We have a building that has a 350 mcm ground around the perimeter of the building. It has 2/0 tails taped off for attachment to columns and motor bases. The tails come up through the concrete in a 1" rigid conduit. Do these conduits have to have a grounding bushing on them and if so what size ground wire attached to them?
Ground tails through slab
- Thread starter dkuc
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Re: Ground tails through slab
Since the concrete is already poured you can only bond one end of the RMC to the GEC. I don't know if the clamps that are made for attaching raceway enclosed GECs to pipe are suitable for encasement in concrete.
250.64 Grounding Electrode Conductor Installation.
Grounding electrode conductors shall be installed as specified in 250.64(A) through (F).
(E) Enclosures for Grounding Electrode Conductors. Metal enclosures for grounding electrode conductors shall be electrically continuous from the point of attachment to cabinets or equipment to the grounding electrode and shall be securely fastened to the ground clamp or fitting. Metal enclosures that are not physically continuous from cabinet or equipment to the grounding electrode shall be made electrically continuous by bonding each end to the grounding electrode conductor. Where a raceway is used as protection for a grounding electrode conductor, the installation shall comply with the requirements of the appropriate raceway article.
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Tom
Since the concrete is already poured you can only bond one end of the RMC to the GEC. I don't know if the clamps that are made for attaching raceway enclosed GECs to pipe are suitable for encasement in concrete.
250.64 Grounding Electrode Conductor Installation.
Grounding electrode conductors shall be installed as specified in 250.64(A) through (F).
(E) Enclosures for Grounding Electrode Conductors. Metal enclosures for grounding electrode conductors shall be electrically continuous from the point of attachment to cabinets or equipment to the grounding electrode and shall be securely fastened to the ground clamp or fitting. Metal enclosures that are not physically continuous from cabinet or equipment to the grounding electrode shall be made electrically continuous by bonding each end to the grounding electrode conductor. Where a raceway is used as protection for a grounding electrode conductor, the installation shall comply with the requirements of the appropriate raceway article.
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Tom
Re: Ground tails through slab
Tom, I may be wrong, but I do not think these are GEC's. My interpetation of a GEC is the cable from the grounded circuit conductor to the electrode. Seems to me what these conductors are bonding jumpers.
I spec a lot of these type of conductors in the same manner. Sometimes I use PVC, RMC or naked. I agree in practice you should bond to the conduit if used (personally I use #6 AWG on 2/0 AWG), but it may or may not be required by code. It depends on whether or not it is likely to come in electrical contact. And if the cable is bare I do not think you have to bond.
It is true a GEC has to be bonded at both ends if it feeds service equipment, but what he is describing is not service equipment IMHO.
Dereck
Tom, I may be wrong, but I do not think these are GEC's. My interpetation of a GEC is the cable from the grounded circuit conductor to the electrode. Seems to me what these conductors are bonding jumpers.
I spec a lot of these type of conductors in the same manner. Sometimes I use PVC, RMC or naked. I agree in practice you should bond to the conduit if used (personally I use #6 AWG on 2/0 AWG), but it may or may not be required by code. It depends on whether or not it is likely to come in electrical contact. And if the cable is bare I do not think you have to bond.
It is true a GEC has to be bonded at both ends if it feeds service equipment, but what he is describing is not service equipment IMHO.
Dereck
Re: Ground tails through slab
Your point is well taken. I can see how you could argue that they are not GECs.
The reason that I took them to be GECs is the language in 250.64 (F). That language permits you to run the GEC to any convenient electrode and from the electrode to other electrodes. I was looking on the structural steel as a grounding electrode and the ground ring as a grounding electrode with multiple GECs to interconnect them.
If the base of the metallic columns are bolted to tie bolts that are properly tied using tie wire to the rebar in the foundation then the structural steel is a grounding electrode.
This may be a hole in the NEC language. Any single conductor that is run through a metallic raceway will have a high impedance to AC currents if it is not bonded to the metallic raceway. The metallic raceway will behave as a choke. That physical law will not be altered by the name we give to the conductor.
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Tom
Your point is well taken. I can see how you could argue that they are not GECs.
The reason that I took them to be GECs is the language in 250.64 (F). That language permits you to run the GEC to any convenient electrode and from the electrode to other electrodes. I was looking on the structural steel as a grounding electrode and the ground ring as a grounding electrode with multiple GECs to interconnect them.
If the base of the metallic columns are bolted to tie bolts that are properly tied using tie wire to the rebar in the foundation then the structural steel is a grounding electrode.
This may be a hole in the NEC language. Any single conductor that is run through a metallic raceway will have a high impedance to AC currents if it is not bonded to the metallic raceway. The metallic raceway will behave as a choke. That physical law will not be altered by the name we give to the conductor.
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Tom
Re: Ground tails through slab
Dereck and Tom: I have always questioned the popular belief that a metal conduit acted as a magnetic inductive choke, as being the reason for bonding at both ends.
A ground electrode current flow of any large value, can only be from a lightning induced surge, natural electrostatic charges, or contact with a high voltage line.
I don't understand how the metal conduit, will act as a choke for these events.
Is there an explanation that makes sense? that I may understand.
[ August 26, 2003, 01:58 PM: Message edited by: bennie ]
Dereck and Tom: I have always questioned the popular belief that a metal conduit acted as a magnetic inductive choke, as being the reason for bonding at both ends.
A ground electrode current flow of any large value, can only be from a lightning induced surge, natural electrostatic charges, or contact with a high voltage line.
I don't understand how the metal conduit, will act as a choke for these events.
Is there an explanation that makes sense? that I may understand.
[ August 26, 2003, 01:58 PM: Message edited by: bennie ]
Re: Ground tails through slab
Bennie, I will try to explain. When metallic enclosure are made of magnetic material such as steel, the "choke effect" is much greater than in metallic non-magnetic materials, such as alluminum. The reason for this is that the magnetic field associated with the wire is "amplified" when it is brought within the vicinity of the magnetic material(the amount of magnetic flux availible to induce current in the iron increases). This "amplification" effect occurs becaause of the "permability" of magnetic materials is greater than 1. "Permability" is, simply, a number that measures the ability of the material to "amplify" a magnetic field in the presence of current. The greater the permability, the greater the amplification effect.
What does it all mean? It means if you run a GEC in steel conduit without bonding, you would incur a greater voltage drop during a surge event, as opposed if you did bond it.
Here are some interesting mathmatical models of voltage drop on conductors in free air and then conduit based on a 3000 amp in 8 microseconds strike which is a modest number.
1 inch free air = .75 volts 1 inch in conduit = 394 volts.
1 foot in free air = 75 volts, 1 foot in conduit = 4875 volts.
10 foot in free air = 1200 volts, 10 foot in conduit = 48,000 volts.
100 foot in free air = 17,250 volts, 100 foot in conduit = 486,000 volts.
If the models are correct it is clear flashover can and will occur. Hope that helps. Dereck
Bennie, I will try to explain. When metallic enclosure are made of magnetic material such as steel, the "choke effect" is much greater than in metallic non-magnetic materials, such as alluminum. The reason for this is that the magnetic field associated with the wire is "amplified" when it is brought within the vicinity of the magnetic material(the amount of magnetic flux availible to induce current in the iron increases). This "amplification" effect occurs becaause of the "permability" of magnetic materials is greater than 1. "Permability" is, simply, a number that measures the ability of the material to "amplify" a magnetic field in the presence of current. The greater the permability, the greater the amplification effect.
What does it all mean? It means if you run a GEC in steel conduit without bonding, you would incur a greater voltage drop during a surge event, as opposed if you did bond it.
Here are some interesting mathmatical models of voltage drop on conductors in free air and then conduit based on a 3000 amp in 8 microseconds strike which is a modest number.
1 inch free air = .75 volts 1 inch in conduit = 394 volts.
1 foot in free air = 75 volts, 1 foot in conduit = 4875 volts.
10 foot in free air = 1200 volts, 10 foot in conduit = 48,000 volts.
100 foot in free air = 17,250 volts, 100 foot in conduit = 486,000 volts.
If the models are correct it is clear flashover can and will occur. Hope that helps. Dereck
Re: Ground tails through slab
Bennie, I have a copy NFPA 780 at work. I design lightning protection systems per UL-96A and have them master labled by UL. I think both documents are very similar, and both require down conductors to be bonded at both ends if metallic conduit is used to protect the conductors.
Bennie, I have a copy NFPA 780 at work. I design lightning protection systems per UL-96A and have them master labled by UL. I think both documents are very similar, and both require down conductors to be bonded at both ends if metallic conduit is used to protect the conductors.
ron
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Re: Ground tails through slab
Appendix J of NFPA 780 summarize the need for bonding as Resistive Effect and Inductive Effect with regard to a nearby metalic object like a water pipe. It does not seem to explain the reasoning to bond both ends of a metalic conduit enclosing a conductor, ie choke.
Some pertinant articles from that standard:
3.4 Mechanical Damage or Displacement.
...... Where metal pipe or tubing is used around the conductor, the conductor shall be electrically connected to the pipe or tubing at both ends.
3.9.11 Protecting Down Conductors.
..... Metallic guards shall be bonded at each end.
Appendix J of NFPA 780 summarize the need for bonding as Resistive Effect and Inductive Effect with regard to a nearby metalic object like a water pipe. It does not seem to explain the reasoning to bond both ends of a metalic conduit enclosing a conductor, ie choke.
Some pertinant articles from that standard:
3.4 Mechanical Damage or Displacement.
...... Where metal pipe or tubing is used around the conductor, the conductor shall be electrically connected to the pipe or tubing at both ends.
3.9.11 Protecting Down Conductors.
..... Metallic guards shall be bonded at each end.
Re: Ground tails through slab
Bennie, are you familar with ferrite beads? You add ferrite to a wire or cable to raise its inductive impedance. It is the same principle.
Another example would be is how do you make a RF choke coil? You could take a wire a coil it up. But you could take the same lenght and wind it around an iron or ferrite rod and obtain a much higher inductance.
Bennie, are you familar with ferrite beads? You add ferrite to a wire or cable to raise its inductive impedance. It is the same principle.
Another example would be is how do you make a RF choke coil? You could take a wire a coil it up. But you could take the same lenght and wind it around an iron or ferrite rod and obtain a much higher inductance.
Re: Ground tails through slab
Dereck: I have a hard time comparing one single conductor to a coil. The one conductor will have DC current with an AC component, but no reversal in direction, from a low side surge created by a lightning strike on the transmission line.
The high side surge will transfer to the secondary by capacitance coupling, not magnetic inductance. Therefore I don't think there will be any great amount of self inductance to produce the choke effect. I can see there will be an energy field, but no magnetic field of any consequence. The energy field will create a charge of potential difference, not a high impedance like a choke coil.
I think technology has gone into the "Twilight Zone", maybe along with my head
Dereck: I have a hard time comparing one single conductor to a coil. The one conductor will have DC current with an AC component, but no reversal in direction, from a low side surge created by a lightning strike on the transmission line.
The high side surge will transfer to the secondary by capacitance coupling, not magnetic inductance. Therefore I don't think there will be any great amount of self inductance to produce the choke effect. I can see there will be an energy field, but no magnetic field of any consequence. The energy field will create a charge of potential difference, not a high impedance like a choke coil.
I think technology has gone into the "Twilight Zone", maybe along with my head
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Re: Ground tails through slab
Derekbc thanks for the data on current over GEC in conduit.
Do you feel the reason metallic conduit is required to be bonded at both ends is:
>minimize inductive choke
>flash over
I feel it is both, depending on the use and location.
Derekbc thanks for the data on current over GEC in conduit.
Do you feel the reason metallic conduit is required to be bonded at both ends is:
>minimize inductive choke
>flash over
I feel it is both, depending on the use and location.
Re: Ground tails through slab
I will buy the flashover part, but have to be convinced about the magnetic induction.
The flashover is created by the e field charging the conduit. e fields create voltage, magnetic fields create current.
Years ago, when I was active in ham radio operation and experimentation, I hand wound inductors. Testing to cut off was by applying a signal from an RF generator, and noting the point of no current flow.
I would like to see a white paper on this phenomenon of a single wire having enough self inductance at 60 Hertz to reach cut off, even if surrounded by a ferrous material.
I will buy the flashover part, but have to be convinced about the magnetic induction.
The flashover is created by the e field charging the conduit. e fields create voltage, magnetic fields create current.
Years ago, when I was active in ham radio operation and experimentation, I hand wound inductors. Testing to cut off was by applying a signal from an RF generator, and noting the point of no current flow.
I would like to see a white paper on this phenomenon of a single wire having enough self inductance at 60 Hertz to reach cut off, even if surrounded by a ferrous material.
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