Trying to understand the applicable uses of main breaker vs main lug load centers
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  1. #1
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    Default Trying to understand the applicable uses of main breaker vs main lug load centers

    Guys,

    The link below contains a description of the differences between main breaker and main lug load centers:

    ElectricSmarts Network - Story: Main Breaker vs. Main Lug: What?s the Difference?

    I gather the impression that all sub-panels downstream from the service entrance are typically main lug and that these have to have a "service disconnect".

    In another recent post here, I mentioned some Square D QO 200 amp "feed-thru" load centers I'm using in our shop. If I'm not mistaken, these load centers can be converted from main breaker to main lug by simply removing the main breaker.

    My concern is that I may be using the downstream boxes incorrectly because I have passed the entire 200 amps from the service entrance box to an identical box on the opposite side of an outside wall. Both are outside boxes. I'm just using one on the outside beneath the service entrance and meter can and the second (identical) box back to back with the first box, on the inside of an exterior wall.

    Both of these boxes still have main disconnects. Other than possibly spending money unnecessarily, what I'd really like to know is:

    Is there anything WRONG (such as a NEC code violation) with doing it this way?

    The reason I ask is that I'm now contemplating doing it again. If there is a compelling reason to remove the "main breakers" from the sub-panels (converting them to "main lug") then I will do so.

    I hope this makes sense.

    Thanks

    V

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    There is no need to remove the main breaker from a subpanel. The breaker will give you the ability to turn off all circuits in the subpanel.

    The difference between the main panel (service disconnect) and a subpanel is that in the main panel neutrals and grounds can be connected to the same bus. In a subpanel the grounds and neutrals are connected to different buses and the neutral bus is isolated from the panel.

    You can convert any panel to a subpanel by simply adding a ground bus and moving all the grounds to the added bus. If the panel was designed to be a main panel, there would be a lug that connects the neutral/ground bus to the panel. You need to remove that lug if converting to a subpanel. You should only have one main panel. All others should be subpanels.

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    Quote Originally Posted by ciladog View Post
    There is no need to remove the main breaker from a subpanel. The breaker will give you the ability to turn off all circuits in the subpanel.
    YAY!!!

    Quote Originally Posted by ciladog View Post
    The difference between the main panel (service disconnect) and a subpanel is that in the main panel neutrals and grounds can be connected to the same bus. In a subpanel the grounds and neutrals are connected to different buses and the neutral bus is isolated from the panel.
    . Understood!

    Quote Originally Posted by ciladog View Post
    You can convert any panel to a subpanel by simply adding a ground bus and moving all the grounds to the added bus. If the panel was designed to be a main panel, there would be a lug that connects the neutral/ground bus to the panel. You need to remove that lug if converting to a subpanel. ...
    When you say "that lug" are you talking about what I would call the "bonding screw". If so, I understand. If not, I'm lost.

    But generally, it looks like I'm good to go. Thanks!

    VT

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    Yes the bonding screw is what he means. Sounds like you've got it under control.

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    Default Just a note...

    Hi Vernon!

    The reason WHY the bonding screw is a concern...

    In a 'main service panel', the utility neutral comes in through the meter, to the neutral busbar in the panel... and the ground lead from your grounds stake comes in through the panel, to the meter (through meaning... it starts at the ground stake, and passes through the panel ground lug, to the meter, without being cut or broken)...

    And the panel neutral busbar is connected to the ground pin... this is the neutral bonding point.

    In a subpanel, there's no bonding- the neutrals from your circuits all land on the neutral busbar, and the neutral busbar is connected to the main panel by your neutral wire... and there's a dedicated ground wire going from the subpanel to the main panel... no additional ground pin.

    The concept is that the ground wire is not a 'current carrying conductor', it's there for safety... and if the neutral was bonded to ground at both panels, the ground wire could carry neutral current.

    So from another perspective... if you have three wires coming to it (neutral and two hots), it's a main panel application... and if you have four (neutral, two hots, and a ground) it's a subpanel, and should have a separate ground busbar and no bonding.

    I prefer to use main panels as my subpanel, specifically for the reason of having that disconnect. They used cost a little more on account of the main breaker and buying the busbar kit, BUT... nowdays, the pre-configured 'kits', and the presence of busbar kits on the big-box store shelves make it a pretty easy thing to snag...

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    Just for clarrification. The subpanel then needs its own ground stake? Is that correct? Reason I ask is that we have this setup, three hots and a neutral coming from the main panel and then the sub panel has its own ground stake??? Is that the way it ius supposed to be?
    Bob

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    Quote Originally Posted by byawor View Post
    Just for clarrification. The subpanel then needs its own ground stake? Is that correct? Reason I ask is that we have this setup, three hots and a neutral coming from the main panel and then the sub panel has its own ground stake??? Is that the way it ius supposed to be?
    Bob
    USA NEC says that a subpanel must NOT, as part of single-point-ground rule. I frankly don't agree with this aspect, because it is possible to have a subpanel which is a substantial distance from the main (like, an outbuilding 300' away), but what NEC is trying to avoid, is a situation where neutral current at one subpanel, feeds through the ground, to another point... and conversely, ground current flowing through the neutral of a service.

    My personal experience has been, that when lightning strikes some point at the far side of the farm, and the outbuilding there has no ground stake, really bad things happen at the main panel.

    Being in Canada, your national code may have a different point of view, and of course, it doesn't matter one bit what the code says, or what we THINK, or KNOW to be wise, prudent, safe, or possible... the INSPECTOR has the final say, because our world is not based on the laws of physics, nature, or common sense, it is based on the iron fist of the bureaucrat.

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    Dave, I must be really thick and our utility is not much help. My subpanel is in another building about 300 feet from main panel ground at sub. I don't know if the installation was ever inspected, probably not, and I don't want to get an inspector out as I am getting different answers from different inspectors. One guy says it is ok to ground the subpanel at the remote location another guy says to run ground wire back to main panel. I am inclined to just let sleeping dogs lie as it seems to be ok as is
    Bob

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    That would be the best inclination to have. The fortunate thing, is that the INSPECTOR makes the call, and if it's pre-existant and undisturbed, it is generally grandfathered. In MY case, there were so many things worse (and by worse, I mean daily-deadly-worse), that my local guy was happy-as-heck to see that I was WORKING on it... (I think he was darned near ready to offer help!)


    The whole concept of single-point-ground protection between distant farm buildings, with respect to safety, is frequently more argued from a standpoint of academics, than well-founded in reality. There's a book out specifically on Bonding and Grounding, and the two concepts should NOT be confused... but I've found that there's many inspectors wandering about, that really don't have a grip on the concept, NOR the reality... they just read the wording of the code, then red-ink your inspection.

    The first time I was really tangled-up in a grounding/bonding disagreement, was when an inspector found that my nearby radio tower had a grounding field installed. He did not like the fact that I had twenty-four 4' ground stakes, all tied to copper wire leading to a copper ring round the base of my transmitting tower, which included a solid copper strap going from said ring, up 72' to the spike at the top of the tower. He didn't like the fact that my tower was shunt-fed energized during the transmit mode, nor did he like the fact that the guy wires had insulators at several intervals along the way...

    It would have taken a year of schooling and education in electronics, before he would understand what that tower was actually doing, and probably another year of atmospheric physics before he understood the difference between static discharge and lightning... but by some condition of official public blessing, he became an expert on using a red pen.

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    Quote Originally Posted by DaveKamp View Post
    That would be the best inclination to have. The fortunate thing, is that the INSPECTOR makes the call, and if it's pre-existant and undisturbed, it is generally grandfathered. In MY case, there were so many things worse (and by worse, I mean daily-deadly-worse), that my local guy was happy-as-heck to see that I was WORKING on it... (I think he was darned near ready to offer help!)


    The whole concept of single-point-ground protection between distant farm buildings, with respect to safety, is frequently more argued from a standpoint of academics, than well-founded in reality. There's a book out specifically on Bonding and Grounding, and the two concepts should NOT be confused... but I've found that there's many inspectors wandering about, that really don't have a grip on the concept, NOR the reality... they just read the wording of the code, then red-ink your inspection.

    The first time I was really tangled-up in a grounding/bonding disagreement, was when an inspector found that my nearby radio tower had a grounding field installed. He did not like the fact that I had twenty-four 4' ground stakes, all tied to copper wire leading to a copper ring round the base of my transmitting tower, which included a solid copper strap going from said ring, up 72' to the spike at the top of the tower. He didn't like the fact that my tower was shunt-fed energized during the transmit mode, nor did he like the fact that the guy wires had insulators at several intervals along the way...

    It would have taken a year of schooling and education in electronics, before he would understand what that tower was actually doing, and probably another year of atmospheric physics before he understood the difference between static discharge and lightning... but by some condition of official public blessing, he became an expert on using a red pen.
    "Mine is not to reason why. Mine is but to do.. and do.. until I get her done".

    Dave,

    Your description of the grounding ring around your tower is very very close to the way I grounded our farmhouse turned shop. Some years ago, lightning struck the meter pole to our water well. It was astounding. And, although I'm digressing here... let me tell you this.

    One of my earliest memories -- I was literally a toddler -- was that I witnessed ball lightning. I was looking out the giant "picture window" in the living room of our tiny house, during a raging lightning storm. I saw lightning strike the transformer pole at the corner of our lot. The creosoted pine pole INSTANTLY became a glowing live ember. Sparks exploded in all directions.

    But the real show was only beginning. I saw two glowing gobs of molten light jump from the pole to the wire. One followed the other down the entire length of the wire. Perhaps 100' away, in rapid succession, both orbs hit a SECOND power pole. It also burst into a solid glowing ember as sparks exploded in all directions.

    That happened probably because I even learned to talk. Yet the imagery is seared into my memory. Perhaps 30 years later, I was now a practicing court interpreter. In that capacity, I was involved with lots of insurance adjusters and such people. One day, I was helping an adjuster with an investigation. He had a young associate with him. I don't remember the exact context of our conversation, but they were talking about "weird investigations" they'd been involved with. The younger guy mentioned something about a claim for a television damaged by lightning or something similar, and was inferring that the claim was bogus, saying that "I just don't believe lightning could do that".

    The older guy said: "Well, you'd better believe it! Have you ever heard of "BALL LIGHTNING?" That was the first time I ever heard the term for what I saw way back then. It was cathartic to finally have a name for something I had not thought of in many many years.

    Anyway, back to grounding, and the aftermath of the water well strike. Sometime after that happened, I upgraded the service to the old farm house to 200 amps. I also moved the service entrance from one side of the house to the other so that it would be much closer to the transformer pole. The electric utility company heartily approved of this.

    About that time, and because I was a clueless newby doing the work myself, I acquired a book on electricity. I don't remember the title - and I cannot find the book, to my great dismay. But I think it was called "Extreme Wiring" or something similar. If somebody recognizes the book please chime in because I'd love to buy another copy to replace the one that disappeared.

    But one of the salient aspects of the book was the guy's almost religious advocacy of grounding rings for buildings. So, when the time came to install the 200 amp service entrance, I took what he recommended to heart and soul. Instead of buying a single, wimpy ground rod, I bought a BUNDLE of the really good, commercial duty ones. Instead of being 1/2" x 8' they were 5/8" x 10'. I drove those suckers into the ground all the way around the house at evenly spaced intervals. When I came out of the service panel with a bare copper wire, I came out with a ROLL of heavy zinc coated bare copper wire. I connected every single rod to this roll. The last rod stops just short of the point of origin of the grounding ring.

    A few years ago, we had a severe storm. There was a tremendous flash and simultaneous KABOOM. The lights dimmed and the service entrance clicked. But the lights did not fail. We had another storm early this morning. I happen to believe that this house may be the safest place in this county during an electric storm. YMMV.

    Vern

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    A panel in a remote building is treated as a new service. It requires both a main disconnecting means (a main breaker or fused disconnect switch) and a ground rod. The neutral and ground busses are to be bonded together in the panel.

    A panel is a Branch panel when it is in the same building.

    Good examples with drawings are shown in the NEC code handbook. I only use the code handbook because it has the complete code along with the examples. More information and no BS.....



    ...add. If you are going to use multiple ground rods IIRC space them at least 5 feet apart and connect them together with the full sized conductor called for in the code book. If you install a separate Lightning ground, Bond it to the electrical ground in ONE place. As stated in other posts the Single bonding connection made for various reasons is to eliminate circulating currents which might cause a voltage drop in the grounding conductor reducing the ability of the protecting circuit breakers to trip.

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    Your local building codes will dictate if you should drive a ground rod or not at your sub panel. The local code here is: If the sub is used with a 3 wire feed ( Two hots, and a neutral or 3 phase three hots and a neutral ) a ground rod MUST be installed. If panel is used with a neutral, and a ground wire a ground rod is not needed. However most of the time if the sup is in a outbuilding the inspector will ask for a rod to be driven as a safety in case the ground to the main building becomes damaged.

    Im most shops I have done electrical work in I drive rods for each sup panel if in another bay from the main, and have been known to drive a rod for CNC machines.


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