If you’re reading this, chances are you have enjoyed the sense of accomplishment that comes from driving an 8-foot long, mostly steel rod into the earth and connecting a ground wire to it. It’s tough work. At the end of the day, you know that your effort was justified because you have installed a system that will protect people from electrical shock, help safeguard expensive electronic equipment, limit neutral-to-ground voltage and satisfy the NEC along the way. There is an increasing awareness in the electrical industry of the benefits of a low-resistance, high-quality grounding system. This article takes a nuts-and-bolts approach to examining one the most commonly installed components: the ground rod.
Copper-bonded, galvanized, and stainless steel ground rods are available in many different sizes. We will not focus on stainless steel rods as their high cost prohibits widespread use. More commonly used are copper-bonded and galvanized steel ground rods. Besides price, what really makes these rods different? Both rods are composed of a steel core with a tensile strength ranging from 58,000 psi for galvanized rods to >90,000 psi for copper-bonded steel rods. From a theoretical standpoint, the higher the tensile strength, the less likely the rod is to “mushroom” or spread when being driven. This is a concern when rods are being coupled or when connections are being made to the top of the rod. Practically speaking, we all know that any ground rod will mushroom if you hit it without using a drive sleeve specifically designed to prevent this. So, the steel used in a copper-bonded rod may give it a slight edge in “driveability,” but not enough to classify it as a superior electrode.
The main difference between the two rods is the thickness and type of material used to cover the steel core. Galvanized ground rods are coated with zinc to a thickness of 3.9 mils or .0039 inches. Copper-bonded ground rods are coated with copper to a thickness of 10 mils or .010 inches. It is the thickness and type of material coating that primarily determines the rod’s corrosion resistance and service life. In essence, we are comparing zinc to copper and 3.9 mils to 10.0 mils. I think everyone would agree that, regardless of the material, a thicker coating would provide better corrosion protection and, therefore, longer service life.
Perhaps a less intuitive leap is that copper is inherently more resistant to corrosion than zinc. We’ve all used galvanized steel products and paid a premium for them. Chances are, you didn’t have any major corrosion problems with these items. Why should we expect anything different from a galvanized ground rod? The reason is that galvanized ground rods are exposed to the much harsher below-grade environment.
It is an entirely different corrosion ballgame when metals are buried. Aluminum illustrates this point perfectly. Aluminum displays good corrosion resistance above grade. In fact, many boats that are subject to corrosive saltwater are made using aluminum. However, aluminum is prohibited for below-grade use in Article 250 of the NEC due to its lack of corrosion resistance in this environment. While not as drastic as aluminum, galvanized metal experiences a similar drop off in corrosion resistance when placed underground.
Comprehensive direct burial studies done by the National Bureau of Standards showed that 3.9 mils of galvanizing could be expected to provide 10-13 years of protection in most soils. This same study showed that 10 mils of copper could be expected to last more than 40 years in most soil types and is the basis for the 10 mils of copper required for a rod to be UL listed. Furthermore, independent ground rod testing performed by the Navy and the National Electrical Grounding Research Project back up the data gathered by the National Bureau of Standards. Because of these studies, a service life of 10 to 15 years can be assigned to galvanized rods and 40-plus years for 10 mil copper-bonded rods in most soil types.
These results may lead you to believe that copper-bonded rods are better than galvanized rods. Sometimes this is true and sometimes not. I want to emphasize the importance of matching the appropriate ground rod to the application. If the facility being grounded has a life expectancy of less than 15 years, a galvanized ground rod is appropriate and will provide the most cost-effective solution. For installations with a longer service life, copper-bonded ground rods are the best fit. For many years, the copper cold water pipe has served as the primary grounding electrode for commercial & residential grounding.
With non-conductive PVC piping used more extensively these days, the supplemental ground rod is becoming the primary electrode. It only makes sense that it should be required to perform as long as the copper water pipe that came before it. As such, I strongly encourage the use of UL-listed copper-bonded ground rods on new home construction.
Galvanized ground rods
Recently, UL-listed galvanized ground rods have shown up in the market. You may have wondered why now and what does this mean? Underwriters Laboratories had never listed galvanized ground rods in the past and there are no listing requirements for galvanized rods in the existing UL 467 Standard, unlike copper-bonded and stainless steel rods which have clear listing requirements. These rods use the higher tensile strength steel found in copper-bonded rods, which is a plus, but we’ve already established that as a minor benefit. These galvanized rods have a smaller diameter than non-UL listed galvanized rods requiring special accessories and different exothermic welding equipment.
Most importantly, these rods are coated with the same amount of zinc as their non-UL listed cousins (3.9 mils). Since the coating is the same, there is no increase in service life. So what makes them better? It has been suggested that the UL listing will make the inspector’s job easier by allowing them to visually inspect for the UL mark. Inspectors that had trouble qualifying galvanized rods in the past may appreciate this, but I believe this to be a small minority of the dedicated individuals in this profession.
While the initial inspection of the rod serves a purpose, the bigger issue is inspection of the rod 5, 10 or 30 years after it is buried. Who performs this important role? Nobody. UL marks are not helpful once the rod is buried. The long-term performance of the rod is more important than its initial inspection.
The length and diameter of the ground rod not only affect its resistance but also its driving characteristics. Although larger diameter ground rods do not have an appreciably lower ground resistance value, they do have a larger steel core that makes them easier to drive in harder soil by providing extra rigidity. It's probably no coincidence that most rods driven in Canada, with its harder soil, are 3/4 inch in diameter as opposed to 5/8-inch rods which dominate in the United States.
The length of a ground rod plays a much bigger role in its final ground resistance measurement, and it goes without saying that it takes longer to drive a longer ground rod. The NEC and UL require a ground rod to be at least 8 feet in length. This specification was obviously created by engineers that had never driven a ground rod or noticed that most people are not 8’ tall. Longer rods are more dangerous to install and bow more when being driven. The more a rod bows or shudders, the less efficient the driving process is. Shorter rods are safer and easier to drive. In fact, I would love to see the industry standardize on using two 4-foot rods and a coupler to achieve the required 8 feet total length. Installations would be faster, easier, and safer not to mention that the logistics of transporting and storing a 4-foot rod are much simpler than longer 8- or 10-foot rods.
Hopefully, this article has given you a little more insight on a product that you use everyday—the ground rod. Not all ground rods are created equal nor are the techniques used to install them. Installing a grounding system is best-done right the first time as it is every expensive to rework, and failure of the grounding system puts people and expensive electronic equipment jeopardy. Understanding the functions of a grounding system and the characteristics of the various components used to implement it will allow the electrical contractor to make educated buying decisions. Exceptional contractors will understand that finding the cheapest way to meet code can end up costing more.