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some things about wiring panels.

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  • some things about wiring panels.

    The panels in separate buildings are not really subpanels as far as code is concerned. They are the service equipment for the structure. You can have a subpanel from the service equipment in each building if you want. Between buildings, section 250.32 applies and if there are metal interconnections between buildings such as water piping, or air lines, or any metal interconnection at all, then you must install an equipment ground wire so if you have a 120/240 system, you would have four wires. The neutral would be separated in each building and a grounding bar would be installed, just like a subpanel. Then the code requires a grounding electrode conductor (GEC) to a grounding electrode (usually a ground rod). The GEC will be connected to the equipment ground bar at each building. This is not to clear overcurrent devices, this is for two reasons. One is lightning, the more important one is to put the equipment ground at the same relative potential as the earth. This is for step potential or touch potential voltages so that what you touch in the building is at the same potential as what you are standing on. Now the tricky part. If you do not have any interconnecting metal between buildings, the code allows you to install three conductors between buildings. When you do this you bond the neutral and ground the neutral just like a new service. Some inspectors think that every panel in a separate building must be treated as a subpanel, but this is not true. In past codes (1996 and older) these rule were in section 250-24 and Exception 2 addressed the grounding bus.

  • #2
    "Section 250.4(D) (5) Effective Ground-Fault Current Path. Electrical equipment and wiring and other electrically conductive material likely to become energized shall be installed in a manner that creates a permanent, low-impedance circuit capable of safely carrying the maximum ground-fault current likely to be imposed on it from any point on the wiring system where a ground fault may occur to the electrical supply source. The earth shall not be used as the sole equipment grounding conductor or effective ground-fault current path." This section requires a low impedance fault return path. The only way to achieve this is to have the equipment ground wire in the same cable, or raceway with the circuit conductors. You cannot separate current flow and maintain a low impedance, it is impossible. Notice one more thing in the above statement. The fault return is go back to the electrical supply source, not where else. This next section tells us what we are to ground equipment to.
    Believe it or not, no equipment goes to a ground rod, and no equipment whatever is ever connected to a ground rod to ground it.
    ) Section "250.4(A)( (3) Bonding of Electrical Equipment. 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." The electrical supply source is the system from which the circuit originated, actually the supply neutral.
    Remember this statement: When the code requires a piece of equipment to be grounded, it is grounded (bonded really) to the system grounded circuit conductor, the neutral. It is never connected to a ground rod, a water pipe, building steel or anything else. It goes directly to the system grounded circuit conductor. (in the case of delta systems it goes to the grounded service equipment).
    Then, we ground the system grounded conductor, the neutral, to earth, no equipment to earth, the system neutral to earth. Stop for a minute and consider where you put all the equipment grounding conductors at home. Most of you wired with Romex and the bare ground is landed directly on the neutral bar in the main service disconnect. Not in a subpanel, but at the main itself. Then you grounded the neutral. Thus all the equipment in you house is grounded to the neutral, just like is supposed to be.
    Remember this also, the code requires a low impedance ground-fault return path for fault current. In order to obtain this, we must keep all the circuit conductors and the equipment grounding conductors in close proximity in the same raceway or cable. The is also required in "300.3 (B) Conductors of the Same Circuit. All conductors of the same circuit and, where used, the grounded conductor and all equipment grounding conductors and bonding conductors shall be contained within the same raceway, auxiliary gutter, cable tray, cablebus assembly, trench, cable, or cord, unless otherwise permitted in accordance with 300.3(B)(1) through (4)."


    • #3

      A good example of these are mall parking lot lights, or any large area lighting. A lot of these have no ground wire to them, they are grounded by way of a rod. If one of these lights develops a ground-fault, the current flow is down the rod, then back to the source through the earth. There are many references in the code that prohibit using the earth for an equipment grounding conductor, but these installations exist by the hundreds. If a ground rod is driven into an area that has any of these conditions, the current will be imposed on the equipment connected to this "isolated ground rod". This creates a shock hazard when touching the equipment grounded this way, and any equipment in the building that is connected to the building grounded system. I strayed, back to 250.54. This section permits a supplemental grounding electrode at the equipment, but the electrode must be connected to the equipment grounding conductor in the circuit to the equipment. A lot of mall lighting is installed this way. The supplemental electrode is bonded to the equipment ground of the light, and it provides added safety for lighting hits. A lot of the lightning energy will dissipate down the rod. Otherwise it would go back on the equipment ground in the circuit and since the insulation is only 600 volts, it causes a lot of damage to conductors. Thanks to Bob Keis for writing this up.


      • #4
        After 40+ years of pulling wire, hanging wire, and making power distribution systems run from here to KheSahn, I'm still puzzling out witch of 3 "professions" is the dumbest, and witch is funniest,
        Jr Engineering Officers, Electrical Inspectors, or Design Engineers.
        The majority of Inspectors I've come across tended to evolve from prior carears as parts counter men, or household "electricians", with a few old timers who were just too wore out or busted up to still work at the trade. Having met a few hundred of them, I don't recall 3 who knew what a ground resistance meter was, or how to set one up and understand the readings. The good inspectors will meet you on the job, and ask you why you did something the way you did, and talk it over if they have a question, and the counter men will just check off code sections on a rejection slip and leave. I can't tell you the number of phone calls I've made to Chief Inspectors over rejection slips, but I also don't recall one that I didn't get voided.
        Design engineers, well, I long ago concluded they got their education by falling out of the cab of a locomotive and landing on their heads. I've seen more machines with undersized splice connection boxes and raceways than I can count.
        Grounds are very interesting things, especially when you factor in the resistance of soil cover and conductivity between earth and ground rods. Grounds and Neutrals are NOT the same, although back in the 60s, people tended to think of them interchangibly. Ground conductors carry fault current, and Neutrals carry the imbalance of load between the 2 (120v) halves of the (240v) transformer winding. When inductive loads are involved, there will very often be voltage between Neutral and Ground.
        I'll not argue that ALL grounds should be bonded together, other than to say that the bonding should be accomplished by exothermic welding. Corrosion on grounds can cause tremendous problems.
        Grounding has become even more important with the evolution of PVC conduit systems, although it still tends to be addressed casually.
        From where I sit, there can never be too many grounds on a system. Ground rods are easy to drive, and don't cost much money, and may be the difference between life and death.
        Code books are wonderful things, but they will never be a substitute for common sense and experience when it comes to installing a grounding network.


        • #5
          I am going to post this in both threads because its important. Yes, there tends to be some confusion but there doesnt need to be. For 99% of the people on this forum there is only one thing that is really important about grounding. When you read the instructions it says "Must be connected to an electrically grounded circuit." This doesnt mean water pipes or ground rods but only one thing. That the grounding conductor comes with the current conductors and ends up back at the neutral bar at service entrance. If you are on a properly wired subpanel you will hook the ground wire to the seperate equipment ground bar not the neutral bar. Its really that simple for the most part.


          • #6
            Franz, there are allowances for installing extra ground rods in certain circumstances provided they are connected to the primary grounding system of the service entrance. Most of that is beyond the need of the average guy here and may do more harm than good and have nothing to do with the equipment saftey ground of these welding machines. Talking about ground rods here even adds more confusion. That thread that was done by bnelson in practical machinist eluded to that and as he sugested,, just use the cord. I dont really see how pounding some ground rods is going to solve a life and death issue, but may actually even bring on stray currents and create a loop. For practical purposes here we are talking about clearing faults and I know you know that the only way is to get the current back to the transformer via the entrance. I think we can confuse more than we help sometimes.