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Thread: Why is a sub*panel not "effectively" bonded when connected to main panel by emt?

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    Why is a sub*panel not "effectively" bonded when connected to main panel by emt?

    Specific situation: 200 amp main service panel with bonded neutral bar and ufer ground. Main panel has 100 amp 240V breaker feeding 2 #3 Cu wires that are connected to 100 amp main disconnect of 100 amp sub-panel. Additional #3 Cu wire connecting sub-panel and main service panel neutral bars and a #6 bare Cu wire connecting sub-panel ground to neutral bonded bar in main 200 amp panel. Sub-panel is approximately 3 feet from main panel, and is connected by EMT conduit. Sub-panel neutral bar is not bonded to panel chassis with a bonding screw.

    All of the above meets NEC, but even without the bonding screw, isn't the sub-panel effectively bonded to the chassis via 2 separate paths? The EMT conduit between the main and sub-panel provides a direct path for electricity from the sub-panel neutral to main panel neutral via the neutral wire, then to the main panel chassis via bonding screw, then to EMT, and finally to sub-panel chassis. Additionally, the bare Cu ground can provide a direct path for current from sub-panel neutral to the chassis by contacting either the chassis or EMT directly. Why is a 1/4 inch path through a bonding screw an issue when neither of the above 3-6 foot long pathways appear to be a concern? Thanks!

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    I'm approving this thread even though the OP is not in the electrical industry. I believe the answer to this would benefit other readers. Please keep all replies to the question asked and not any "how to" answers.
    If you aim at nothing, you will hit it every time!

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    Quote Originally Posted by cowdr4 View Post
    Specific situation: 200 amp main service panel with bonded neutral bar and ufer ground. Main panel has 100 amp 240V breaker feeding 2 #3 Cu wires that are connected to 100 amp main disconnect of 100 amp sub-panel. Additional #3 Cu wire connecting sub-panel and main service panel neutral bars and a #6 bare Cu wire connecting sub-panel ground to neutral bonded bar in main 200 amp panel. Sub-panel is approximately 3 feet from main panel, and is connected by EMT conduit. Sub-panel neutral bar is not bonded to panel chassis with a bonding screw.

    All of the above meets NEC, but even without the bonding screw, isn't the sub-panel effectively bonded to the chassis via 2 separate paths? The EMT conduit between the main and sub-panel provides a direct path for electricity from the sub-panel neutral to main panel neutral via the neutral wire, then to the main panel chassis via bonding screw, then to EMT, and finally to sub-panel chassis. Additionally, the bare Cu ground can provide a direct path for current from sub-panel neutral to the chassis by contacting either the chassis or EMT directly. Why is a 1/4 inch path through a bonding screw an issue when neither of the above 3-6 foot long pathways appear to be a concern? Thanks!
    I am a bit confused by your train of thought in the second paragraph. Maybe its just too late for me. The sub panel should have neutrals and ground separated, is that what you have? Indeed the installer could have used the EMT as the EGC and skipped the wire EGC. There is nothing that prohibits multiple Equipment grounding conductors. Using an approved raceway as an EGC has fallen greatly out of favor in the last 30 years or so, despite no history or data showing it is less safe.
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

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    Quote Originally Posted by cowdr4 View Post
    Specific situation: 200 amp main service panel with bonded neutral bar and ufer ground. Main panel has 100 amp 240V breaker feeding 2 #3 Cu wires that are connected to 100 amp main disconnect of 100 amp sub-panel. Additional #3 Cu wire connecting sub-panel and main service panel neutral bars and a #6 bare Cu wire connecting sub-panel ground to neutral bonded bar in main 200 amp panel. Sub-panel is approximately 3 feet from main panel, and is connected by EMT conduit. Sub-panel neutral bar is not bonded to panel chassis with a bonding screw.

    All of the above meets NEC, but even without the bonding screw, isn't the sub-panel effectively bonded to the chassis via 2 separate paths? The EMT conduit between the main and sub-panel provides a direct path for electricity from the sub-panel neutral to main panel neutral via the neutral wire, then to the main panel chassis via bonding screw, then to EMT, and finally to sub-panel chassis. Additionally, the bare Cu ground can provide a direct path for current from sub-panel neutral to the chassis by contacting either the chassis or EMT directly. Why is a 1/4 inch path through a bonding screw an issue when neither of the above 3-6 foot long pathways appear to be a concern? Thanks!
    The clear distinction, if you follow the current paths for both cases, is this:
    If you bond with the screw at the sub panel normal neutral current has an available path back to the main through the EGC as well as through the neutral wire.
    If you only have a bond at the main panel all normal neutral current can flow only on the neutral wire. There is no reason for current to flow down the neutral to the main panel, then through the bond and EGC back to the sub panel. There is nowhere for it to go at the sub panel as the parallel path is not complete.

    In both cases an ohmmeter reading will be very close between the two cases, but the current flow will be very different.

    Note that from the main panel (service disconnect) back to the POCO transformer there will be a parallel path through the earth, but that is not considered obectionable and is also not part of the jurisdiction of the NEC.

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    Quote Originally Posted by cowdr4 View Post
    Specific situation: 200 amp main service panel with bonded neutral bar and ufer ground. Main panel has 100 amp 240V breaker feeding 2 #3 Cu wires that are connected to 100 amp main disconnect of 100 amp sub-panel. Additional #3 Cu wire connecting sub-panel and main service panel neutral bars and a #6 bare Cu wire connecting sub-panel ground to neutral bonded bar in main 200 amp panel. Sub-panel is approximately 3 feet from main panel, and is connected by EMT conduit. Sub-panel neutral bar is not bonded to panel chassis with a bonding screw.
    This is code compliant, you have a redundant EGC with both the EMT and #6 conductor which is not a problem because as Ef stated you can have more than one EGC (#6 and EMT) even if they're in parallel with each other.

    Quote Originally Posted by cowdr4 View Post
    All of the above meets NEC, but even without the bonding screw, isn't the sub-panel effectively bonded to the chassis via 2 separate paths? The EMT conduit between the main and sub-panel provides a direct path for electricity from the sub-panel neutral to main panel neutral via the neutral wire, then to the main panel chassis via bonding screw, then to EMT, and finally to sub-panel chassis. Additionally, the bare Cu ground can provide a direct path for current from sub-panel neutral to the chassis by contacting either the chassis or EMT directly. Why is a 1/4 inch path through a bonding screw an issue when neither of the above 3-6 foot long pathways appear to be a concern? Thanks!
    Since the subpanel neutral is not bonded the part in bold is incorrect.
    Rob

    Moderator

    All responses based on the 2014 NEC unless otherwise noted

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    To put it simply, at no point beyond the point in the main disconnect (usually) where the EGC system (enclosures, GECs, etc) is bonded to the neutral, should the neutral ever be grounded again. It should be insulated from anything grounded as if it were an energized conductor.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

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    Quote Originally Posted by LarryFine View Post
    To put it simply, at no point beyond the point in the main disconnect (usually) where the EGC system (enclosures, GECs, etc) is bonded to the neutral, should the neutral ever be grounded again. It should be insulated from anything grounded as if it were an energized conductor.
    That is an excellent explanation.
    Cheers and Stay Safe,

    Marky the Sparky

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    Quote Originally Posted by LarryFine View Post
    To put it simply, at no point beyond the point in the main disconnect (usually) where the EGC system (enclosures, GECs, etc) is bonded to the neutral, should the neutral ever be grounded again. It should be insulated from anything grounded as if it were an energized conductor.
    And a major reason for that is the grounded conductor (often also a neutral conductor) can carry current during normal operation. When a conductor carries current there is a voltage drop across that conductor, may not be much at all in some instances but there is a voltage drop. If you connect exposed conductive objects to that conductor you can be exposed to that voltage potential. If there were a 3 volt drop on the grounded conductor (and everything else bonded to it) when you touch one of those bonded objects and are standing on "earth" you will be subjected to that 3 volts.

    Then there is the fact that all possible paths back to the source will carry some of the current - each will also have a voltage drop across them. By not bonding beyond the service or first disconnect of separately derived systems, we don't create parallel paths for (grounded) conductors that were intended to carry current in normal operation.
    I live for today, I'm just a day behind.

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    Quote Originally Posted by cowdr4 View Post
    Specific situation: 200 amp main service panel with bonded neutral bar and ufer ground. Main panel has 100 amp 240V breaker feeding 2 #3 Cu wires that are connected to 100 amp main disconnect of 100 amp sub-panel. Additional #3 Cu wire connecting sub-panel and main service panel neutral bars and a #6 bare Cu wire connecting sub-panel ground to neutral bonded bar in main 200 amp panel. Sub-panel is approximately 3 feet from main panel, and is connected by EMT conduit. Sub-panel neutral bar is not bonded to panel chassis with a bonding screw.

    All of the above meets NEC, but even without the bonding screw, isn't the sub-panel effectively bonded to the chassis via 2 separate paths? The EMT conduit between the main and sub-panel provides a direct path for electricity from the sub-panel neutral to main panel neutral via the neutral wire, then to the main panel chassis via bonding screw, then to EMT, and finally to sub-panel chassis. Additionally, the bare Cu ground can provide a direct path for current from sub-panel neutral to the chassis by contacting either the chassis or EMT directly. Why is a 1/4 inch path through a bonding screw an issue when neither of the above 3-6 foot long pathways appear to be a concern? Thanks!
    If you are questioning suitability of that 1/4 inch bonding screw - you do have to use the factory supplied screw in the hole it was intended to go in. It was listed and tested for the purpose.

    I have seen such bonding screws fail though. Probably because it wasn't properly tightened when installed. One was fairly recently. I had a service call for a couple breakers that were tripping. They both had short circuit/ground fault conditions and tripped immediately when you tried to turn them on. I happened to notice burn marks around the bonding screw - tried to tighten it and it wouldn't tighten -stripped threads. My guess is it wasn't tightened properly and the fault current was enough to damage threads to where they won't work right anymore. I had to use alternate means to bond this panel and of course find their problems causing the tripping breakers.
    I live for today, I'm just a day behind.

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    Quote Originally Posted by LarryFine View Post
    To put it simply, at no point beyond the point in the main disconnect ... should the neutral ever be grounded again. ...
    Single-point grounding prevents many, many problems. Not just in buildings, but also in vehicles, boats, aircraft, audio-video, concert stages ... almost everywhere electricity is used.
    Rock star pastor is electrocuted as he conducts baptism service

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