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Thread: Bonding Jumper Required?

  1. #1
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    Bonding Jumper Required?

    I've looked through the 2017 NEC, SOARES Book on Grounding & Bonding, and Mike Holt's printed book, Illustrated Guide to Understanding NEC requirements for Grounding Vs. Bonding, but I'm unable to locate my exact scenario.

    Setup: Branch Circuit Conductor from Panel via 20LF EMT conduit transitioning to short section (~2LF) LFNMC, and then transitioning back to (~2LF) EMT before terminating in Metallic Enclosure. The Branch circuit conductor has an EGC run in the conduit through the entire section terminating in the metallic enclosure.

    The installation location is interior and not located in a hazardous location.

    Is a bonding strap required across the LFNMC if I'm not using the EMT as an EGC to keep the system electrically continuous?

  2. #2
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    Assuming that both pieces of EMT are bonded to the enclosures that they start/end at, I do not see why a bonding jumper is required.
    "Electricity is really just organized lightning." George Carlin


    Derek

  3. #3
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    Quote Originally Posted by jumper View Post
    Assuming that both pieces of EMT are bonded to the enclosures that they start/end at, I do not see why a bonding jumper is required.
    They are; I could see this being an issue on a much larger scale (hundreds/thousands of feet on either side of this LFNMC conduit), as if you had a fault on the EMT side of the LFNMC furthest away from the Panel side, you could introduce a step potential hazard, as the fault would go back to the enclosure, and then via EGC back to panel. It's a stretch, but was the line of thinking that had me worried.

    Basically 250.90

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    I don't understand the step potential suggestion. Planet Earth would not be part of the fault current path, so nobody's left foot would be at a different potential as their right foot. Can you clarify?

    My opinion is that a bonding jumper is not required. Any fault on either side of the LFNMC would cause enough current to flow in the EGC that the upstream breaker would trip and quickly terminate the event. The EGC will cause both enclosures (start and end of branch circuit run) to be at the same potential as the dirt below the building. No other bonding would change or improve that.
    Charles E. Beck, P.E., Seattle
    Comments based on 2017 NEC unless otherwise noted.

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    Quote Originally Posted by tw1156 View Post
    They are; I could see this being an issue on a much larger scale (hundreds/thousands of feet on either side of this LFNMC conduit), as if you had a fault on the EMT side of the LFNMC furthest away from the Panel side, you could introduce a step potential hazard, as the fault would go back to the enclosure, and then via EGC back to panel. It's a stretch, but was the line of thinking that had me worried.

    Basically 250.90
    Gimme a sec. Your question is the opposite of what is usually asked.

    You have a short section of non metallic conduit in the middle of a metallic conduit run.

    Usually the problem is reversed.

    I am approaching it as something I have encountered. RMC from panel going down into a trench, transition to PVC, when coming up to subpanel, back to RMC. Each piece of RMC bonded to cabinets.

    No bonding jumper between the pieces of RMC.
    "Electricity is really just organized lightning." George Carlin


    Derek

  6. #6
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    Quote Originally Posted by tw1156 View Post
    I've looked through the 2017 NEC, SOARES Book on Grounding & Bonding, and Mike Holt's printed book, Illustrated Guide to Understanding NEC requirements for Grounding Vs. Bonding, but I'm unable to locate my exact scenario.

    Setup: Branch Circuit Conductor from Panel via 20LF EMT conduit transitioning to short section (~2LF) LFNMC, and then transitioning back to (~2LF) EMT before terminating in Metallic Enclosure. The Branch circuit conductor has an EGC run in the conduit through the entire section terminating in the metallic enclosure.

    The installation location is interior and not located in a hazardous location.

    Is a bonding strap required across the LFNMC if I'm not using the EMT as an EGC to keep the system electrically continuous?
    You do not get a choice of using the EMT or not. If it is present it is an EGC.

    The two seperate sections of EMT have to be bonded together somehow. In your case they probably are by some kind of connection to a metallic box at the source and to the metallic box at the end. The wire EGC bonds the destination box back to the originating box and thus bonds the two pieces of EMT. You could just as easily skipped the wire EGC and just run a bonding wire between the two pieces of EMT.
    Bob

  7. #7
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    Quote Originally Posted by charlie b View Post
    I don't understand the step potential suggestion. Planet Earth would not be part of the fault current path, so nobody's left foot would be at a different potential as their right foot. Can you clarify?

    My opinion is that a bonding jumper is not required. Any fault on either side of the LFNMC would cause enough current to flow in the EGC that the upstream breaker would trip and quickly terminate the event. The EGC will cause both enclosures (start and end of branch circuit run) to be at the same potential as the dirt below the building. No other bonding would change or improve that.
    You are correct in that I misused the term step potential in describing this situation; I've attached a rough diagram illustrating the point I was attempting to describe. Hypothetical person touching either side of LFNMC. Apologize for rotation as I was unable to successfully insert the image rotated. Name:  Diagram_example.jpg
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Size:  125.3 KB

  8. #8
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    Quote Originally Posted by petersonra View Post
    You do not get a choice of using the EMT or not. If it is present it is an EGC.

    The two seperate sections of EMT have to be bonded together somehow.
    He has a wire EGC.

    They are bonded together, through the EGC.
    "Electricity is really just organized lightning." George Carlin


    Derek

  9. #9
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    If you have an effective GF path, there will be no potential across the LFNMC because the OCPD at the source will trip.

    If you don't have an effective GF path, it's possible the OCPD won't trip and then you have a real problem. I think you would need to calculate the impedance from the origination point to the LFNMC and make sure it is adequate to trip the OCPD reliably. BTW, this is a required part of the installation. Usually it is not an issue because distances are usually short enough that it does not matter any. But you are not releived from the requirement to provide an effective ground fault path just by using the EMT as the EGC or providing an EGC that is of the typical size for the OCPD rating supplied.


    Effective Ground-Fault Current Path. An intentionally
    constructed, low-impedance electrically conductive path
    designed and intended to carry current under ground-fault
    conditions from the point of a ground fault on a wiring system
    to the electrical supply source and that facilitates the operation
    of the overcurrent protective device or ground-fault detectors.
    250.4 (A)
    (3) Bonding of Electrical Equipment. Normally non–current carrying
    conductive materials enclosing electrical conductors
    or equipment, or forming part of such equipment, shall be
    connected together and to the electrical supply source in a
    manner that establishes an effective ground-fault current path.
    Bob

  10. #10
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    The drawing does not match the info in the OP. Length is way different.
    "Electricity is really just organized lightning." George Carlin


    Derek

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