Subpanel Grounding

Discussion in 'Electrical Forum discussion & Blog' started by drick, Jan 2, 2009.

  1. drick

    drick In the Trades

    Messages:
    392
    I am installing a subpanel next to my main panel as I am running out of breaker space. The house is only 3 years old, but the electrician only put in a 32 space panel.

    Anyway, I connected my subpanel to the main panel with a short length of 2 inch diameter EMT with compression fittings rated for use with EMT at each end. Do I still need to run a grounding conductor to the subpanel? Article 358.2 and 358.60 makes me think no. However I ran #4 bare anyway to the grounding bar in the subpanel just to be sure I had a good ground. I'm a bit leery of the panel board enclosures providing a constantly reliable ground unless you are actually threaded into them as the grounding bars are. I suppose as long as the compression fittings are tight it doesn't matter.

    Thoughts? Did I waste a couple of bucks on the #4?

    -rick
    Last edited: Jan 2, 2009
  2. Chris75

    Chris75 Electrician

    Messages:
    608
    Location:
    Litchfield, CT
    Emt is an acceptable ground path.
  3. sixlashes

    sixlashes Plumber in Previous Life

    Messages:
    83
    Location:
    Pensacola. FL
    There are some electricians with serious credentials on this forum that can speak to the legality of solely using EMT for a subpanel ground circuit. I will not even try.

    From an electronics background, I would not use EMT as the sole ground circuit for a subpanel and here's why. Yes, the NEC lists EMT is a ground conductor, but... To use it as a ground for a subpanel, there may be issues. Assuming it is legal, you would have to ensure the paint on both boxes was broken to ensure a zero-ohm resistance connection between the two panels. You would not be able to utilize concentric knockouts and it assumes the "proper connector" makes complete continuity with the locking ring biting into both the panels and the screws in the connectors are completely tight. If the conduit is loose anywhere in the slightest, your zero ohms is gone.

    Where a low resistance in the ground would show its ugly head would be if there was a surge produced by a lightning strike. Any surge protection equipment relying on this ground would be compromised by voltage produced by the resistance in the ground circuit. Using the 4 ga is easy insurance, IMHO. For this same reason, I would also want a ground conductor inside EMT for any outlet serving computers and pricey entertainment electronics.

    I am not an electrican, but I don't think you wasted the $2.

    Another question is, did you separate the neutral and the ground in the subpanel? :D
  4. codeone

    codeone Code Enforcement

    Messages:
    160
    Location:
    North Carolina
    Bonding jumpers meeting the other requirements of this
    article shall be used around concentric or eccentric knockouts
    that are punched or otherwise formed so as to impair
    the electrical connection to ground. Standard locknuts or
    bushings shall not be the sole means for the bonding required
    by this section.
    (A) General. Metal raceways, cable trays, cable armor, cable
    sheath, enclosures, frames, fittings, and other metal non–
    current-carrying parts that are to serve as grounding conductors,
    with or without the use of supplementary equipment
    grounding conductors, shall be effectively bonded where necessary
    to ensure electrical continuity and the capacity to conduct
    safely any fault current likely to be imposed on them.
    Any nonconductive paint, enamel, or similar coating shall be
    removed at threads, contact points, and contact surfaces or be
    connected by means of fittings designed so as to make such
    removal unnecessary. (These are some things to consider.)
  5. drick

    drick In the Trades

    Messages:
    392
    Thanks all for the replies. I missed the part on concentric or eccentric knockouts, and that is an issue here since there is one knockout ring remaining. I'll be keeping the #4 bare.

    And yes the grounds are separated from the neutrals in the sub panel.

    -rick
  6. codeone

    codeone Code Enforcement

    Messages:
    160
    Location:
    North Carolina
    rick just a note the statement on concentric or eccentric ko's comes out of the method of bonding at the service. Thid is not what you are doing to the sub panel. Didnt have enough room to put that all in on the last post.
    Thats why the statement of some things to consider. You do have a better system by leaving your bond wire in. (makes things more sure.)
  7. Chris75

    Chris75 Electrician

    Messages:
    608
    Location:
    Litchfield, CT

    Okay, here are the actual requirements for bonding jumpers. :)


    All metal raceways containing service conductors, and metal raceways with277/480v AND terminated at ringed knockouts...



    So... if I ran 50' of emt out of a main panel 120/240v, to a subpanel or piece of equipment, I would not need bonding jumpers.

    And just for fun, EMT will ALWAYS be the better grounding path, as long as installed correctly. But someone can easily make mistakes installing a wire under a lug as well. I would not waste the time pulling EGC's in metal conduit.
    Last edited: Jan 7, 2009
  8. codeone

    codeone Code Enforcement

    Messages:
    160
    Location:
    North Carolina
    Key words for emt ground path are* "installed correctly".Article 250.92 B for services Bonding jumpers meeting the other requirements of thisarticle shall be used around concentric or eccentric knockoutsthat are punched or otherwise formed so as to impairthe electrical connection to ground. Standard locknuts orbushings shall not be the sole means fot the bonding requiredby this section.
    This was in reply to statement- All metal raceways containing service conductors.
  9. Chris75

    Chris75 Electrician

    Messages:
    608
    Location:
    Litchfield, CT
    I realize this, but whos to say the wire gets installed correctely as well? get my point? A hack is a hack. I just dont get why people think using a metal raceway as the grounding means is so taboo.
  10. codeone

    codeone Code Enforcement

    Messages:
    160
    Location:
    North Carolina
    Metal raceway is not taboo. As I stated on DIY Again the most important thing is to aquire permits and have the work inspected. Even this will not assure every little thing is correct. However it usually assures the work is closer to correct. An extra pair of eyes always helps.

    In commercial work Ive seen many of a set screw connector not tightened properly. Espically if the work is high and the workers are pushed as usual and working alone. They put a connector on a pipe go upto the far end push it on the first one and go to the next without going back to tighten the other side.

    Again as you said installed correctly.

    Either they are in a hurry and forget or dont care.

    Always hope its the first.
  11. drick

    drick In the Trades

    Messages:
    392
    Thanks for the explanation. Explains why I didn't see it. I'm still leaving the #4 bare. Its only a few feet and makes me feel better about the grounding between the panels. If I had to run 100 feet of it I'd probably feel differently.

    Thanks again,
    -rick
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