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  • Electric again

    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
    For replacement of non-grounding-type receptacles with grounding-type receptacles and for branch-circuit extensions only in existing installations that do not have an equipment grounding conductor in the branch circuit, connections shall be permitted as indicated in 250.130(C). C) Nongrounding Receptacle Replacement or Branch Circuit Extensions. The equipment grounding conductor of a grounding-type receptacle or a branch-circuit extension shall be permitted to be connected to any of the following: (1) Any accessible point on the grounding electrode system as described in 250.50 (2) Any accessible point on the grounding electrode conductor (3) The equipment grounding terminal bar within the enclosure where the branch circuit for the receptacle or branch circuit originates (4) For grounded systems, the grounded service conductor within the service equipment enclosure (5) For ungrounded systems, the grounding terminal bar within the service equipment enclosure If you use the water pipe, the connection must be within 5 feet of where the pipe enters the building.
    You are permitted to replace a two-wire nongrounding type receptacle with a GFCI under the following in section 406.3 (3) Nongrounding-Type Receptacles. Where grounding means does not exist in the receptacle enclosure, the installation shall comply with (a), (b), or (c). (a) A nongrounding-type receptacle(s) shall be permitted to be replaced with another nongrounding-type receptacle(s). (b) A nongrounding-type receptacle(s) shall be permitted to be replaced with a ground-fault circuit interrupter-type of receptacle(s). These receptacles shall be marked "No Equipment Ground." An equipment grounding conductor shall not be connected from the ground-fault circuit-interrupter-type receptacle to any outlet supplied from the ground-fault circuit-interrupter receptacle. (c) A nongrounding-type receptacle(s) shall be permitted to be replaced with a grounding-type receptacle(s) where supplied through a ground-fault circuit interrupter. Grounding-type receptacles supplied through the ground-fault circuit interrupter shall be marked "GFCI Protected" and "No Equipment Ground." An equipment grounding conductor shall not be connected between the grounding-type receptacles. When you make this change be very, very careful that you get the hot "black" wire on the right side of the outlet.

    (B) With Circuit Conductors. By an equipment grounding conductor contained within the same raceway, cable, or otherwise run with the circuit conductors.
    One of the functions of an equipment grounding conductor is to provide a low-impedance ground-fault path between a ground fault and the electrical source. This path allows the overcurrent protective device to actuate, interrupting the current. To keep the impedance at a minimum, it is necessary to run the equipment grounding conductor within the same raceway or cable as the circuit conductor(s). This practice allows the magnetic field developed by the circuit conductor and the equipment grounding conductor to cancel, reducing their impedance.
    Magnetic flux strength is inversely proportional to the square of the distance between the two conductors. By placing an equipment grounding conductor away from the conductor delivering the fault current, the magnetic flux cancellation decreases. This increases the impedance of the fault path and delays operation of the protective device.


    • #3
      About "isolated grounds". There are no such things and there cannot be such things. I know they are out there, and I know some of you make the installation under the guidance of the design engineer, or because the electronic guy said it had to be this way or else. I will repeat, THERE ARE NO SUCH THINGS AS ISOLATED GROUNDS.
      You will probably find that there is no particular continuity to what I am about to start on. There is no easy way in 20 words or less to do this. So, lets just start. Section 250.146(D) covers "Isolated Receptacles". This section address the IG grounds and states in part; "The receptacle grounding terminal shall be grounded by an insulated equipment grounding conductor run with the circuit conductors." Let's look at this sentence first. The insulated grounding conductor is to run with the circuit conductors --- How can it do this if it goes to a water pipe, or a separate ground rod someplace?
      Next sentence "This grounding conductor shall be permitted to pass through one or more panelboards without connection to the panelboard grounding terminal as permitted in 408.20, Exception, so as to terminate within the same building or structure directly at an equipment grounding conductor terminal of the applicable derived system or service." This sentence tells you where the insulated ground is to terminate. How can it terminate within the same building, on the grounding terminal of the service, or the separately derived system (transformer), if it goes somewhere else?
      "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.
      Where do we ground equipment? And yes, electronic equipment is equipment under the code and must comply with all the rules. I know some electronic people think they are special, and are exempt from certain aspects of the code, but believe me they are not. They must follow the same rules as the rest of us. Section 250.6(D) contains the following statement: "(D) Limitations to Permissible Alterations. The provisions of this section shall not be considered as permitting electronic equipment from being operated on ac systems or branch circuits that are not grounded as required by this article. Currents that introduce noise or data errors in electronic equipment shall not be considered the objectionable currents addressed in this section."
      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. (Yes, there is an exception for 'supplemental grounds') 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)."
      Everything stays together, including the IG ground.
      Section 250.54 allows for what the code calls "supplementary grounding electrodes". Some electronic equipment in the installation instructions require this electrode. Why they want these I have no idea, but some electronic people think that if it isn't connected to earth, then it isn't grounded. They don't understand that the earth is loaded with stray currents from many things, but so be it. These stray currents come from many sources. One is the fact that between your grounding electrode at home the system transformer, there are currents through the earth in parallel with your service neutral. These are a fact of life and how much current depends on a lot of things. Also, you, or your neighbors could have a piece of UF going out to a yard light that has a nick in the insulation. Black or white, it don't matter. Some current is leaking out of this nicked insulation and will find it's way back to the electrical system through the earth. Then there are the installations that now grounded to a ground rod because that is the way they were put in. 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.
      Take at least one example: A piece or equipment is grounded to a ground rod to satisfy the electronic people. They insist that the equipment be connected to an isolated ground. The average ground rod will megger well over 100 ohms. But just suppose you are lucky and manage a 10 ohm ground rod. 120 volt divided by 10 means that leas than 12 amperes will flow in case of a ground-fault. Will this clear a 15- or 20-amp overcurrent device? No way, but there is 120 volts going down the rod, and 120 volts on the metal of the equipment, just waiting for someone to come in contact with it. Where is this current going from the ground rod. Pretty much where ever it wants, and if there is a swimming pool nearby that for some reason wasn't properly bonded, or has a bad bonding connection, this could be time for an electrocution. About the overcurrent device, in order to clear a standard 20 amp circuit breaker in a reasonable time, it must have close to 80 to 100 amperes pass through it. Kind of hard to do when the rod will only pass 10 or 12 amperes.
      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.
      As for isolation transformers, these are no different electrically than any other transformer. All transformers, other than autotransformers, are isolation transformers. By this I mean the primary is isolated from the secondary. The difference between a standard and an isolation transformer is the addition of a grounded metal shield between the primary and the secondary winding. All separately derived systems, transformers, must be grounded and bonded to comply with section 250.30. No exceptions are made for isolation transformers, nor for electronic equipment Again, some people feel that these transformers are neither bonded or grounded because they are special. If the bonding connection is not made, there is no way at all to complete the ground-fault return circuit for opening the overcurrent device. You wind up with phase voltage imposed on the entire structure, and this can be deadly


      • #4
        Whew. I think my eyes got dizzy before I could get a clear picture of what you are saying. So let me ask what you are referring to in regards what others are calling "isolated grounds."

        To me, an "isolated ground" is usually run with a dedicated receptacle and the isolated ground does NOT bond to anything on the way, but it is ALSO run with a normal ground wire which does. Both are landed at the ground bus in the panel, and at the receptacle, the normal ground (which is bonded to the conduit, building, etc) is landed to the box. The isolated ground is landed to the receptacle itself and nowhere else.

        We've run a lot of what are called "isolated grounds." We use conduit with THHN or MC cable with an extra ground wire (green with yellow stripe usually) in addition to the hot, neutral, and normal ground. The isolated ground is treated like an ungrounded conductor as far as care in installation. It is only stripped of insulation at the ground bus in the panel and at the grounding screw of the receptacle or wirenuts to an isolated ground tail on the receptacle. They are usually orange, hospital-grade (green dot), and have a triangle on the other side of the green dot. Here's an example of what we install:

        Maybe this isn't what you are getting at at all, though?

        EDIT: In our large commercial installations, we run the isolated ground system to the service entrance or main electrical room for that building. From there, it might be a larger grounding conductor that even has isolated ground buses in subpanels, and then run as described above, with grounds and isolated grounds never bonded again, not even in the subpanels; only in the main electrical room for that building or the service entrance panel.

        I've never seen an "isolated ground" mean that it goes directly to a ground rod, if that's what you were describing.


        • #5
          Right, for the purpose of our discussion ground has nothing to do with ground rods and yes an iso is insulated all the way back to the main. You are right, iso doesnt mean a rod which is illegal if it used as the sole grounding means. With computers and iso it can be accomplished at home easily, run a new circuit back to the main panel with romex and make sure the recept box is insulated or plastic and you have an iso circuit. In this type of circuit you wouldnt need an iso recept any would work, if its piped or has metallic cable then you need the hospital iso type recept.


          • #6
            We focus a lot on wire size and breakers, recepts but this is the number one saftey issue with DIY wiring in my book. Almost any wire will fault a breaker, even an undersized one but without a return parth it will not and there could be 120V potentials on equipment you may not be aware of until,, zap


            • #7
              I believe the NEC isnt even clear on 'bonding' the iso gnd bar to the neutral. With an iso gnd, you just dont want unwanted EMI between the source and the recep.(being picked up). Ive installed them but always bonded to neutral. In theatres, ive used isolated transformers which did the job better and had it tested by an audio engineer to see the before and after...night and day.In my experience, i only see an iso being neccesary when it's a 3phase service or with multiple circuits in conduits. Best test...notice how your tic tracer or current tracer acts very erratic or inaccurate in these instances.
              BAKERY MECHANIC


              • #8
                It would be a good question for a code forum, I need to credit Bob Keis with writing that, I should attatch a note to it, I saved them from forum posts, beats typing it all every time.


                • #9
                  Definitly...anything i looked up mentioned the 'no connection to neutral' problem. Knowing this is what makes guys like you and Mac better mechanics. That ability to say..."something here just doesnt sound right"...L
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