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I am thinking about installing two five-foot ground rods in place of one ten-foot ground rod. What would be the relevant comparison(s) and contrast(s)?

Let me know if you need further information. Thanks in advance.

PS: My reason for the replacement strategy is because I would dig two five-foot holes rather than one ten-foot hole. My main concern is whether I would be losing total resistance by doing so.

PSS: I am using enhanced/chemical ground rods. I am sorry for the confusion led by my omission of this information regarding digging holes for ground rods.

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3 Answers 3

You don't generally dig holes for ground rods - they are intended to be driven (hammered) into place, and make better contact with the soil that way than if placed in a dug hole.

In addition, a shorter rod will be more prone to having the soil around it dry out .vs. a longer rod, leading to higher resistance to soil. Ground rod length is mostly about reaching damp soil.

Mind you, code only calls for them to be below 3 feet and is perfectly happy if they are laid horizontal in a 3 foot deep trench (actually, that now is only the case if there is rock preventing them from being driven deeper.) However, code also calls for them to be a minimum of 8 feet long, so 5 foot rods do not qualify as meeting current code (in the USA.) But for best effect and contact with moist soil, deeper is better, and exceeding code minimums is perfectly acceptable.

You can dig a bit of a posthole (2-3 feet) to make it easier to get started driving the rod. If you dig a trench, you can make it easier to start driving the rod and actually finish driving it down near the bottom of the trench, so the whole rod (and ground cable) is deeper than it would be if driven from the surface. You need to use connectors rated for burial, but most ground rod clamps are.

Multiple ground rods (spaced 6 or more feet apart) can help reduce total resistance, but the individual rods can't be too short.

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Many tool rental companies will have a ground rod driving adapter that installs onto a handheld jackhammer or hammer drill. This makes driving the rods far easier than doing so by hand. Current US National Electric Code more or less requires two rods. (You can use one but you have to test its resistance to ground using a tool no one really has. Installing two lets you skip the test.) –  mfarver Jul 13 at 14:33
    
Do you take the multiple-rod case and connect them all in parallel, like resistance in a circuit? –  sherrellbc Jul 13 at 15:13
    
@sherrelbc: Multiple rods in parallel aren't necessarily as effective as one big rod, due to overlapping voltage fields. I believe the general guideline is that earth rods should be separated by a distance at least twice their depth. –  Li-aung Yip Jul 13 at 16:12
    
Please, see my PSS. –  Hal_Eng Jul 14 at 16:50
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Two five foot rods are not necessarily equivalent to one ten foot rod.

We usually model the soil as having multiple layers, each with a characteristic resistivity . For example, the first five foot of the ground soil might have a high resistivity of 1000 ohm-metres, while the layer under that might have a low resistivity of 10 ohm-metres.

The reason for the 10-foot ground rod is to reach through the high-resistivity layer and into the low-resistivity layer, in order to form a low-resistance connection to ground.

The above is a general, theoretical consideration. As I live in Australia, my local electrical code (AS/NZS 3000) has different requirements to the US electrical codes, so I can't comment on specific requirements in your country.


Regarding physical installation - the usual method for installing earth stakes is to hammer them in. You only need to bore a hole if you are installing especially deep earth rods, or if you are installing earth rods into solid rock.

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What exactly is an Ohm-meter, or how would you characterize that in words? Also, would a ground rod driven into soil be more or better suited than one driven into solid rock? –  sherrellbc Jul 13 at 15:15
    
1) An ohm-meter is the measure of the ground resistance. I believe it is something like ohms × cubic metres ÷ square metres. 2) solid rock usually has high resistance compared to soil, so earthing installations built on solid rock can be difficult. That said, you usually don't get a choice - if you build something on a rock, you will have to find a way to earth it into the rock. –  Li-aung Yip Jul 13 at 16:16
    
Please, see my PSS. –  Hal_Eng Jul 14 at 16:52
    
@Li-aungYip and Ohm*Meter is a measure of resistivity NOT resistance. Your first paragraph should be edited to say resistivity where ever you say resistance. Your second paragraph is fine. And NO it is not the formuale you describe, it is resitivity * length/area = units of resistance. –  placeholder Jul 14 at 17:42
    
@placeholder: duly noted and edited answer, thanks. –  Li-aung Yip Jul 14 at 17:59
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One has bore the hole when using enhanced/chemical treatment. Space the short ground rods as far apart from each other as possible to lower the resistance. Longer ground rods are better, though.

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