Rods

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Esthy

Senior Member
I always put the second rod, but an exam is coming and the questions are confusing for me - non-native and very old gal - so, what are those versions? (See only #8 in one of the versions)
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
Maybe I don't understand the question, but the dwg, made sense,,, a ground rod that has greater than 25 ohms resistance must be supplemented by one additional electrode. NEC 250.56
 

Esthy

Senior Member
The writing (#8) is from EC&M and is confusing as are all the questions on those new tests, so, less than 25, I don't need the second one and more than 25, I need the second one. So, why are different professional opinions?
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
I've never heard any opinion other than that.
With the introduction of the concrete-encased-electrode, ground rods are rarely mentioned here.
 
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winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Only a single ground rod is required.

However if that single ground rod is not 'good enough' by a fairly arbitrary and difficult to properly test metric, then a second ground rod is required.

In many locations, it is common practice to simply install 2 ground rods as more economical than putting the effort into the testing to show that a single ground rod is acceptable.

-Jon
 

iwire

Moderator
Staff member
Location
Massachusetts
However if that single ground rod is not 'good enough' by a fairly arbitrary and difficult to properly test metric, then a second ground rod is required.

:D

They should add that to the code section. :thumbsup:


In many locations, it is common practice to simply install 2 ground rods as more economical than putting the effort into the testing to show that a single ground rod is acceptable.

That has been my SOP for a long time.
 

MasterTheNEC

CEO and President of Electrical Code Academy, Inc.
Location
McKinney, Texas
Occupation
CEO
Honestly, I did not find anything confusing or wrong about question #8. Looks like an accurate statement to me.
 

kwired

Electron manager
Location
NE Nebraska
The writing (#8) is from EC&M and is confusing as are all the questions on those new tests, so, less than 25, I don't need the second one and more than 25, I need the second one. So, why are different professional opinions?
I think you are just not seeing the opinions as being about the same.

A little history on this area - The intention was always that if the resistance was more then 25 ohms that they wanted an additional electrode, but the way it was worded didn't necessarily force one to use the second electrode. In 2008 NEC is says "....that does not have a resistance to ground or 25 ohms or less shall be augmented by one additional electrode...." This wording kind of leaves the installer with the ability to ask the inspector to prove the resistance or I am not installing the second electrode.

2011 they rewrote this area and it now says you must install the second electrode, but they added an exception afterwards allowing you to omit the second electrode if there is 25 ohms or less on the first. By wording it this way instead of the installer asking the inspector to prove it is low enough resistance - the inspector can instead ask the installer to prove the resistance if they only install one electrode.

Since such testing takes some time and/or expensive equipment many installers just put in two electrodes knowing it passes that way and move on. Six feet to the second electrode is only the NEC minimum and you likely do not get much more performance out of the second electrode if it is any closer then 6 feet but you generally would bet better performance with more distance between them.
 

Pharon

Senior Member
Location
MA
Six feet to the second electrode is only the NEC minimum and you likely do not get much more performance out of the second electrode if it is any closer then 6 feet but you generally would bet better performance with more distance between them.
"Performance" in this context is relative. In practice, adding a second electrode will do very little no matter what the ground resistance is, or how far away it is arbitrarily placed. Just another example of Code panel whims that have no basis in fact or common sense.
 

kwired

Electron manager
Location
NE Nebraska
"Performance" in this context is relative. In practice, adding a second electrode will do very little no matter what the ground resistance is, or how far away it is arbitrarily placed. Just another example of Code panel whims that have no basis in fact or common sense.
I understand, but I think the NEC looks at six feet as the closest because you get the sphere of influence of each far enough apart that you will get more performance out of two rods spaced at six feet then at one or two feet apart. You may get more conductivity out of 12 feet then 6 feet but not as significant of a change as you likely get when going from 1 foot to 6 feet apart.
 

tom baker

First Chief Moderator
Staff member
"Performance" in this context is relative. In practice, adding a second electrode will do very little no matter what the ground resistance is, or how far away it is arbitrarily placed. Just another example of Code panel whims that have no basis in fact or common sense.

Not true. The optium distance for separation is twice the length. Adding a second rod drops the resistance by 60%. The reasoning for this was worked out many many years ago. if you have documentation on your statement I and others would like to see it. My documentation is at home so I don't have it handy.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
In order to call something optimum, you have to specify how you are evaluating.
Two rods at infinite distance would have only 50% of the resistance.
What you are calling optimum is merely an arbitrary point along the curve of diminishing returns. To get an actual optimum you would need to assign a value to each ground ohm improvement and a cost to each added foot of separation.
 

brian john

Senior Member
Location
Leesburg, VA
In many locations, it is common practice to simply install 2 ground rods as more economical than putting the effort into the testing to show that a single ground rod is acceptable.

-Jon

We test grounding electrode systems and for what it cost you to drive a second electrode, would barely cover me stopping at 7-11 to get a cup of coffee on my way to your job.

Much cheaper to drive an electrode.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Sorry, this picture is better and I added a bonus for my mistake
:)
The wording on the hand drawn picture is unclear. The "> 25 ohms" is meant to apply to the first rod only. It does not matter what the resistance to earth of the second rod is, nor what the resistance to ground of the combined system is. If you have two rods, you do not need to measure in the first place.
 

Pharon

Senior Member
Location
MA
Rods

Not true.
Of course it's true. Let's say you're actually able to get 25 ohms with two rods - still not even close to tripping a breaker. One rod at, say, 40 ohms is going to give you the same relative performance - that is, still not enough to trip a breaker.

Whether every house has one ground rod or two - won't matter much for doing what they're really for - giving utility and lightning lots of points for reference.

The one ground rod vs. two may arguably be the most arbitrary and meaningless requirement in the entire Code.
 

jxofaltrds

Inspector Mike®
Location
Mike P. Columbus Ohio
Occupation
ESI, PI, RBO
Only a single ground rod is required.

However if that single ground rod is not 'good enough' by a fairly arbitrary and difficult to properly test metric, then a second ground rod is required.

In many locations, it is common practice to simply install 2 ground rods as more economical than putting the effort into the testing to show that a single ground rod is acceptable.

-Jon

I was not aware that proof was require in the code only that the contractor was to use the requirement.

IMHO testing after the installation is the inspectors job. Exactly why when guys came from KY where the inspector MADE them install two, they would as me how many that I wanted. After letting them know that it was not the code according to Mike, I explained that I had no way to test it so I would pass one or two properly installed rods. Same if they wanted to make the #6 continuous to the 2nd rod.
 

jxofaltrds

Inspector Mike®
Location
Mike P. Columbus Ohio
Occupation
ESI, PI, RBO
:lol::lol::lol:


That is some funny stuff. :p

90.4 Enforcement. This Code is intended to be suitable
for mandatory application by governmental bodies that exercise
legal jurisdiction over electrical installations, including
signaling and communications systems, and for use by
insurance inspectors. The authority having jurisdiction for
enforcement of the Code has the responsibility for making
interpretations of the rules, for deciding on the approval of
equipment and materials, and for granting the special permission
contemplated in a number of the rules.
 

mwm1752

Senior Member
Location
Aspen, Colo
Never tried it but 120v @ 25 ohms = 4.4 amps amprobe & 20 amp breaker -- we all know the grounding electrode system is about protection from induced voltage/lighting strikes
 
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