Installing Grounding Rods

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  #1  
Old 08-21-14, 06:28 AM
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Installing Grounding Rods

I just bought a multifamily home with two seperate electrical meters and panels each at 100 AMP. There was never any grounding rods put in and they were never bonded to the water main. Can I just install one grounding rod and run two seperate lines to the panels or do I have to install two grounding rods? As for bonding to the copper pipe do I have to run two seperate lines for that?
 
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Old 08-21-14, 07:30 AM
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Can I just install one grounding rod and run two seperate lines to the panels or do I have to install two grounding rods?
Just one rod should be fine.

As for bonding to the copper pipe do I have to run two seperate lines for that?
In my opinion you need a GEC from each panel to the metallic water service within 5 feet of where it enters the building. You need to jump around any pressure reducing valves and/or meters with the continuous conductors.
 
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Old 08-21-14, 07:36 AM
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CASUAL JOE WROTE : Just one rod should be fine.
It should, but...

250.56 Resistance of Rod, Pipe, and Plate Electrodes. A
single electrode consisting of a rod, pipe, or plate that does
not have a resistance to ground of 25 ohms or less shall be
augmented by one additional electrode of any of the types
specified by 250.52(A)(4) through (A)(8). Where multiple
rod, pipe, or plate electrodes are installed to meet the requirements
of this section, they shall not be less than 1.8 m
(6 ft) apart.


Further you might want to contact your local building department to see exactly what grounding/bonding requirements must be met in your area.
 
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Old 08-21-14, 08:00 AM
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Further you might want to contact your local building department to see exactly what grounding/bonding requirements must be met in your area.
While it is never a bad idea to consult the local building department, I am not personally aware of any jurisdiction that requires the resistance test to be performed. In my area the resistance with a new 1/2" X 8 Ft ground rod and a new #6 GEC typically runs about 5 to 6 ohms. Soil conditions do vary so conditions in the OP's area could be quite different. I still feel one ground rod would be fine. If the AHJ asks for the resistance test just add a second rod 6 feet away from the first rod and be done with it.
 
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Old 08-21-14, 08:12 AM
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I agree Joe, I have never been required to do a resistance test by a local jurisdiction. However, not doing a resistance test and just driving one rod and guessing is technically a code violation.
 
  #6  
Old 08-21-14, 10:13 AM
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The "rule" is two 8' ground rods at least 6' apart. The "exception" is if a resistance test gives certain results specifically a 25 or less ohms resistance to the earth (soil, dirt, ground*) for one rod.

The same two rods and water pipe can serve both panels. The NEC calls for a continuous wire (grounding electrode conductor) from panel to ground rod and continuous wire from panel to water pipe so you can daisy chain the wire from one panel to the other and then to the ground rods etc. Alternatively the wire can be run piecemeal using approved crimp connectors for splicing or branching or continuing.

The NEC has a table for the size of the GEC which I don't have handy now. The maximum size ever needed to ground rods is #6 copper but in some cases the GEC to the water pipe may need to be fatter.

* As in (customer) "This coffee tastes like mud." (waiter) "Well, it was ground this morning."
 
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Old 08-21-14, 10:18 AM
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While it is never a bad idea to consult the local building department, I am not personally aware of any jurisdiction that requires the resistance test to be performed. In my area the resistance with a new 1/2" X 8 Ft ground rod and a new #6 GEC typically runs about 5 to 6 ohms. Soil conditions do vary so conditions in the OP's area could be quite different. I still feel one ground rod would be fine. If the AHJ asks for the resistance test just add a second rod 6 feet away from the first rod and be done with it.
you lucky dog........in the center of fl. an 8 ft rod can be 200-300-- ohms.




I agree Joe, I have never been required to do a resistance test by a local jurisdiction. However, not doing a resistance test and just driving one rod and guessing is technically a code violation.
NO .........not doing a resistance test and just driving one MORE rod 6 ft away IS CODE.

SO WE DO 2 RODS HERE.....................but....2X0=0 I LIE 2 RODS AT 200 MIGHT GIVE YOU150.

IF I WANT A 10 OHM GROUND I HAVE TO DRIVE A 3/4 ROD 30-50 FEET

MY POWER COMPANY WILL DO A RESISTANCE TEST ON THE GROUND.

THEY SAID THE TOY CoST OVER $9,000.00
 
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Old 08-21-14, 10:34 AM
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The maximum size ever needed to ground rods is #6 copper but in some cases the GEC to the water pipe may need to be fatter.
Or smaller if protected in conduit as we do around here. Its more common to see #8 in emt than #6 without. And unfortunetly most guess with driving just one rod knowing that nobody (the poco or the ahj) is not going to do any sort of resistance test. Poor practice.
 
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Old 08-21-14, 12:29 PM
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Hopefully, if you're using metal conduit for physical protection, it's bonded at both ends.
 
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Old 08-21-14, 01:37 PM
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The "rule" is two 8' ground rods at least 6' apart. The "exception" is if a resistance test gives certain results specifically a 25 or less ohms resistance to the earth (soil, dirt, ground*) for one rod.
Where exactly is that rule found in the NEC? What version?

not doing a resistance test and just driving one rod and guessing is technically a code violation.
What section of the NEC would that be violating?

NO .........not doing a resistance test and just driving one MORE rod 6 ft away IS CODE.
Ok, show me the code. If I am wrong, I'll admit it.
 
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Old 08-21-14, 01:55 PM
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casual joe said:

Not doing a resistance test and just driving one rod and guessing is technically a code violation.

What section of the nec would that be violating?
nec 250.56 ................................
 
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Old 08-21-14, 02:03 PM
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casual joe said:

Not doing a resistance test and just driving one rod and guessing is technically a code violation.

What section of the nec would that be violating?
nec 250.56 ................................
I am familiar with that requirement, but it doesn't require that the contractor do the test. My opinion is that if the inspector wants to see proof of the resistance the test can either be performed or an additional rod added 6 feet away from the first one. I see no reason to just arbitrarily add the cost of the second rod to every job unless it is required by the AHJ.
 
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Old 08-22-14, 03:47 AM
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The NEC calls for a continuous wire (grounding electrode conductor) from panel to ground rod and continuous wire from panel to water pipe so you can daisy chain the wire from one panel to the other and then to the ground rods etc.
Wouldn't I want to run two seperate lines from each panel to the rods and copper piping? Lets say if there is a lightening strike or ground fault then it will run through both panels if they are daisy chained toghether or does that not matter?
 
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Old 08-22-14, 05:44 AM
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You are required to have two grounding electrodes. The only time we need to install two rods is when there is not another electrode available such as a building that does not have any water service. Your water service is the primary electrode (one), the ground rod is supplemental (two).

Of course each state is different and the local AHJ should be consulted.

See 250.52 and 250.53 (2011)
 
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Old 08-22-14, 06:20 AM
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CJ, it sounds like your inspector has a different interpretation. For me unless I can prove less than 25 ohms I need the second rod.I am not going to hold up a job or buy an expensive meter when I can drive a second rod and be done.
 
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Old 08-22-14, 07:25 AM
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Is #6 wire is good for 100 AMP panels to the ground rod?
 
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Old 08-22-14, 07:50 AM
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Is #6 wire is good for 100 AMP panels to the ground rod?
yes.................from the first panel only
 
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Old 08-22-14, 08:22 AM
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What do you mean the first panel? I thought I have to run one line from each panel to the rod.
 
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Old 08-22-14, 08:23 AM
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CJ, it sounds like your inspector has a different interpretation. For me unless I can prove less than 25 ohms I need the second rod.I am not going to hold up a job or buy an expensive meter when I can drive a second rod and be done.
PC, what you do and my opinion are not so far apart. To the best of my knowledge there hasn't been an inspector who asked for the restance test results here, but I suspect that could be because of the soil conditions. Like I said before, a new 1/2" X 8 foot ground rod and new #6 copper GEC tests at 5 to 6 ohms. The soil conditions in this county and adjacent counties west and north of here are very similar in all but a small portion of the southwest county where it starts getting very rocky. In that part of the county there might be some requests for the resistance test results, I cannot say for sure. If challenged on the test results, the quickest and cheapest thing to do is just add the second rod and be done with it.

yes.................from the first panel only
The OP has two services, both panels are the first panel.
 
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