250.53 B ground rod spacing

[raider1]

If the NEC is NOT an engineering document, I wonder why this restriction exist. There is no safety issue here, so why is the Code taking a stance? Electrodes placed closer than 6' to each other can also produce the desired value.

This is a very good question that I don't have the answer too.

Chris

 

[charlie]

. . . Electrodes placed closer than 6' to each other can also produce the desired value.

Yes they can Laszlo. However, the code also permits a single electrode to be used if it is 25Ω or less. A second ground rod is not required if you have convinced the AHJ that the first is 25Ω or less but no one does that. Since the code permits a second one and you walk, it is less expensive to drive the second one. OK, this is basic.

Now, assume you are following the code and only plan on driving a single ground rod. You measure the resistance and get 31Ω. You now have to drive the second one and walk. Since the code is a prescriptive document and it has to work for all installations, it requires six feet of separation. In my opinion, it should have said the length of the rod separation but that is another issue. You could drive another rod six inches away and get the 25Ω of resistance to the pair but the code requires six feet.

Now I will get to the answer. I am driving a ground rod in the Sonora Desert and get 100Ω for a new service. In order to meet the code and get far enough away from the first rod's influence to be somewhat effective, I now have to be six feet away. If this were a performance document, you would be correct. :smile:

 

[bphgravity]

If the NEC is NOT an engineering document, I wonder why this restriction exist. There is no safety issue here, so why is the Code taking a stance? Electrodes placed closer than 6' to each other can also produce the desired value.

Following in line with Charlie's comments, section 250.4 lays out some performance requirements that the rest of Article 250 is going to hopefully accomplish.

Most of the prescriptive rules found in Article 250 are attempting to best meet the performance needs identified in Section 250.4, while also attempting to be a minimum standard at the same time.

This is not easy to do, especially considering many of the conceptual aspects of Article 250 come from old telegraph operators and engineers during the early 1900's.

In short, it's very hard to assign prescriptive requirements to a performance demand that is not quantified with minimums and maximums.

 

[weressl]

Yes they can Laszlo. However, the code also permits a single electrode to be used if it is 25Ω or less. A second ground rod is not required if you have convinced the AHJ that the first is 25Ω or less but no one does that. Since the code permits a second one and you walk, it is less expensive to drive the second one. OK, this is basic.

Now, assume you are following the code and only plan on driving a single ground rod. You measure the resistance and get 31Ω. You now have to drive the second one and walk. Since the code is a prescriptive document and it has to work for all installations, it requires six feet of separation. In my opinion, it should have said the length of the rod separation but that is another issue. You could drive another rod six inches away and get the 25Ω of resistance to the pair but the code requires six feet.

Now I will get to the answer. I am driving a ground rod in the Sonora Desert and get 100Ω for a new service. In order to meet the code and get far enough away from the first rod's influence to be somewhat effective, I now have to be six feet away. If this were a performance document, you would be correct. :smile:

Thanks Charlie.

My point is that the Code has the right - based on its stated purpose - to proscribe an Ohmic value. It has the right to specify the material - although it should give leeway about soils with different corrosive properties - but it outh not to lay down design parameters.

Let's say that you are at an urban housing area. The architect was smart enough to live a 3' radius halfmoon opening for you in the concrete walkway under the service entrance point, but you drive a single rod and you get 27Ohms. So how are you going to drive another rod 6' away from the first one?

 

[infinity]

Let's say that you are at an urban housing area. The architect was smart enough to live a 3' radius halfmoon opening for you in the concrete walkway under the service entrance point, but you drive a single rod and you get 27Ohms. So how are you going to drive another rod 6' away from the first one?

If you want to comply with the NEC you will or you can use another GE like a grounding plate. Start digging. :grin:

 

[iwire]

Let's say that you are at an urban housing area. The architect was smart enough to live a 3' radius halfmoon opening for you in the concrete walkway under the service entrance point, but you drive a single rod and you get 27Ohms. So how are you going to drive another rod 6' away from the first one?

Break concrete, life is harsh.