600 amp service

[iwire]

Re: 600 amp service

Originally posted by bennie:
The earth connections properly bonded constitute one ground point regardless of number of electrodes.

Bennie please explain how if these electrodes are each say 70' apart why there would not be current flow on the bonding conductor?

And if there is current flow on this bonding conductor why that is OK, but having the same current flow on the panel grounding bus is bad?

 

[eprice]

Re: 600 amp service

Well, this thread has split off into two threads it seems, but to comment on the question concerning the size of the GEC to a rod:

I don't see anything confusing about the wording of 250.66(A). If a GEC connects more than one electrode, then it is sized according to Table 250.66, but if there is a portion of it that only leads to a rod, then "that portion" only needs to be #6. If there is a portion that only leads to a concrete-encased electrode, then "that portion" only needs to be #4 [250.66(B)]. If a particular GEC only connects to rods, then "that portion" that only needs to be #6 will be the entire conductor.

Suppose we run our conductor first to a metal water pipe and from there to a pair of rods. The first portion leads to both the pipe and the rods, so it must be sized according to table 250.66. But from there, "the portion" that goes from the pipe to the rods only needs to be #6.

Technically, I believe in this case, the portion from the pipe to the rods would actually be a bonding jumper, but according to 250.53(C) this bonding jumper is sized the same way as a GEC

[ January 02, 2004, 01:40 PM: Message edited by: eprice ]

 

[don_resqcapt19]

Re: 600 amp service

Bennie,
If you are correct, then 250.54 should not be in the code book?
Don

 

[Ed MacLaren]

Re: 600 amp service

Bennie said

Multi point grounding is when more than one connection is made to earth.

I maintain - Multi-point grounding is when more than one grounding connection is made to the grounded (neutral) circuit conductor.

A "ground loop" can only exist if it includes a circuit conductor.

Ed

[ January 02, 2004, 02:00 PM: Message edited by: Ed MacLaren ]

 

[bennie]

Re: 600 amp service

Jim: You got it

 

[bennie]

Re: 600 amp service

The above drawings are electrically identical. Take a voltage reading between the two electrodes. There will be a voltage reading. Only when there is no potential difference is there proper bonding.

Only when the electrodes are daisy chained, not separately fed, will there be no voltage.

Take a high resistance input voltmeter and read between any two places 4 feet apart in the earth. I read 1 millivolt at the surface. This reading will increase with length and depth increase.

Now bond the two tester probes with a 4 foot piece of #12 cu. There will not be a voltage difference in the earth points. Bonding is therefore short circuiting. If you don't short circuit the ground electrodes there will be a difference in potential.

 

[hurk27]

Re: 600 amp service

Bennie does have something as That is exactly what I was shown in school. And this is what the NSI web site say's too. But I have seen where a lightning strike had blown a single wire ground feeding the bussbar usealy located outside apart. and I think redundancy would be good.


The grounding system must address low earth impedance as well as low resistance. A spectral study of lightning's typical impulse reveals both a high and a low frequency content. The high frequency is associated with an extremely fast rising "front" on the order of 10 microseconds to peak current. The lower frequency component resides in the long, high energy "tail" or follow-on current in the impulse. The grounding system appears to the lightning impulse as a transmission line where wave propagation theory applies.

A single point grounding system is achieved when all equipment within the structure(s) are connected to a master bus bar which in turn is bonded to the external grounding system at one point only. Earth loops and differential rise times must be avoided. The grounding system should be designed to reduce ac impedance and dc resistance. The shape and dimension of the earth termination system is more important a specific value of the earth electrode. The use of counterpoise or "crow's foot" radial techniques can lower impedance as they allow lightning energy to diverge as each buried conductor shares voltage gradients. Ground rings around structures are useful. They should be connected to the facility ground. Exothermic (welded) connectors are recommended in all circumstances.

Cathodic reactance should be considered during the site analysis phase. Man-made earth additives and backfills are useful in difficult soils circumstances: they should be considered on a case-by-case basis where lowering grounding impedances are difficult an/or expensive by traditional means. Regular physical inspections and testing should be a part of an established preventive maintenance program.

Somthing else to think about is how short you need to keep your grounding conductors.

Under fast rise time conditions, cable inductance becomes important and high transient voltages can be developed across long leads

This whole article can be read at : NSI Lightning Info

 

[hurk27]

Re: 600 amp service

Ed what your describing is multi-point grounding of a current carrying conductor. What Bennie is describing is a multi-point earth connection that in the event of a lightning strike will place a very high voltage across the different connections and can cause step potential between the different electrodes that would exceed any thing beyond your wildest dreams. Some of these events can place as high as 400,000 volts in between electrodes. I wouldn't want it. This is why I think NFPA should take lightning protection out of the NEC and keep it to a separate code all on it's own but have it enforceable along with other fire codes that can be adopted by units of government. Because most grounding electrodes are made inefficient by the lenght of GEC to the electrode anyway. Lightning only sees the path of impedance to earth not resistance. This is what most misunderstand. and unless you have had training in radio frequency systems it is hard to understand. Like I said before. What is a dead short to DC can be a open circuit to HF AC. and visa-versa.

P.S. Removing it from the NEC would also maybe remove some people's thinking that the electrode is for fault current too :roll:

[ January 02, 2004, 07:46 PM: Message edited by: hurk27 ]

 

[Ed MacLaren]

Re: 600 amp service

Ed what your describing is multi-point grounding of a current carrying conductor

Of course. What else is there. When electricians talk about grounding something, they don't mean grounding the earth. :roll:

Ed

 

[hurk27]

Re: 600 amp service

Huh? I know you know the differance between a EGC and a GEC. The bottom of the last diagram you showed a multi-point connection of the neutral to EGC bond, but Bennie is talking about the multi-point connection to earth of the GEC. or am I missing somthing?

 

[wattey]

Re: 600 amp service

I get the impression Bennie is an industrial Electrician. The system I worked on most was a common Daisy Chained Earth. Simply because Multiple Earths would have a potential Difference and cause Noise .. Thus screwing our HF systems and PLC/PC hybrid systems.

A Lightning strike would have been catastrophic on the system, in any event, our Lightning protection was handled by separate rods and grounding on Water-towers. (Lightning strikes are pretty rare in England thankfully)

I am enjoying reading the discussion though because the *small* amount of domestic work I did do in England was also done with redundancy built in (Multi-Bondings)

Horses for courses I guess. The home does not generally have HF or other sensitive systems so redundancy is a good thing. Industry using HF systems wont tolerate *noise* though. Interesting to see different opinions of the wording of the code though.

Shaine~

 

[tim]

Re: 600 amp service

I can't figure out how my simple original post got to this point. Thank you to those who didn't get away from my question. Does anyone even remember what it was? I think if the only grounding means I have is a gnd rod theen #6 copper is the size to use. I would probably also need to put two rods down. Also, since each of the five units has it's own independant water supply being fed with plastic water mains, then I would bond each of these units water lines from thier own units sub-panel. Pretty sraight forward I would say. Thank you Tim. :roll:

 

[pierre]

Re: 600 amp service

Current will follow all paths, as in a fault most current will follow the grounded conductor back to the transformer, and some will follow the GEC - through earth and back to the transformer. Ohms law will tell us just how much would travel in each path.

If I take a conductor and place it between two points, it is 'shorted' as Bennie has stated. If I short it at the location of the ground rods or at the same location in at the service, it is still shorted. That is why the NEC permits both of the installation methods.

Pierre

 

[iwire]

Re: 600 amp service

Tim posts take on a life of there own and often we end up learning much more because of that.

You said you have no available electrodes.

So as testing one rod is more of a pain than just driving two rods, just drive two rods run a 6 AWG to them and you are done with that.

Yes you will have to bond the water pipes for each unit sizing the bonding jumper by table 250.66 and for you that means a 2/0 bonding jumper.

Bob

 

[bennie]

Re: 600 amp service

Tim: Thanks for opening this subject, sorry you are not satisfied with the forums response.

Bob answered your original question, in a very professional manner, on post one, right after your question.

His answer did not leave any room, or need, for a further discussion. What developed was from everyones open mind thinking.

This is what these forums do.

Again thanks, post more simple problems, we all learn from them.

Regards: Bennie

 

[tim]

Re: 600 amp service

Once again guys, thank you very much. Everyone have a great 2004. Tim Hausmann-Champaign Illinois.

 

[tim]

Re: 600 amp service

Iwire- I thought I was done but I just read your last post and have to respond. Each sub-panel is fed with a #3 thhn. I would in my opinion bond each units water pipes with #8 copper using table 250.66. Each sub-panel is fed with 3 #3 thhn and a #6 gnd. I don't see why I would need t
o bond with 2/0. Here we go again Tim. LOL Of course, the bonding would come from each sub-panel for each unit. The water suppies are completly seperate from each other.

 

[bwyllie]

Re: 600 amp service

the bonding at the individual units is due to the water meter for each apartment? therefore, if a building had just one main water meter, the only bonding of the water piping is at that meter? correct?

 

[iwire]

Re: 600 amp service

Here we go again

In this case size the bonding jumper by 250.122

I was thinking of 250.104(A)(1) which would require sizing to 250.66.

Your situation and you should look for yourself to make sure, sounds like 250.104(A)(2) would be the article for you and table 250.122 will be your guide.

 

[tim]

Re: 600 amp service

Thanks Bob, have a great weekend. I really do appreciate yours and everyone elses opinion. As someone who usually does just single family homes I don't get into these situations very often. Again thanks, Tim Hausmann