# earthing calculation

#### hanz

##### New member
Hi all,

Currently i'm doing earthing calculation. our plant has limited area, only 18m x 22m. soil resistivity is 244 ohm-meter.
according user requirement, resistance shall be max 4 ohm.

after calculate with the hollow square, 9 rods, i just got 5,565 ohms.

thank you

#### Sahib

##### Senior Member
It is possible to reduce the ground resistance to the desired level by using a network of closely spaced rods connected together. The formulas for doing so is given A.S.Pabla's Electric Power Distribution, TATA-MCGRAW HILL Edition.

#### petersonra

##### Senior Member
this is not a NEC issue as here are not NEC requirements for minimum earth resistance.

#### kwired

##### Electron manager
this is not a NEC issue as here are not NEC requirements for minimum earth resistance.
True, but this is not the NEC forum either it is the Electrical calculations / engineering forum so I think it is a valid question for here.

Most applications knowing the exact resistance of a grounding electrode is probably not all that important, but sometimes it maybe is.

#### Julius Right

##### Senior Member
I calculated the Rmat resistance using 1/0[95 sqr.mm] stranded bare copper 0.5 m depth [only 80 m total length] and 15 driven rods of 12 m [4 ft.] length and 17 mm dia and I get 3.94 ohm.[calculation according IEEE 80/2000]

#### kwired

##### Electron manager
I wonder if people creating these grounding schemes realize that the electrode to earth connection is not a constant.
Same here, and unless there is some extreme importance to have a low resistance, IMO is not even worth any attempt to do any calculations or take any action beyond NEC minimum requirements.

#### K8MHZ

##### Senior Member
Same here, and unless there is some extreme importance to have a low resistance, IMO is not even worth any attempt to do any calculations or take any action beyond NEC minimum requirements.
Also, trying to use ground rods to achieve that lofty goal is not the correct method, IMHO.

Better to use an electrolytic (chemical) electrode.

http://www.erico.com/products/ChemRod.asp

#### kwired

##### Electron manager
Also, trying to use ground rods to achieve that lofty goal is not the correct method, IMHO.

Better to use an electrolytic (chemical) electrode.

http://www.erico.com/products/ChemRod.asp
true, even a concrete encased electrode puts you far ahead at the start of the game as compared to using ground rods.

#### kingpb

##### Senior Member
I calculated the Rmat resistance using 1/0[95 sqr.mm] stranded bare copper 0.5 m depth [only 80 m total length] and 15 driven rods of 12 m [4 ft.] length and 17 mm dia and I get 3.94 ohm.[calculation according IEEE 80/2000]
Isn't 12-m almost 40-ft in length? That's one hell of a ground rod!

Typical ground grid conductor we design with is #4/0 AWG bare copper, maybe even #2/0 AWG but can't remember when we ever went smaller.

Is it being assumed that you have a uniform soil resistivity? Probably not realistic.

If you have to have 4-ohms or less, wasn't a soil resistivity test completed, if not, may be beneficial because if this is a contract requirement, I would think you will have to demonstrate it meets spec before project completion. (payday)

#### Julius Right

##### Senior Member
Isn't 12-m almost 40-ft in length? That's one hell of a ground rod!

Typical ground grid conductor we design with is #4/0 AWG bare copper, maybe even #2/0 AWG but can't remember when we ever went smaller.

Is it being assumed that you have a uniform soil resistivity? Probably not realistic.

If you have to have 4-ohms or less, wasn't a soil resistivity test completed, if not, may be beneficial because if this is a contract requirement, I would think you will have to demonstrate it meets spec before project completion. (payday)
You are right, kingpb: 95 sqr.mm it is close to 4/0[107 sqr.mm] not to 1/0[53 sqr.mm] and 12 m it is 40 ft. not 4 ft. as I translate wrongly! However, since the calculation was entirely in IS units [and not in Imperial units] the result is correct. I have to be more circumspect in translations!:ashamed1:
I did not consider 12 m rod as a huge one.I think the ground is soft enough?at 244 ohm.m-resistivity-and will not problem to drive it.
Ok,I am only a designer of power station and I purchased hundreds of 12-20 m electrodes and I don?t remember the grounding grid contactor ever complains.

#### topgone

##### Senior Member
You are right, kingpb: 95 sqr.mm it is close to 4/0[107 sqr.mm] not to 1/0[53 sqr.mm] and 12 m it is 40 ft. not 4 ft. as I translate wrongly! However, since the calculation was entirely in IS units [and not in Imperial units] the result is correct. I have to be more circumspect in translations!:ashamed1:
I did not consider 12 m rod as a huge one.I think the ground is soft enough?at 244 ohm.m-resistivity-and will not problem to drive it.
Ok,I am only a designer of power station and I purchased hundreds of 12-20 m electrodes and I don?t remember the grounding grid contactor ever complains.
It could be just another typo there! 1.2m is around 3.936 feet AFAIK.

#### Sahib

##### Senior Member
Same here, and unless there is some extreme importance to have a low resistance, IMO is not even worth any attempt to do any calculations or take any action beyond NEC minimum requirements.
The NEC allowable maximum ground resistance of 25 ohms does not apply to outdoor substations and other outdoor switching apparatus belonging to a consumer. The ground resistance in such cases should be maintained in the range of 4 ohms or less.

#### hurk27

##### Senior Member
Generally soil resistivity is measured in ohms per cm which is a volume of soil, not just in meters which is linear, placing rods closer then twice their length will cause a low gain over installing just over this, this is because you want each rod to be out of the sphere of influence of the other rods, longer rods over 4.5m or 15 deep will not gain much unless a steady moisture level can be obtained at reaching a greater depth, even a CCE might not reach the desired ohms if the soil's ohms per cm doesn't support it,, removing the high resistance soil and replacing it with a much higher conductive soil can sometime be the best option but not always an available option, chem rods are also not always a good choice because they require constant maintenance of adding chemicals to keep them working.

But as others have said, there is no scientific bases for requirements like this unless it in being used for a single wire earth return electric supply to a remote area, if this is for a lightning protection system then someone needs to learn a little more about lightning because earth electrode resistance plays very little to no part in a lightning protection system, it is more important to have a low impedance path around what ever that is being protected then how good a connection to earth is, the reason for the above is that lightning is an opportunity event, as the clouds move across an area and the charge builds up to the point the air breaks down, then what ever is under that point is going to be hit, you don't attract lightning, its going to take a path to earth that may include what ever low impedance paths that might be in that route, you build paths around what you want to protect, the resistance of the soil plays no part in this, look at aircraft, if it is within the path of the break down of the air the lightning can and will take the path through the metal body of it because it provides a little less impedance in the stroke in the path of it to the earth, the aircraft was just in the wrong place at the wrong time.

If this is for other electrical purposes, keep in mind current does not want to go to earth, it wants to return to source from which it came, most times this is the last transformer it was fed from, except the SWER system mention above that actually uses the earth as one of the conductors of a circuit earth plays no part in the operation of electrical systems, most of these kind of requirements are built around old myths that keep on circling around in the electrical world.

#### Smart \$

##### Esteemed Member
The NEC allowable maximum ground resistance of 25 ohms does not apply to outdoor substations and other outdoor switching apparatus belonging to a consumer. The ground resistance in such cases should be maintained in the range of 4 ohms or less.
1) If owned by consumer, the NEC applies provided it is adopted by the AHJ.
2) The NEC 25 ohm maximum only applies to a single ground rod, pipe, or plate in a grounding electrode system.

#### Smart \$

##### Esteemed Member
Generally soil resistivity is measured in ohms per cm which is a volume of soil, not just in meters which is linear...
Ummm... ohm-cm, not ohms per cm and a simple conversion from the ohm-meter unit.

#### Sahib

##### Senior Member
1) If owned by consumer, the NEC applies provided it is adopted by the AHJ.
2) The NEC 25 ohm maximum only applies to a single ground rod, pipe, or plate in a grounding electrode system.
If the maximum 25 ohms resistance is not met even after addition of a second electrode, the NEC does not care further even when ground resistance per electrode is more than 25 ohms. This is not the case with outdoor substations and switching apparatus.

#### Smart \$

##### Esteemed Member
If the maximum 25 ohms resistance is not met even after addition of a second electrode, the NEC does not care further even when ground resistance per electrode is more than 25 ohms. This is not the case with outdoor substations and switching apparatus.
As noted, if consumer owned (which is what you stated), it falls under NEC purview when the NEC has been adopted by the AHJ. Please cite an NEC reference to back up your claim.

EDIT...

FWIW, the following section was added to the NEC 2011 edition:

250.191 Grounding System at Alternating-Current
Substations. For ac substations, the grounding system shall
be in accordance with Part III of Article 250.
Informational Note: For further information on outdoor ac
substation grounding, see ANSI/IEEE 80-2000, IEEE
Guide for Safety in AC Substation Grounding.

Last edited:

#### Sahib

##### Senior Member
As noted, if consumer owned (which is what you stated), it falls under NEC purview when the NEC has been adopted by the AHJ.
Suppose in an outdoor substation, the maximum ground resistance of 25 ohms is achieved by a single ground rod. Is this installation safe from touch and step potentials when a HV ground fault occurs? I think not.