Electrical Grounding Help Information
Loresco Vertical Resistance Calculator

Data Input
Note: Do not use commas in the input values. If the decimal is not entered, it is assumed to be just to the right of the right-most digit entered.

English Units or Metric Units: You may choose Metric or English input units by use of the input unit select button prior to entering variables. For example, if you wish to enter variables in English units and English units (ft & in) appear next to the input variable entry slots, simply enter the proper numbers and continue. However, if you wish to enter the variables in English units and Metric units (m & cm) appear next to the input variable entry slots left click the mouse on the "English Units" button. Note: the required units for Resistivity are ohm-cm regardless of the input units selection.

Environment Resistivity, (ohm-cm): Enter the average resistivity of the environment (earth) around the grounding electrode in ohm-cm. This is the value of soil resistivity determined though field testing. Soil resistivities may vary from a low of 100 to a high of 1,000,000 ohm-cm. Regardless of the input units selected, the units for resistivity must be in ohm-cm.

Length of single electrode column, (m or ft): Enter the length of a single grounding electrode or electrode column in meters or feet. If LORESCO grounding backfill is used, the length is the length of the backfill column. If no backfill is installed, the length is the actual electrode length. Where several electrodes are installed in a single hole, enter the length of the grounding backfill column, if used , or the total length of all of the individual electrodes, if no backfill is used. Multiple electrodes in a single hole are considered a single electrode.

Number of electrodes: Enter the total number of electrode holes installed. This is the number of LORESCO grounding backfill columns. Regardless of the number of electrodes installed in a single hole or grounding backfill column, this is considered one electrode.

Diameter of electrode columns, (cm or in): Enter the diameter of a single electrode or electrode column in centimeters or inches. Again, this is the diameter the LORESCO grounding backfill, if used, or the actual electrode diameter, if no backfill is used.

Spacing between columns, (m or ft): Enter the center-to-center spacing in meters between individual electrode holes or columns in meters or feet. Typical spacings between surface electrodes vary between 1.5 to 10 meters (5 to 30 feet). Typical spacing between deep electrode systems vary between 7 to 30 meters (25 to 100 feet).

If the number of electrodes entered in one, no spacing value is required. If a value for spacing is entered with the number of electrodes equal to one, the spacing is ignored.

Deep or shallow: Select one of the two choices from the drop-down menu. Surface electrodes generally are installed with the top of the active electrodes within 15 meters (50 feet) from the surface. Deep electrodes are generally installed with the top of the active electrode below 15 meters (50 feet) from the surface.

Result: This is the resistance-to-earth in ohms of the electrode system described by the input data. If the estimated resistance is higher than the required value, one or more of the design variables may be changed in order to determine its effect on the expected resistance. The greatest decrease in resistance is achieved either by increasing the number of grounding electrode holes or increasing the length of a single electrode column.

The most recent result along with the input data is displayed in output column 1. You may recalculate by reentering the required variables while changing any one or all of the input data values for the next calculation. As additional calculations are undertaken, the output results automatically scroll to the right. In other words, at any time you may compare the three most recent calculation results.

General Information

This calculator employs the surface and deep electrode equations developed by Erling D. Sunde and found in his book Earth Conduction Effects in Transmission Systems. In the case of multiple electrode holes or electrode systems, correction for the mutual interference between the electrodes is accomplished using the general formula developed by Sunde. These equations apply to equally spaced, vertical electrodes placed along a straight line. Development of the equations assumed a homogeneous electrolyte with a single resistivity. The equations ignore any attenuation effects along the grounding electrode. Finally, although the equations were derived for direct current flow, they may be used for low frequency alternating current systems and even lightning surge resistance if the length of interconnecting wire is relatively short (less than 100 meters). As the lengths of interconnecting wires become greater in AC systems, the impedance of the wires becomes more significant and should be taken into account.

These calculations do not take into account the contact resistance between each electrode and the LORESCO grounding backfill. However, the contact resistance for each electrode is small, being on the order of a few tenths of an ohm or less. Since all of the electrodes in a grounding system are in parallel, the total contact resistance is the contact resistance for one electrode divided by the total number of electrodes. Therefore, for practical cases this contact resistance is negligible and may be ignored.