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Old 07-01-2012, 11:23 AM   #1
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8 unit apartments and grounding rods


Recently I helped install service equipment on an 8 unit apartment complex. There were two disconnect boxes. Each box housing 4 disconnects. I assume it was 1 service lateral supplying both service disconnect boxes. We also installed two separate ground rods, one for each box. Question: why can't we just bond the two boxes together and use one ground rod instead of the two that we installed 6ft apart?

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Old 07-01-2012, 11:34 AM   #2
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8 unit apartments and grounding rods


You didn't install one rod for each meter stack. You installed two rods for the service which are required when you can't prove one single rod's resistance to earth is less than 25 ohms.

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Old 07-01-2012, 12:26 PM   #3
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8 unit apartments and grounding rods


(You also bond the two service disconnect boxes to each other; this can be accomplished using a grounding electrode conductor between the rods.)
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Old 07-01-2012, 12:42 PM   #4
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8 unit apartments and grounding rods


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(You also bond the two service disconnect boxes to each other; this can be accomplished using a grounding electrode conductor between the rods.)
Well we didn't bond them together. It was one ground wire to each ground rod per box. I guess I was wondering why we couldn't just run a ground wire from each box to one single rod.

I don't clearly understand the first reply about proving 25 ohm resistance. Aren't they rated that way to begin with? I'm missing something here......
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Old 07-01-2012, 12:47 PM   #5
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8 unit apartments and grounding rods


What you did was essentially the same as running from one of the stacks to the first rod, then from the first rod to the second rod. The bolt together design of the the stacks bonds both stacks to both rods.

The resistance between the rod and the ground depends on the earth. Is the soil sandy? Rocky? You would have to prove the resistance which is a much larger PITA than just driving a second ground rod.
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Old 07-01-2012, 12:53 PM   #6
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8 unit apartments and grounding rods


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What you did was essentially the same as running from one of the stacks to the first rod, then from the first rod to the second rod. The bolt together design of the the stacks bonds both stacks to both rods.

The resistance between the rod and the ground depends on the earth. Is the soil sandy? Rocky? You would have to prove the resistance which is a much larger PITA than just driving a second ground rod.
Ok it's starting to come together now. So am I correct in saying that the stacks are bonded to each other and both rods by the brick structure they are bolted too? Could you explain why only one rod is needed for a single housing disconnect or main panel?
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Old 07-01-2012, 01:38 PM   #7
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8 unit apartments and grounding rods


Also for a single first disconnect box (or breaker panel with first main disconnect), you need to drive enough rods to get the resistance to earth down to 25 ohms or less or drive a second rod whichever comes first.

Running #6 copper wire (as grounding electrode conductor) from one meter stack to one ground rod over to the other ground rod and up to the other meter stack will bond the meter stacks. Simply fastening both stacks (with no firm metal to metal contact with each other) to a brick wall won't bond them.

From each meter stack neutral bus to one ground rod or qualifying water pipe exiting the building underground must be a continuous GEC. Other GECs needed to complete the bonding can be clamped to/on or spliced.
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Old 07-01-2012, 02:02 PM   #8
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8 unit apartments and grounding rods


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Also for a single first disconnect box (or breaker panel with first main disconnect), you need to drive enough rods to get the resistance to earth down to 25 ohms or less or drive a second rod whichever comes first.

Running #6 copper wire (as grounding electrode conductor) from one meter stack to one ground rod over to the other ground rod and up to the other meter stack will bond the meter stacks. Simply fastening both stacks (with no firm metal to metal contact with each other) to a brick wall won't bond them.

From each meter stack neutral bus to one ground rod or qualifying water pipe exiting the building underground must be a continuous GEC. Other GECs needed to complete the bonding can be clamped to/on or spliced.
So what does it mean when we do not bond each meter stack together? All we did on this job was run a grounding electrode conductor from one stack to a ground rod and another conductor from the other stack to its own individual ground rod. We did not run a conductor between ground rods. Each meter stack is essentially grounded to its own rod with no bonding between stacks that I can think of.
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Old 07-01-2012, 02:31 PM   #9
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8 unit apartments and grounding rods


There has to be something connecting the two stacks... Metal conduit will bond the two stacks.
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Old 07-01-2012, 03:36 PM   #10
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8 unit apartments and grounding rods


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There has to be something connecting the two stacks... Metal conduit will bond the two stacks.
What are some of the consequences of not connecting the two stacks?
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Old 07-01-2012, 03:46 PM   #11
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8 unit apartments and grounding rods


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Originally Posted by Hexamexapex View Post
Recently I helped install service equipment on an 8 unit apartment complex. There were two disconnect boxes. Each box housing 4 disconnects. I assume it was 1 service lateral supplying both service disconnect boxes. We also installed two separate ground rods, one for each box. Question: why can't we just bond the two boxes together and use one ground rod instead of the two that we installed 6ft apart?

So you have more than 6 disconnects without a main?
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Old 07-01-2012, 04:56 PM   #12
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8 unit apartments and grounding rods


Easiest thing to do would be to run a ground wire from one disconnect to both rods and then back to the other disconnect.
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Old 07-01-2012, 04:58 PM   #13
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8 unit apartments and grounding rods


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So you have more than 6 disconnects without a main?
Well it's an 8 unit apartment with two separate enclosures. Each enclosure houses 4 unit disconnects.
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Old 07-01-2012, 05:06 PM   #14
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8 unit apartments and grounding rods


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Easiest thing to do would be to run a ground wire from one disconnect to both rods and then back to the other disconnect.
I see in the code where a single service requires a continuous grounding system. But I am starting to think it might be two separate services......since a single service supplies only up to six disconnects and there are 8 disconnects split into two separate enclosures. So I think the utility might run two separate services in this case. Which two separate ground rods ....6 ft apart would be permissible.
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Old 07-01-2012, 05:09 PM   #15
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8 unit apartments and grounding rods


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Originally Posted by Hexamexapex View Post
I see in the code where a single service requires a continuous grounding system. But I am starting to think it might be two separate services......since a single service supplies only up to six disconnects and there are 8 disconnects split into two separate enclosures. So I think the utility might run two separate services in this case. Which two separate ground rods ....6 ft apart would be permissible.
I doubt it has two services, check out 230.2 Number of Services

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