10 foot ground rod

[Carultch]

Grounding electrodes are not about clearing faults. If a rod is the only available electrode doesn't matter if you have 100 amps of service or 4000 amps of service, that rod (plus a supplemental rod if you don't want to measure resistance) is good enough for either application as a grounding elecrode.

An acceptable 25 ohm rod installation will not let enough current flow to even trip a 15 amp breaker. Even @ 277 volts the 25 ohm rod only lets about 11 amps of current flow.

Ok, that makes sense.

So to summarize:
The ohms of the ground rod system is all that ultimately matters in any application, and in most cases you achieve it with two ground rods at min 8 ft embedment and the standard 6 ft separation.

 

[iwire]

Well you can drive it all the way in and then use a clamp approved for burial, but I don't see that very often.

I don't understand, you don't see it done as required?

Do you fail them for it?

 

[kwired]

Ok, that makes sense.

So to summarize:
The ohms of the ground rod system is all that ultimately matters in any application, and in most cases you achieve it with two ground rods at min 8 ft embedment and the standard 6 ft separation.

Rods are considered one of the poorest electrodes. They have less surface to earth contact surface then most other electrodes is main reason why. Consider how much surface area there is on a 1/2 inch by 8 foot rod, and compare that to the surface area of a building footing - even a fairly small building is much more surface area in contact with earth.

The reason they let you drive a second one and forget about it is if a second rod doesn't lower resistance enough a third and fourth usually don't improve it much either.

 

[cowboyjwc]

I don't understand, you don't see it done as required?

Do you fail them for it?

Lot's of concrete around here and our panels are always outside, so you hardly ever see one that doesn't have at least two to three inches sticking up.

No, it's become such a common practice that no one that I know is calling it.

 

[petersonra]

Ok, that makes sense.

So to summarize:
The ohms of the ground rod system is all that ultimately matters in any application, and in most cases you achieve it with two ground rods at min 8 ft embedment and the standard 6 ft separation.

"The ohms" does not matter at all for most electrical installations.

There is no "ohms" requirement at all if you have two rods, only if you have just one rod.

The 6 ft separation you mention is the minimum the code allows, it is not any kind of standard, although my guess is most people do it this way.

 

[Carultch]

"The ohms" does not matter at all for most electrical installations.

There is no "ohms" requirement at all if you have two rods, only if you have just one rod.

The 6 ft separation you mention is the minimum the code allows, it is not any kind of standard, although my guess is most people do it this way.

My point is, the reason why it is 6 ft, 8 ft, and two rods, is in order to achieve not more than a given number of ohms of contact resistance.

The NEC doesn't "open the books" on the calculation that determines these requirements, so it isn't immediately obvious that ohms are a function of all these values, or how you can adjust the configuration and still achieve the requirement.

 

[iwire]

My point is, the reason why it is 6 ft, 8 ft, and two rods, is in order to achieve not more than a given number of ohms of contact resistance.

The NEC doesn't "open the books" on the calculation that determines these requirements, so it isn't immediately obvious that ohms are a function of all these values, or how you can adjust the configuration and still achieve the requirement.

No one knows.

Some members here and I think Mike Holt have traced the 25 ohms all the way back to the days of telegraph installations.

Grounding the electrical system to dirt is voodoo as far as the NEC is concerned.