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Old 02-27-2008, 07:17 AM   #61
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Sub Panel Grounding Question.


I haven't had a return call from the inspector yet. I am going to talk to a friend that is an engineer. I am an electronics tech by my first trade. All of this talk about current paths, and grounding protection is great until you get to the grounding electrode at the main panel neutral/ground bar/electrode. The center tap of the transformer is at the same potential as the equipment ground, and neutral at this point. The neutral path should only have current/amperage when there is a load applied. Be it a high impedance or low, it is a load. Low impedance usually causes smoke, or a tripped breaker, or both. Now, if I go to an old house with a 200 amp main panel that looks good, and put in a detached sub-panel in a garage, and put in a nice new shiny copper clad ground rod, everything is up to code. If the ground rod at the main is an old galvanized/ brass acorn ground rod with corrosion from age, and electrolysis that makes the ground at the sub-panel the least path of impedance(AC)/resistance(DC) for lightening. It will also make it carry neutral potential for the 200 amp main, and also the transformer. So if we put in a sub-panel should we update the ground electrode/rod at the main panel, and is there any code I am missing about this. I would call it common sense. Don't disconnect the main ground electrode without pulling the meter.

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Last edited by RobertS; 02-27-2008 at 10:23 AM. Reason: Add stuff.
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Old 02-27-2008, 09:56 AM   #62
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Sub Panel Grounding Question.


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Originally Posted by Robert Shields View Post
I haven't had a return call from the inspector yet. I am going to talk to a friend that is an engineer. I am an electronics tech by my first trade. All of this talk about current paths, and grounding protection is great until you get to the grounding electrode at the main panel neutral/ground bar/electrode. The center tap of the transformer is at the same potential as the equipment ground, and neutral at this point. The neutral path should only have current/amperage when there is a load applied. Be it a high impedance or low, it is a load. Low impedance usually causes smoke, or a tripped breaker, or both. Now, if I go to an old house with a 200 amp main panel that looks good, and put in a detached sub-panel in a garage, and put in a nice new shiny copper clad ground rod, everything is up to code. If the ground rod at the main is an old galvanized/ brass acorn ground rod with corrosion from age, and electrolysis that makes the ground at the sub-panel the least path of impedance(AC)/resistance(DC) for lightening. It will also make it carry neutral potential for the 200 amp main, and also the transformer. So if we put in a sub-panel should we update the ground electrode/rod at the main panel, and is there any code I am missing about this. I would call it common sense.
As has been pointed out to you, the only current of consequence we care about on the ground rods you mention are lighting strikes and high voltage surges on the POCO lines. Those high voltage pulses will not be greatly affected by the resistance of the ground rods. Shiny or not shiny will make little difference. The difference in resistance or impedance to a high voltage potential is of no concern.

Because of this, there is nothing in the code about updating the grounding electrode system due to normal corrosion. Remember, in many cases, no ground rod at all will be used. Another grounding electrode type will be used. Ground rods are an "install them where required and forget them" part of the service.

The neutral current flowing through your ground rods is also inconsequential because you have provided a low impedance path back to the transformer on the service entrance grounded (neutral) conductor.

Current will flow along EVERY path available to it, in proportion to it's resistance or impedance.

I suggest you take a course on grounding and bonding. Here is a starting point for you.

http://www.mikeholt.com/index.php?id=homegeneral
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Old 02-27-2008, 10:39 AM   #63
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Sub Panel Grounding Question.


No, the new ground rod will not "carry the neutral potential" from the 200 A main. Some small neutral current will flow, true, but the service neutral carries the neutral current.

Bottom line:

A detached building needs a 4-wire feeder. Grounds and neutrals separated. Grounding electrode installed and attached to the equipment ground in the panel. Disconnect rated at 60 amps or greater on incoming feeder, regardless of the feeder overcurrent size a the main. That's it.

The inspector is WRONG if he says a 4-wire feeder doesn't need a grounding electrode.
He is RIGHT if he says a 3-wire feeder does.

If he insists he is right,tell him, politely, to show you where in the LOCAL CODE they have this written. In writing, not "because he said so". He can't just make up codes on the spot.

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Old 02-27-2008, 02:50 PM   #64
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Sub Panel Grounding Question.


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Don't disconnect the main ground electrode without pulling the meter.
Pulling the meter will prevent the premise wiring from placing neutral current on the return paths but it will not prevent current from other sources from being present on the grounding conductors. So you are correct you do not want to treat the grounding conductors as dead wires. It isn't unusual at all to have a large neutral current using your water pipe which is common to your neighbors. His water pipe (common to yours) is also bonded to his service equipment and service neutral back to the transformer. If his dwelling had a failing neutral or worse an open neutral it is very possible to have lethal current flowing on your water pipe bond consequently allowing an dangerous shock hazard to exist. This can also happen with earth return from a neighbors loss or partial loss of his service neutral coming in on both the water pipe bonding wire and the supplemental electrode grounding conductors. Many a plumber has found this out the hard way.

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