Electrode resistance

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bthielen

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I'm going to be wiring a new 60A service to a pole building on my premises. I have purchased a grounding electrode and I am not sure I understand 250.56. How do I measure the resistance to ground of the grounding electrode to determine if the resistance is less than 25 ohms? Won't the resistance change depending on soil type, point of measure, moisture, etc.?

Any assistance would be much appreciated.

Thank you,

Bob
 
dereckbc

dereckbc

Moderator
Staff member
Location
Plano, TX
Re: Electrode resistance

Very simple, install two ground rods and walk away. If you install two there is no need to measure impedance. Otherwise it takes some fairly expensive test equipment to measure the ground resistance
 
ryan_618

ryan_618

Senior Member
Location
Salt Lake City, Utah
Re: Electrode resistance

To make sure you don't think that Dereck is simply blowing you off, he is dead right. If you were to do a search on this subject you would find that it has been hashed out several times on this very forum. Installing the second rod is a much less painful experience. :)
 
iwire

iwire

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Staff member
Location
Massachusetts
Re: Electrode resistance

To make sure you don't think that Dereck and Ryan are simply blowing you off, they are dead right. :)

We have a tester as many times the job specs exceed the NEC requirements, but in the absence of job specs we drive two rods and do not bother with a three point ground test.

That eliminates the need to do the test while the inspector is there.

Keep the rods at least 6 feet apart from each other.

Bob
 
G

Guest

Guest
Re: Electrode resistance

Per 250.56 if you install two ground rods at least six feet apart then you don't have to prove the 25 Ohms. Two rods-- you're done. Be sure to leave the code bug revealed on the daylighted (or buried) portion of your rods. The bug is on the end you drive from.

../Wayne
 
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bthielen

Guest
Re: Electrode resistance

Thank you very much for your help. Looks like my best bet is as you suggest.

Again, many thanks,

Bob
 
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