect boxes. I have a main panel with 208V coming into it and I have setup 3 disconnect boxes after that. I have ground wire from the main panel running to all the disconnects. Do I need to use the box bonding screw and also attach my ground to that bar and from there run out to my devices? If it's already grounded in the main panel, isn't that enough?

    Thanks for the help.
  2. Chris75

    Chris75 Electrician

    Messages:
    608
    Location:
    Litchfield, CT
    What are you doing with 208?
  3. Speedy Petey

    Speedy Petey Licensed Electrical Contractor

    Messages:
    996
    Location:
    NY State, USA
    This does not sound like a DIY job, and it sounds like you are in over your head.
    If this is commercial CALL AN ELECTRICIAN!
  4. Bill Arden

    Bill Arden Computer Programmer

    Messages:
    584
    Location:
    MN, USA
    You must remove the bonding screw from all sub panels and run both a neutral and a ground to each sub panel or disconnect box.

    Also note that on 120V/208V systems, the neutral MUST be just as heavy as the power wires since the two voltages are not at the same phase.
  5. jdh

    jdh New Member

    Messages:
    2
    Location:
    AZ
    To those that helped....thanks. To those that referred me to a "professional", no thanks. Forums exist so people with experience can give tips and help prevent hard working people from blowing their entire paycheck on a otherwise simple job. Besides, an electrician friend ran all the wire from the main panel.

    Bill, I'm confused by your reply. This is 3 phase 208V which should only be using 3 hots and 1 ground. Also, I purchased the disconnect boxes new and the bonding screws were included but not installed. My question is, what would be the purpose of running a ground from the main panel to the neutral/bonding bar in each of the disconnect boxes without using the bonding screw? After all, doesn't this serve the same purpose as a ground screw in a J-box?

    Thanks.
  6. Billy_Bob

    Billy_Bob In the Trades

    Messages:
    422
    If you have no neutral to the subpanels and the subpanel boxes are grounded via the bonding screws (all metal boxes of course need to be grounded for safety), then I would see no reason to also connect an additional ground wire to each of the subpanels neutral/ground bars from the main panel (provided the ground wire which is run from the main panel to each subpanel is of sufficient size).

    The key here is that you are not running a neutral to the sub panels. And that the subpanel metal boxes will be in fact grounded via the bonding screw and the ground wire to the main panel.

    Basically the neutral bar would become just a ground bar in the subpanels. I would run the ground wires to the 3 phase "gizmos" from the subpanel's ground bars rather than from the main panel though.

    The thing with subpanels and separate neutral and ground bars with the neutral bar not using the bonding screw is in case the neutral wire accidentially becomes disconnected. But you have no neutral wire to become accidentially disconnected!

    FYI-There are all sorts of 3 phase configurations. Read more here...
    http://www.federalpacific.com/university/transbasics/chapter3.html
  7. Bill Arden

    Bill Arden Computer Programmer

    Messages:
    584
    Location:
    MN, USA
    The problem is that you can't pull any 120V power out of the sub panel if you don't have a neutral.

    To prevent problems in the future, you should leave the bonding screw out and not use the neutral bar.

    On the other hand if any of the circuits need 120V, then you have to run a neutral wire back to the main panel.

    All these rules are designed so that you are NOT running current threw the grounding conductors and thus don't make the grounding wires "hot" if there is a break in a wire.

    Personally, I like 120V/208Y 5 wire, but I only have single phase power and thus need to use a rotary converter to get it.
  8. Speedy Petey

    Speedy Petey Licensed Electrical Contractor

    Messages:
    996
    Location:
    NY State, USA
    Then you sir are a fool. It' not always about money. :mad:

    THIS:
    ........is NOT a DIY job and NOT any place you should be concerned about saving money.
    Is this a commercial setting? If so you are even more the fool. Unlike your own home any mistakes you make will haunt you and follow you.

    Good luck. You are going to need it!
  9. jbfan74

    jbfan74 Electrical Contractor

    Messages:
    131
    Location:
    Newnan, GA
    "Besides, an electrician friend ran all the wire from the main panel."

    I like this part. Ran the wires and now won't tell him how to hook it up!
  10. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    This "get a pro" debate comes up a lot on these forums. We walk a balance betweet helping anyone with anything, and trying to keep a homeowner from hurting himself.

    A good example in the plumbing arena is GAS. Some folks take the stand that gas is too dangerous, and should be done by pros. I tend to take that attitude that since at least here in CA it is LEGAL for a homeowner to do gas work himself, we should try to help him do it right. This will include the advice to get the permit and inspections. If we blow him off, he is going to do it anyway, and maybe blow himself up!

    In this thread, as I understand it, 208 implies a 3 phase service, probably a commercial building, or a large hotel, something like that. ( Am I right on that??) Three phase power is a different animal, and although I feel reasonalby comfortable on regular residential electric based on extensive training and experience in Navy elecricity and electronics, and having done a lot of work in homes I owned ( WITH PERMITS AND INSPECTIONS), I would not touch 3 phase. So I understand the stance that folks take on that. I suspect that teaching 3 phase may be beyond the scope of a few posts on a bulletin board.
  11. Billy_Bob

    Billy_Bob In the Trades

    Messages:
    422
    I've seen Laundromats with a couple of large front loading washers which are 3 phase. Everything else is single phase.

    Then the home "shop" thing. A saw or whatever which is 3 phase.

    I've seen small hamburger stands with 3 phase. I have no idea what they have that is 3 phase? Maybe walk-in fridge? Ice cream machine? (Hummm that sounds good as it is quite hot here!)

    Some airplanes even have 3 phase. One thing I have seen this used for is a gyroscope artificial horizon. This has an incredibly fine balanced 3 phase motor (innards look like fine watch work) and it rotates at something like 12,000 rpm!
  12. Gary Swart

    Gary Swart In the Trades

    Messages:
    7,358
    Location:
    Yakima WA
    I have to take exception on the comment that home shop "saw or whatever" is 3 phase. Three phase saws do exist and are used in factories that have 3 phase service. Home shops do not have 3 phase service and while there are phase converters available, the home shop tools are not 3 phase so unless Joe Homeowner finds a deal on a 3 phase machine, there is no use for a converter in 99.9% of home workshops. The more powerful tools such as cabinet table saws are often wired for 220 vac although even then some are wired for 120. Many times tool motors come with the option of 220 or 120 just be properly connecting the wires. In my shop, the table saw, band saw, jointer, dust collector, and air compressor either required 220 or had the option. Three phase is strictly an industrial thing and is usually not even a possibility in home workshops.
  13. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,134
    Location:
    New England
    Three-phase power is often used in military equipment since it is easier to control power supply ripple with multiple phases inputting to the rectifier, plus, 400Hz is often a part of that for the same reason...caps are easier than inductors for size and weight.
  14. Gary Swart

    Gary Swart In the Trades

    Messages:
    7,358
    Location:
    Yakima WA
    Thanks for adding that Jim. I of course knew all of that but I didn't want to show off:D
  15. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,134
    Location:
    New England
    A three phase motor theoretically would have a smoother rotation for the same power input because of the additional field coils.
  16. Mikey

    Mikey Aspiring Old Fart, EE, computer & networking geek

    Messages:
    2,716
    Location:
    Central Florida
    The old IBM mainframes used to used 3-phase power. I was told once that one reason was to minimize the amount of gear that disappeared into homes, but I suspect that was a (welcome) side-effect.
  17. Bill Arden

    Bill Arden Computer Programmer

    Messages:
    584
    Location:
    MN, USA
    I've seen two phase 120/208V used in residential houses. I know its rare but it does happen.

    By using a transformer you can convert two phase 120/208V into full three phase 208Y.

    I also know of a power utility where all the transformers can be used at either 7.2KV or 14KV due to a planed voltage conversion.

    Then there was the one company that wanted 6 phase 120v/63v/208Y/240v :cool:
    It used three 120/240V services, but each one was coming from a different phase.

    Also don't forget that due to all the surplus it's cheaper to run phase converters than to convert equipment to single phase. so you could have 208Y three phase in a home workshop.

    -
    Back to the topic at hand.
    The guy has three wires, Hot, Hot, Ground

    This means that
    1. You have to remove the bonding screw per UL
    2. You don't have a neutral and thus can't use the neutral bar.
    3. You need to use a ground bar that is screwed to the back of the panel.

    This leaves the neutral bar unused for future use and to prevent someone else from getting confused.
    Last edited: Jul 3, 2008
  18. Speedy Petey

    Speedy Petey Licensed Electrical Contractor

    Messages:
    996
    Location:
    NY State, USA
    Since when does UL write the codes???
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