This topic can be extremely complicated. So here is a quick run down. Many people often ask, “how far out do you need to run leads to the furthest rod?” This is not a linear calculation.
If you are talking about a grounding delta that is 10′ x 10′ x 10′, you could be running your leads as much as 10-16x the hypotenuse, which could be 160′. But if you are talking about a ground system with a diagonal measurement of 500′, you could be running your leads 2-4x this measurement which puts you around 1000-2000′.
This is a significant amount of cable to have on hand. So when possible, we try to break down the ground system into segments and make multiple measurements. After we get our measurements, we can then add the resistances in parallel.
Who would have thought that adults actually use algebra and geometry in today’s world?! Anyone who does any type of ground testing knows these formula’s.
Addition of Parallel Resistors:
Typical Ground Resistance Test Setup
For distances greater than those illustrated, refer to the Megger book appendix II and III.
You can follow this link to download a great PDF on ground testing: Megger – Getting Down To Earth