The 25 ohm rule

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joker50

New member
I may be asking something very insignificant but I have to know.
I am reading 28 ohms between the neutral which is bonded to a 8' copper ground rod and the ground at a cold water pipe(it runs 65 feet horizontally @8'. Is this perfectly acceptable???

Thanksa
 

earlydean

Senior Member
Re: The 25 ohm rule

Is this water pipe bonded to the neutral? If it is, then you,ve got problems. If it isn't yet, then it's fine. Bond it, and life goes on.
 

eprice

Senior Member
Location
Utah
Re: The 25 ohm rule

Originally posted by earlydean:
... If it isn't yet, then it's fine. Bond it, and life goes on.
Except that the ground rod would be the supplemental electrode covered by 250.53(D)(2). That section referes to 250.56 which requires the ground rod to have a resistance of 25 ohms or less. It is not clear that the ground rod meets that requirement. A second rod may be required.
 

bill addiss

Senior Member
Re: The 25 ohm rule

joker,

As Earl said this is not the type of reading you should have. Please don't confuse the reading you are taking with the 25 ohms or less required for the Ground Rod in 250.56

Bill
 

charlie b

Moderator
Staff member
Location
Seattle, WA
Occupation
Electrical Engineer
Re: The 25 ohm rule

There?s a bit of confusion here, and it may prove that earlydean correctly identified a problem. Let?s clear up our language.

First, I presume you are talking about the service panel. Otherwise, you do have a real problem.

Secondly, you say the neutral is bonded to a ground rod. Are you certain that that is the case? It need not be. The neutral could be bonded to the ground bus at the service panel, with that ground bus being connected (via a ?Grounding Electrode Conductor?) to the ground rod. But the bottom line is that there is metal (i.e., wire), and not dirt, between the neutral and the ground rod.

Third, the same ground bus should be bonded to the water pipe. This would mean that there is metal (i.e., wire), and not dirt, between the neutral and the water pipe.

Fourth, since dirt does not enter this picture, Bill?s comment is accurate: ignore the 25 ohm rule of 250.66.

Finally, if you are reading 28 ohms along what should be a metal to metal to metal path, then something is wrong. My guess is that the neutral and the ground bus are not bonded (or there is a loose connection) within the panel. That would mean that the path between the neutral and the water pipe does include dirt, and that 28 ohms would be a reasonable reading for this circumstance. My suggestion is that you verify the connection between the neutral and ground (not just by physical inspection ? take a reading with your ohmmeter).

[ October 22, 2003, 04:37 PM: Message edited by: charlie b ]
 
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