Grounding Electrode Conductor Size

[bphgravity]

This is a thought experiment.

I have a 300-ampere service with 350-kcmil service entrance conductors. Table 250.166 requires a 1/0 copper grounding electrode conductor.

If I have no electrodes present, I can drive a ground rod. A mximum #6 AWG is required. It is understood that conductor sizes larger than #6 to a single rod or rods is ineffective due to total surface area of a ground rod its high contact resistance to the soil.

If a concrete-encased electrode is present, I am only required to install a #4 AWG electrode conductor. But wait... Hasn't it been established that the Ufer ground provides the least overall impedance of all permitted electrodes.

If I have a 10-ft metal frame member in direct contact with the earth or encased in concrete, this electrode would have to be connected with a 1/0? How is this electrode any more effective as compared to the concrete encased electrode? The same for the water pipe, it needs supplementing, yet still requires the full sized electrode. Makes no sense to me.

Any ideas or thoughts on the subject?

 

[petersonra]

Re: Grounding Electrode Conductor Size

I have often thought that whomever came up with some of these sizes just pulled numbers out of the air.

I am pretty much convinced that the size of the GEC is really not especially significant, once you get past a certain size.

It does not have to carry any current under any normal circumstances. It does not have to carry any fault current under any fault conditions.

It does have to carry enough current to keep the grounded conductor at the same potential as earth, which ought to amount to a very low number, or something is not done right in the first place.

It needs to be big enough to not be liable to damage from incidental contact, and it does potentially need to be able to take a lightning strike, so a very small wire is not a good idea.

Beyond the lightning strike and physical damage criteria, I can't really come up with a criteria for sizing these things that makes any sense.

 

[roger]

Re: Grounding Electrode Conductor Size

I don't think any one, past or present, (and probably future) has any true explanation of the reasoning behind the GEC size requirements, hence the elusive 25 ohm origin.

I guess at least one person past would know why the sizes are in the code the way they are, but I don't think I'd buy any long winded explanation of why they are still the way they are.

I do wonder if during the days of complete metallic water systems in cities and townships if there was a true design consideration to have the GEC carry the neutral load for even lost neutral conditions.

Roger

[ March 04, 2005, 06:34 PM: Message edited by: roger ]