why NOT larger than #6

Status
Not open for further replies.
Ponchik

Ponchik

Senior Member
Location
CA
Occupation
Electronologist
Code section 250.66(A) permits that GEC doesn't have to be larger than #6 if it is the sole connection to a grounding rod.
But why is it required that we have to follow 250.66 if it is connected to water pipe?

I had a debate with a local inspector that they follow 250.66 even if it is the sole connection to a ground rod.
He said "it doesn't make sense not be larger than #6". The only reason that i see they strictly follow table 250.66 is because of 90.4.
 
The reason the ground rod GEC does not have to be larger than a #6 is because the rod's ability is only as good as a #6. Same is true for CEE where a #4 is all that is required. Building steel and water pipes have a greater capacity than a rod.
 
Dennis Alwon said:
The reason the ground rod GEC does not have to be larger than a #6 is because the rod's ability is only as good as a #6. Same is true for CEE where a #4 is all that is required. Building steel and water pipes have a greater capacity than a rod.
Click to expand...

so hypothetically speaking, if i install a 10sqft of plate electrode i don't need the GEC to be larger than #6 even though plate this electrode has more contact with the earth than 3/4" of copper water pipe.
 
edward said:
so hypothetically speaking, if i install a 10sqft of plate electrode i don't need the GEC to be larger than #6 even though plate this electrode has more contact with the earth than 3/4" of copper water pipe.
Click to expand...

Not true, since the NEC has to cover all installations, including installs where the copper water pipe connects back to the service neutral at the next house, the impedance of it could be very low compared to any Earth connected electrode, the impedance of an Earth electrode is limited by the small area of soil that is in contact with the rod or plate, while the plate might have a little less impedance then a rod, it will never come close to the low impedance of a water service that is connected to many houses, and back to many service neutrals, and this is the reason of the larger conductor requirement for water pipes, also the fact it also serves as the 250.104 bond.

Remember even if you can get 25 ohms which can be imposable is some soils, at 25 ohms the max current that can flow is only 4.8 amps at 120 volts, I would think a #6 would handle that very easily.

Now here is a challenge:

If #6 is good for 60 amps, what would be the impedance required to reach this current @ 120 volts that would overload the #6 to a ground rod.

answer the above and you will see how silly it is to require anything larger.
 
edward said:
so hypothetically speaking, if i install a 10sqft of plate electrode i don't need the GEC to be larger than #6 even though plate this electrode has more contact with the earth than 3/4" of copper water pipe.
Click to expand...
But often a metal underground water pipe is common to other buildings in the area and has other connections to the utility grounded conductor via the main bonding jumpers in the other buildings. This provides a way to flow current on a metallic path and not just through the earth as does a rod or plate electrode.
 
Dennis, Wayne & Don,

Thank you, your explanation makes sense. Now I can see the whole picture why the #6.

Wayne,
If i have done my math correct :), then the Z has to be 2 ohms. So another words the Z of the grounding rod probably will not 2 ohms and if it does then #6 will handle 60Amps.
 
Last edited:
edward said:
Dennis, Wayne & Don,

Thank you, your explanation makes sense. Now I can see the whole picture why the #6.

Wayne,
If i have done my math correct :), then the Z has to be 2 ohms. So another words the Z of the grounding rod probably will not 2 ohms and if it does then #6 will handle 60Amps.
Click to expand...

The NEC only requires a 1/2" or 5/8 ground rod to be in contact with the soil for 8'
I would bet my pay check that it would be almost impossible to ever get 2 ohms or less from a 8' ground rod to Earth.
 
hurk27 said:
I would bet my pay check that it would be almost impossible to ever get 2 ohms or less from a 8' ground rod to Earth.
Click to expand...
At least in 99% of the world anyway. I have got as low as 13 ohms with a ufer but with 2 rods I get about 89 ohms around here. Other areas, I hear , do much better.
 
Actully high carbon soils are about the best you can get, so land around coal mines, and or processing plants would have a low resistance, the Eating Crow thread in which Gary did those test in, I caculated his soil was about 8 ohms at the current he was getting off his test rod, and it makes sense as that area around Houston was known for shipping coal, oil and other high carbon content products, many of those towns in that area were built on top of the old shipping stock piles.
 
K8MHZ said:
I know you are joking but there are places in Michigan where the copper content of the soil is so high that it renders compasses useless.

I wonder how much affect that has on grounding electrodes.
Click to expand...

Ok drive a rod into a vain of copper, that would do also:grin:

You know with all the current being passed through the earth it's amazing any old school compass work, now AC shouldn't mess with a magnetic compass but DC will, and is one of the reasons they try not to run DC S.W.E.R. HV lines where ships might pass, like across straights.

But still it would seem as the more DC current we pump into earth the more it should affect compasses?:confused:

Maybe it has to hit the metal like copper before it can produce enough of a field to affect a compass?
 
Last edited:
Status
Not open for further replies.
Top