Bonding conduit to electrode

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I have a question I was hoping you can answer. 250.64E states " Ferrous metal enclosures for grounding electrode conductors shall be electrically continuous from the point of attachment to cabinets...and shall be securely fastened to the...fitting." Heres my scenario: 6awg CU stranded bare grounding electrode conductor traveling from the main disconnect to the ground rod running within a 1/2" EMT w/compression connectors attached to MD can (direct connection, no coccentrics). The conduit is ran along the face of the building (to protect wire from physical damage) in one complete bonded system (1-LB rigid; 2-connectors on each end of LB; 1-connector on the can; open bottom to allow the electrode conductor to exit to the rods.) Does the mentioned application have any need to install grounding bushings on each end and why?
I recently purchased the NEC handbook for this specific question and noticed that I am required to bond each end. On that article, it is contradicting itself and on the article of using EMT as a grounding conductor when not using a bonding conductor. Looking at the procedure for a mathematical standpoint, if the electrode (A) equals the service disconnect panel (B)-due to the bonding screw; and the service disconnect panel (B) equals the electrode grpounding conductor raceway (C)...then shouldn't A=C? There shouldn't be any potential difference if the conduit is bonded effectively. The concept of using grounding bushings on this application means all conduits coccentric or not, must have a bonding bushing on each end AND cannot be used as a grounding condutor.
 
25064E.jpg



I dont understand all of it, but the emt somehow acts as a choke so you need to bond it...
 
stickboy1375 said:
25064E.jpg



I dont understand all of it, but the emt somehow acts as a choke so you need to bond it...
Click to expand...


Is a bonding bushing really required within the CB enclosure? The neutral is bonded to the case of the enclosure, the GEC is connected to the neutral and the conduit is connected to the enclosure. IMO they're already connected together without the additional bonding bushing.
 
infinity said:
Is a bonding bushing really required within the CB enclosure? The neutral is bonded to the case of the enclosure, the GEC is connected to the neutral and the conduit is connected to the enclosure. IMO they're already connected together without the additional bonding bushing.
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I dont know enough about it, but I think we had this discussion before about it really not being required...
 
Whenever you have current flowing through a wire, you get a magnetic flux around that wire.

Whenever the current flow through a wire is _changing_, the magnetic flux around that wire is changing.

Whenever you have a wire in changing magnetic flux, a voltage will be induced in that wire. (Yes, I know that this sounds like a chicken and egg situation, current creating field creating voltage, but that is the way it works.)

The polarity of this voltage is such that it acts to counter the change in current flow. The magnetic flux around a wire acts to stabilize the current flow, much as the inertia of a flywheel acts to stabilize rotation.

For normal house wiring, this effect can be ignored. The magnetic field is very weak, and is essentially canceled out by the balanced current flow in the other wire, so very little flux gets created and there is very little induced voltage.

This effect is _intentionally_ used in transformers; a wire gets wrapped around a magnetic core, greatly amplifying the effect of the magnetic flux, to the point where the induced voltage is as great as the supply voltage.

When you surround a conductor with a magnetic conduit, you get the same effect; the magnetic conduit amplifies the flux produced, thereby increasing the induced voltage. For ordinary 60Hz current flow at ordinary supply levels, this impedance effect is pretty minimal. But for high frequency, high voltage events such as lightning, this 'inductive impedance' is quite large. If you don't bond the GEC at both ends, then in the event of a high current transient, this inductive impedance will mean significantly more current following other paths.

By bonding the conduit to the GEC at both ends, you don't eliminate the inductive impedance in the GEC itself, but you provide an alternative path: the conduit.

-Jon
 
Ah hahhhh!

Ah hahhhh!

I see one of Holt's pic's with two ground rods. I am not stirring anything up, I have just never seen that one before.
 
bonding bushing requirement

bonding bushing requirement

The need for a bonding bushing has always been clear to me when I consider the danger associated with the pipe becoming energized during a fault condition - where if someone's touching it they could get shocked. The bonding bushing keeps the pipe at ground potential.
JM
 
Rob,
Is a bonding bushing really required within the CB enclosure?
Click to expand...
I asked that question of Phil Simmons a couple of weeks ago at a class and his reply was "yes". (Phil is a former chairman of CMP 5) Not that his answer is any more valid than one of ours.
Don
 
don_resqcapt19 said:
Rob,

I asked that question of Phil Simmons a couple of weeks ago at a class and his reply was "yes". (Phil is a former chairman of CMP 5) Not that his answer is any more valid than one of ours.
Don
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I saw one of Stallcup's books in a book store and he had a bonding locknut in lieu of the bonding bushing (don't know where that requirement came from). I really don't see how the bonding bushing will make any difference if the neutral, GEC and raceway are all connected together.
 
6awg CU stranded bare grounding electrode conductor traveling from the main disconnect to the ground rod running within a 1/2" EMT
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Gotta use PVC. If lighting strikes the EMT acts as a choke....whatever that means. Seems dumb but I didn't go to college so it's easier just to find a way around it.....like PVC.
 
You did the right thing on asking the question rather than just going with PVC. It's the only way you learn your craft. I see pride in you and thirst for the correct knowledge.
 
Bonding Bushing wire Size

Bonding Bushing wire Size

How do you size the wire to bond conduits together? For example you are installing a 1600amp feed from main gear to sub Panel you have 4-runs of pairalle 500 MCM with 4/0 ground. do you need to hit every bushing with the a 4/0 or can you bond them together with a smaller wire
 
stickboy1375 said:
I dont understand all of it, but the emt somehow acts as a choke so you need to bond it...
Click to expand...
220/221 said:
If lighting strikes the EMT acts as a choke....whatever that means.
Click to expand...
It works like the cylindrical lump at one or both ends of most of your computer cords. Inside is a tubular ferrite core that the cord passes through. Think of a CT donut, with a phase wire passing through.

A high-frequency signal/noise causes a magnetic field in the core, which in turn generates an opposing electrical current. Lower frequencies pass through, because their magnetic generation is too weak.

Where does the blocked energy go? It's dissipated by the core as heat. Needless to say, lightning's heat generation is a bit stronger than electronic noise signals, due to the current and myriad frequencies.
 
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