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TerraFill® and TerraDyne® Electrolytic Grounding Electrode Calculator

Alltec has developed a new tool to help users determine the quantity of Ground Conductor, TerraFill® and TerraDyne® Electrolytic Grounding Electrode(s) needed to design a low resistance grounding system.

The “grounding component” is the connection path from the site’s equipment to the earth.  The grounding system resistance, measured in ohms, determines how quickly electrical fault as well as lightning/switching transient currents dissipate to the earth. Grounding is also necessary to maintain a reference point for equipment during normal operating conditions.

We’ll talk about the information you need to use this calculator and how it can benefit you.

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Figure 1:TerraFill Calculator

To start, the calculator asks you to fill out your name, email and the project title.  You can also create a login to save your information, there is no charge for this!

Once you have filled that out, you want to select what measurement system you are using and then decide for what type of grounding component you need to calculate grounding system resistance.  The TerraDyne® Electrolytic Grounding System comes in vertical and horizontal models, and there is of course, horizontal grounding conductor encapsulated with TerraFill® low resistive backfill material.

For each of the calculators provided a key measurement is needed, which is the Soil Resistivity test value at the installation location.  The first step in a low resistance grounding system design is to perform soil resistivity testing.  Let’s discuss that before moving forward.

Soil naturally contains varying amounts of electrolytes that conduct electricity.  One must first determine the soil resistivity.  The soil resistivity value is determined by using the Wenner four-point measurement method, corresponding to IEEE Std. 81.  The Wenner 4 point measurement test employs four equally spaced auxiliary probes inserted into the earth to determine the actual soil resistance at varying depths, traditionally in ohms-cm.  Soil values can range from 500 Ωcm with large amounts of electrolytes to over 1 million Ωcm.  Once you have this measurement, you need to calculate what grounding components you need to achieve the desired low resistance grounding system design.

Let’s first focus on the Vertical Ground Electrode Calculator.  The first field you will need to fill out is the soil resistivity value that we talked about earlier, once you have that, plug the number into the appropriate field.  The next two fields are for the length of the electrode and diameter of the TerraFill® column. The Length is anywhere from 8 to 300 feet depending on the application.  Diameter is the size of the hole the electrode is going into that will be filled with the TerraFill®.  A 6” diameter column is the minimum recommended by Alltec.

  • Please note, these calculators are for single soil resistivity value.  For specific grounding system analysis requiring multiple soil layers, contact Alltec and one of our engineers will be happy to help.

Vertical Model:

  Figure 2: Vertical Installation

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Figure 3: Vertical Calculator

There is an option to select either single or multiple electrodes for vertical TerraDyne® electrodes. The effect of the concentric shell is that it takes a finite amount of earth for an electrode to fully realize its resistance value. This finite amount of earth is known as the sphere-of-influence. The sphere-of-influence for a TerraDyne® is commonly considered a radius around the ground electrode equal to its length. The option to select TerraDyne® electrode spacing for multiple electrodes accounts the sphere of influence factor in the final grounding system resistance calculation.

Figure 4: Sphere of Influence

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Figure 5: Calculation Results

 The calculation will appear as shown in Figure 5 above and you additionally have the option to view a pdf result and email or fax to the Alltec sales department for a quote.

Horizontal Model:

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Figure 6: Horizontal Calculator

 The horizontal model works much the same way as the vertical with the exception of the depth of burial replacing the diameter.  The depth will vary from 24” to 42” based on the electrode riser length.  See the following diagram.

At a site where there is a limitation to install vertical electrodes because of rocky soil conditions, horizontal TerraDyne’s® are normally recommended. The horizontal TerraDyne® should be encapsulated in 6”×4” minimum of TerraFill®.

Figure 7: Horizontal TerraDyne® Installataion

The last calculator option is the ground conductor section, in this one, the soil resistivity value is required as well as the length of the conductor and depth of burial.  The width and depth of the TerraFill® material are required to be inserted in order to determine grounding system resistance as well as the number of TerraFill® bags.

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Figure 8: TerraFill® Soil Enhancement Detail

Ground Conductor Model:

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Figure 9: Ground Conductor Calculator


Good grounding system designs contain both horizontal and vertical components of the grounding system. It is imperative to perform proper technical selection of grounding conductor length as well as depth and placement of ground electrode components.  The Alltec calculator tool allows easy change iterations to be executed in order to optimize material usage and installation costs, while achieving grounding system resistance goals.

The individual grounding system resistance of horizontal ground conductor with TerraFIll® and vertical/horizontal grounding electrode determined from the calculated results can be combined (with parallel resistance formula) to determine the overall effective grounding system resistance.

We hope that you find this new tool helpful in project planning and budgeting.  If you have questions, need assistance using this tool, or have feedback, please feel free to contact us at +1.800.203.2658 or via email at

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