residential grounding

[minton1012]

I have a 100amp residential service main to be grounded. I have the 8' ground rod but a pvc water main to the house. Where do I pick up my earth ground,(water pipe source), or do I need another ground rod within 6'?

 

[Dennis Alwon]

I have a 100amp residential service main to be grounded. I have the 8' ground rod but a pvc water main to the house. Where do I pick up my earth ground,(water pipe source), or do I need another ground rod within 6'?

Just drive 2 ground rods, not within 6", but at least 6' apart.

 

[rexowner]

I have a 100amp residential service main to be grounded. I have the 8' ground rod but a pvc water main to the house. Where do I pick up my earth ground,(water pipe source), or do I need another ground rod within 6'?

I'm not sure if this is your question, but if by
"water pipe source" you are asking about grounding
a copper pipe system, you can ground it at any accessible
point in the system as you are not using the pipe service
as a ground. See 250.104. It would be common to do
it at the water heater if it is nearby the panel. If there is
plastic piping within the house, incidental copper such
as feeding fixtures doesn't need to be grounded.

As Dennis Alwon said, two ground rods will provide
the required ground for your service.

 

[tryinghard]

I have a 100amp residential service main to be grounded. I have the 8' ground rod but a pvc water main to the house. Where do I pick up my earth ground,(water pipe source), or do I need another ground rod within 6'?

"Ufer" conctrete-encased electrode 250-52(A)(3), it's the best if this is new construction

 

[Pierre C Belarge]

I'm not sure if this is your question, but if by
"water pipe source" you are asking about grounding
a copper pipe system, you can ground it at any accessible
point in the system as you are not using the pipe service
as a ground. See 250.104. It would be common to do
it at the water heater if it is nearby the panel. If there is
plastic piping within the house, incidental copper such
as feeding fixtures doesn't need to be grounded.

As Dennis Alwon said, two ground rods will provide
the required ground for your service.

The terminology at this point in the system would be "bond" not ground. (PVC water supply system to the building)

 

[rexowner]

The terminology at this point in the system would be "bond" not ground. (PVC water supply system to the building)

Yes, I should have said "bond" to be more consistent
with industry standard terminology, which is of course
the term that the text in 250.104 uses, along with
other important details such what to bond to.

 

[rexowner]

"Ufer" conctrete-encased electrode 250-52(A)(3), it's the best if this is new construction

If you have a chance to do a Ufer, my experience has
(surprisingly to me at first) better with rebar than
35 feet of copper. Since we built two indentical homes
("A" & "B") next to one another using rebar UFER
grounds, I measured 13 Ohms from GEC(A) to GEC(B),
or about 6.5 Ohms each through our dry Silicon
Valley clay. I believe in fact the rebar was tied to all
the rebar in the footing so, we had considerably more
than the minimum length in contact with concrete.

On the other hand, on a different building site, I observed about 100 ohms between 35 feet of 4 AWG copper
that was being used for a Ufer, and some footing rebar that happened to be sticking out of the concrete. This was
just the resistance within the concrete, and not through
the earth.

I realize these datapoints are not 100% scientific, but I
have asked around to other electricians I work with, and
I haven't found anyone who actually measure ground
resistance.

I just thought it was interesting that the extra length
in the footing rebar seems to provide such an excellent
ground, even vs. the more expensive copper (which
wasn't bonded to the rebar in the concrete, but I suppose
could be if that's the route you went.)

FWIW.

 

[tryinghard]

Around here (central California) most common is a bottom rebar is turned up out of the footing as the electrode. It does need to be longer than 20' in order to comply because 250-52(A)(3) requires 20' on the bottom of the footing, this rebar also ties to the structural steel which does contribute to a lower resistance, #4 copper installed is the same. The point these protrude from the footing is actually the end of the electrode, the part the electrode conductor will terminate to.

I have never measured ground resistance on a ufer but I understand they are lower resistance due to the ties throughout the whole rebar grid. On our school projects we had to measure rod type electrode resistance in the presence of the inspector and a single 3/4"x10' rod would average around 10-15 ohms, I think we did readings at 25', 50', & 75' out.