Number of Ground Rods

[Volt-Amps]

Regards,

I am a designer, can the experts help me understand how to determine the number of ground rods needed for a grounding system?
How do I determine if 1, 2 or 3 rods are needed?
How to determine if I need 10-ft or 20-ft ground rod?
Thank You

 

[roger]

See the exception to 250.53(A)(2)

Roger

 

[kwired]

Regards,

I am a designer, can the experts help me understand how to determine the number of ground rods needed for a grounding system?
How do I determine if 1, 2 or 3 rods are needed?
How to determine if I need 10-ft or 20-ft ground rod?
Thank You

To meet minimum code see section roger referenced.

To acquire a certain resistance - it gets more complicated and requires measurements of the installation and will be different at every installation.

 

[chrisstaker]

Regards,

I am a designer, can the experts help me understand how to determine the number of ground rods needed for a grounding system?
How do I determine if 1, 2 or 3 rods are needed?
How to determine if I need 10-ft or 20-ft ground rod?
Thank You

For residential installations, you will rarely have more than (1) ground rod. The concrete slab rebar should also be connected to the grounding system. This most often will meet the NEC requirement of 25 ohms or less.

But in industrial or manufacturing installations, there should be a ground loop or grid installed consisting of #4/0 or #2/0 grounding electrode conductors (GECs) with ground rods spaced every 50 to 100 feet. I typically space them every 35 feet or in the (4) corners of a ground loop if it is 50 ft x 50 ft or smaller. Structural steel, as well as the rebar in concrete foundations in an area should also be connected to comprise one continuous grounding system.

Places with sandy soils are about the only time one would need longer ground rods, or more frequently spaced ground rods along with ground enhancing material, due to the high insulating, low conductivity properties of sand, which equates to an increased resistance in the grounding system. Although, certain clients have stricter grounding requirements than others, which may require more or less depending on the client’s grounding specs/standards.

Respecrfully,

Christopher D. Staker, PE
Atlanta, GA

 

[ptonsparky]

For residential installations, you will rarely have more than (1) ground rod. The concrete slab rebar should also be connected to the grounding system. This most often will meet the NEC requirement of 25 ohms or less.

But in industrial or manufacturing installations, there should be a ground loop or grid installed consisting of #4/0 or #2/0 grounding electrode conductors (GECs) with ground rods spaced every 50 to 100 feet. I typically space them every 35 feet or in the (4) corners of a ground loop if it is 50 ft x 50 ft or smaller. Structural steel, as well as the rebar in concrete foundations in an area should also be connected to comprise one continuous grounding system.

Places with sandy soils are about the only time one would need longer ground rods, or more frequently spaced ground rods along with ground enhancing material, due to the high insulating, low conductivity properties of sand, which equates to an increased resistance in the grounding system. Although, certain clients have stricter grounding requirements than others, which may require more or less depending on the client’s grounding specs/standards.

Respecrfully,

Christopher D. Staker, PE
Atlanta, GA

Refer to 250.52 (A) Electrodes permitted for Grounding.

Concrete Slab rebar is not on the list.

All the the rest you mention are not an NEC requirement, but may be specified in Addition to as you stated.

 

[MRKN]

I'm sure he meant footing or foundation, we're not all well-versed in civil/structural nomenclature here.

 

[infinity]

If you have a CEE you do not need any ground rods and do not need to have 25 ohms or less on the CEE.

 

[Frank DuVal]

CEE Concrete Encased Electrode, i.e. Ufer.

 

[Andy Delle]

CEE Concrete Encased Electrode, i.e. Ufer.

Yes very popular here on the southwest. They just leave a piece of Rebar sticking out by the meter location and that's your central ground point.

 

[petersonra]

the correct number is anything from 0 on up to as many as you want.

there is no requirement to have any nor any limit to the number you are allowed to have.

 

[Dale001289]

For residential installations, you will rarely have more than (1) ground rod. The concrete slab rebar should also be connected to the grounding system. This most often will meet the NEC requirement of 25 ohms or less.

But in industrial or manufacturing installations, there should be a ground loop or grid installed consisting of #4/0 or #2/0 grounding electrode conductors (GECs) with ground rods spaced every 50 to 100 feet. I typically space them every 35 feet or in the (4) corners of a ground loop if it is 50 ft x 50 ft or smaller. Structural steel, as well as the rebar in concrete foundations in an area should also be connected to comprise one continuous grounding system.

Places with sandy soils are about the only time one would need longer ground rods, or more frequently spaced ground rods along with ground enhancing material, due to the high insulating, low conductivity properties of sand, which equates to an increased resistance in the grounding system. Although, certain clients have stricter grounding requirements than others, which may require more or less depending on the client’s grounding specs/standards.

Respecrfully,

Christopher D. Staker, PE
Atlanta, GA

What you’ve described using #2/0 or #4/0 is an equipotential plane and is not a NEC requirement. It is rather a common practice in heavy industries.
There is no specific quantity of Rods etc other than what may be required by client specifications. Most clients for example, in petro-chem facilities require an overall ‘grid’ of under 5 Ohms which invariably means driving multiple rods, taps to steel, rebar etc.


Sent from my iPhone using Tapatalk

 

[Volt-Amps]

Thank you all for your help. I will review the code sections referenced above. This forum is a great help, I truly appreciate all the help.

 

[tom baker]

For residential installations, you will rarely have more than (1) ground rod. The concrete slab rebar should also be connected to the grounding system. This most often will meet the NEC requirement of 25 ohms or less.

Before a Ufer ground, we would always have two ground rods, as one in Seattle area is typically 3,000 ohms if tested.
Ufer ground is not option (should be), its required to be part of the GES if its present
There is no minimum resistance with a Ufer ground and be careful about stating the NEC requires 25 ohms

 

[tom baker]

Regards,

I am a designer, can the experts help me understand how to determine the number of ground rods needed for a grounding system?
How do I determine if 1, 2 or 3 rods are needed?
How to determine if I need 10-ft or 20-ft ground rod?
Thank You

One ground rod is measured resistance is 25 ohms or less. If use two then no test is needed, so if ground rods are all you have, drive two and go home
8ft ground rod in contact with soil is minimum.
Deeper is lower resistance, or by specs.

 

[jeff48356]

One ground rod is measured resistance is 25 ohms or less. If use two then no test is needed, so if ground rods are all you have, drive two and go home
8ft ground rod in contact with soil is minimum.
Deeper is lower resistance, or by specs.

If you have one ground rod in addition to the copper water pipe entering the house at the water meter, would those two electrodes combined meet the 25 ohm or less requirement? Or would I still need two ground rods in addition to the water pipe?

 

[infinity]

If you have one ground rod in addition to the copper water pipe entering the house at the water meter, would those two electrodes combined meet the 25 ohm or less requirement? Or would I still need two ground rods in addition to the water pipe?

The single rod on its own needs to meet the 25 ohms or less threshold or it does not qualify as a grounding electrode.

 

[Coppersmith]

One ground rod is measured resistance is 25 ohms or less. If use two then no test is needed, so if ground rods are all you have, drive two and go home
8ft ground rod in contact with soil is minimum.
Deeper is lower resistance, or by specs.

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