Ground Rods

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dasarmin

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Can a single Ground Rod be used to Bond more than 1 transformer?
I'm installing a new 480-208/120 transformer to feed a Loadcenter; there's a Ground Rod next to this installation used with another transformer.
Can I Bond the new transformer to the existing Ground Rod?
 
dasarmin said:
Can a single Ground Rod be used to Bond more than 1 transformer?
I'm installing a new 480-208/120 transformer to feed a Loadcenter; there's a Ground Rod next to this installation used with another transformer.
Can I Bond the new transformer to the existing Ground Rod?
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Unless it has been tested to show the ground resistance < 25 Ohms two ground rods are required.

All grounding electrodes associated with a single structure have to be bonded together.

When you create a new SDS it has to be connected to the structure's grounding electrode system (GES).

250.30 is the answer.
 
I think the post may be confusining. If there is only one rod then the first transformer probably needs 2- rods at least 6' apart and bonded together. Then, you can use those same rods for the second transformer also.

I
 
Neither transformer needs to connect to a rod.

They need to tie into the grounding electrode system, which 480 volt system that supplies the transformer should already be connected to. Even if you desire to add additional rods or other electrodes for whatever reason - they still need to also be bonded to the GES.
 
petersonra said:
Unless it has been tested to show the ground resistance < 25 Ohms two ground rods are required.

All grounding electrodes associated with a single structure have to be bonded together.

When you create a new SDS it has to be connected to the structure's grounding electrode system (GES).

250.30 is the answer.
Click to expand...


Just a note - 2017 NEC changed the definition of 'Structure' to; "That which is built or constructed, other than equipment."
I take this to mean, pump, skid and transformer pads no longer require bonding of rebar.
 
Dennis Alwon said:
I think the post may be confusining. If there is only one rod then the first transformer probably needs 2- rods at least 6' apart and bonded together. Then, you can use those same rods for the second transformer also.

I
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I know the Code requires 6 feet separation for supplemental electrodes (rods) however rods are frequently driven one on top of the other to achieve 25 ohms. The average, single ground rod in the US is about 40ohms.
 
Dale001289 said:
I know the Code requires 6 feet separation for supplemental electrodes (rods) however rods are frequently driven one on top of the other to achieve 25 ohms. The average, single ground rod in the US is about 40ohms.
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The ones driven one on top of the other are coupled together to create a single longer rod aren't they? You can't do this with the typical 8 foot rod without some sort of coupling method be it integrated into a rod or an additional fitting.

40 ohms average is nothing more then trivia information. Doesn't mean if I go outside my front door and drive a rod there it will be 40 ohms.
 
Dale001289 said:
Mastering the NEC - Mark Shapiro
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So now I would like to see Mark's substantiation. I have tested many rods in the Southeast US and I can honestly say the majority are well above 40 ohms. With a little salt and moisture I can get there but not for the long haul.

Roger
 
kwired said:
The ones driven one on top of the other are coupled together to create a single longer rod aren't they? You can't do this with the typical 8 foot rod without some sort of coupling method be it integrated into a rod or an additional fitting.

40 ohms average is nothing more then trivia information. Doesn't mean if I go outside my front door and drive a rod there it will be 40 ohms.
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Correct; you'd need a driving stud, (such as Erico Cadweld B13718) to drive additional rods. For example, this plant is close to Atlantic Ocean. You'd think soil here would be quite 'friendly' however typically requires three to four rods to get under 25 Ohms. Some places are over 1000 Ohms - i.e. New England rock. All depends on soil resistivity. You'd be lucky to get 25 Ohms with a single driven rod regardless of where you're at geographically.
 
roger said:
So now I would like to see Mark's substantiation. I have tested many rods in the Southeast US and I can honestly say the majority are well above 40 ohms. With a little salt and moisture I can get there but not for the long haul.

Roger
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I don't have Mark's contact info but I am sure this statistic would not have been published without substantiation. I agree that most are above 40 Ohms.
 
Dale001289 said:
I don't have Mark's contact info but I am sure this statistic would not have been published without substantiation. I agree that most are above 40 Ohms.
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If the average is 40 then one should expect that around half are above and around half are below.
 
kwired said:
If the average is 40 then one should expect that around half are above and around half are below.
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Sounds reasonable but I don't think it works that way mathematically. You have to extrapolate between extreme high/low ends - e.g. high ohms for Northeast coast vs low ohms along the Gulf coast.
 
Dale001289 said:
Sounds reasonable but I don't think it works that way mathematically. You have to extrapolate between extreme high/low ends - e.g. high ohms for Northeast coast vs low ohms along the Gulf coast.
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Again what is that 40 ohm figure good for other then trivial information?
 
Ground Rods

kwired said:
Again what is that 40 ohm figure good for other then trivial information?
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Knowledge is power - What is trivial to one may be relevant to another. Never hurts to broaden your horizons unless you’re no longer actively involved in the electrical field.


Sent from my iPhone using Tapatalk
 
Last edited:
Dale001289 said:
Knowledge is power - What is trivial to one may be relevant to another. Never hurts to broaden your horizons unless you’re no longer actively involved in the electrical field.


Sent from my iPhone using Tapatalk
Click to expand...
So average is 40, but at any given location I might see anywhere between 10 and 1000, what does the 40 average mean to me?

Unless I am going to try to achieve a certain resistance for whatever reason - I likely drive two rods, move on and never ever know what the resistance was anyway.
 
kwired said:
So average is 40, but at any given location I might see anywhere between 10 and 1000, what does the 40 average mean to me?

Unless I am going to try to achieve a certain resistance for whatever reason - I likely drive two rods, move on and never ever know what the resistance was anyway.
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“Unless I am going to try to achieve a certain resistance for whatever reason-“

Most clients such as Dow, Chevron, Exxon etc require an equipotential grid not to exceed five ohms plus an Isolated Grounding grid (for DCS systems) not to exceed one ohm. It’s much different than residential or comercial - you can’t simply drive two rods and forget about it.
The 40 ohm value provides a basis for estimating number of Rods.
 
Dale001289 said:
“Unless I am going to try to achieve a certain resistance for whatever reason-“

Most clients such as Dow, Chevron, Exxon etc require an equipotential grid not to exceed five ohms plus an Isolated Grounding grid (for DCS systems) not to exceed one ohm. It’s much different than residential or comercial - you can’t simply drive two rods and forget about it.
The 40 ohm value provides a basis for estimating number of Rods.
Click to expand...
Exactly - those are some of the "whatever reasons". I never run into that requirement though. Though equipotential grid isn't the same thing as a grounding electrode The two ordinarily are bonded to one another.
 
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