Ground rods are an important component of electrical systems to help protect against potential shock hazards and damage from lightning strikes. Installing proper grounding helps ensure electrical safety by providing a low-resistance path for stray electrical currents to dissipate into the earth. There are specific code requirements that regulate the use of ground rods which electricians must follow.
What are ground rods?
Ground rods, also called earth rods or grounding electrodes, are metal rods that are driven into the earth and used to make an electrical connection with the ground. They are typically made of solid copper, galvanized steel, or stainless steel and measure 8 to 10 feet in length and 5/8 to 1 inch in diameter. The purpose of a ground rod is to provide a low impedance path for electricity to travel into the earth. This helps prevent the buildup of dangerous voltages that could pose a risk of electrical shock. Ground rods also serve as a discharge point for lightning strikes, helping protect electrical systems from surges.
Why are ground rods required by code?
Ground rods are required by electrical code because they are a fundamental component of safe electrical installations. Properly grounded electrical systems help safeguard against a number of risks including:
- Electric shock – Grounding provides an alternative path for current to flow away from a person who comes in contact with a live wire.
- Fire – Grounding helps prevent fires by giving stray electrical currents a safe place to dissipate rather than traveling on an unintended path that could generate heat and sparks.
- Surges – Ground rods help disperse the large amounts of energy from lightning strikes into the earth where it can’t harm electrical systems.
- Equipment damage – By draining errant currents into the ground, ground rods help prevent damage to electrical equipment from overloads and surges.
The National Electrical Code (NEC) mandates ground rods for these protective purposes on electrical services, generators, subpanels, commercial buildings, and structures vulnerable to lightning.
NEC ground rod requirements
The key ground rod provisions of the National Electrical Code include:
- Number – At least two ground rods are required for grounding systems with some exceptions for smaller residential services where only one may be permitted.
- Depth – Ground rods must be driven to below permanent moisture level which is usually 8 feet deep minimum.
- Spacing – Where multiple ground rods are used, they must be spaced at least 6 feet apart.
- Resistance – Total resistance to ground should not exceed 25 ohms for a single rod. This can be lowered to 5 ohms or less by using multiple rods bonded together.
- Connections – Ground rods must be connected to the grounding electrode conductor using listed fittings and irreversible compression connectors.
- Materials – Ground rods should be made of materials able to withstand corrosion including copper, copper-clad steel, hot-dipped galvanized steel, and stainless steel.
Typical ground rod installations
Some typical situations where properly installed ground rods are required include:
Main electrical service
At the main electrical service entrance, two ground rods should be used, spaced 6 feet apart and driven to 8 foot depth. These are connected to the ground bus in the panel via a grounding electrode conductor sized according to the amperage of the service.
Subpanels
Subpanels, including outbuildings, require a separate grounding with at least one ground rod.
Generators
Permanent standby generators serving a building’s electrical system need grounding with at least two rods. Portable generators should be grounded to an earth grounding rod when operated for extended periods.
Lightning protection
Lightning protection systems made up of air terminals, conductors, and grounding electrodes require a low impedance grounding connection with a minimum of two ground rods to disperse the high energy of lightning strikes.
Flagpoles
Metal flagpoles installed on a building require grounding with a nearby ground rod both for the safety of operators in storms and to minimize damage to the structure.
Electrical service grounding electrode system
For a main electrical service panel, multiple grounding electrodes work together to form the grounding electrode system required by NEC Article 250. This normally consists of the following components:
- Ground rods – At least two rods spaced 6 feet apart and driven to 8 foot depth.
- Concrete encased electrode – A conductor or rebar within a concrete foundation or footing that is in direct contact with the earth.
- Water pipe bonding – Cold water supply pipes consisting of 10 feet or more of metal pipe in contact with the earth.
Where present, these grounding electrodes must all be connected together using the appropriate size grounding electrode conductor and special clamps that bond to rebar or pipes. The system should achieve a total resistance to ground not exceeding 25 ohms.
Common ground rod materials
Ground rods are commercially available in a variety of different materials that meet code requirements for corrosion resistance and strength including:
Copper
– Offers the best electrical conductivity of available ground rod materials.
– Provides excellent corrosion resistance in most soil conditions.
– Softer material that is easy to cut and clamp connections to.
– More expensive than other options like galvanized or stainless steel.
Copper-clad steel
– Steel core provides strength with copper exterior for good conductivity and corrosion protection.
– Durable rods for driving into a wide range of soil conditions.
– Higher cost than galvanized rods but less expensive than solid copper.
Galvanized steel
– Galvanized coating helps resist corrosion in acidic soils.
– Harder steel construction allows driving into tougher soils.
– Lower cost option compared to copper or stainless steel.
– Not as conductive as copper.
Stainless steel
– Excellent corrosion resistance in acidic, sandy and moist soils.
– Higher strength for driving through rocky or frozen ground.
– More expensive than galvanized steel rods.
– Low electrical conductivity.
Installing ground rods
Proper installation of ground rods is essential for them to function as intended within the electrical system. The basic process includes:
Step 1: Drive into earth
Use a mallet or hydraulic hammer to drive the full length of the rod vertically into the ground until only a couple inches remain above grade. Driving to below permanent moisture level ensures a good year-round connection to earth.
Step 2: Ensure proper spacing
When using multiple rods, verify they are positioned at the required spacing, which is at least 6 feet apart per NEC. Greater spacing is better for achieving lower resistance.
Step 3: Connect conductor
Attach the grounding electrode conductor to the rods using suitable clamps. Avoid sharp bends. Exothermic welds provide the best connection.
Step 4: Test resistance
Measure the earth resistance using a ground resistance tester. Add more rods spaced in a straight line until resistance is 25 ohms or less. Resistance must be tested after installation but before any conductors are attached.
Step 5: Make electrical connections
Connect the grounding electrode system to the ground bus within the service panel, subpanel, or equipment being grounded using the properly sized grounding electrode conductor.
Ground rod alternatives
In some cases, alternative grounding electrodes that don’t require installing rods into the dirt may be used in lieu of ground rods. A few common substitutes include:
Concrete encased electrode
– Conductor or rebar cast within at least 2.5 inches of concrete in direct soil contact.
– Often part of concrete foundation or footing.
Ground ring
– Conductor loop buried at least 30 inches deep.
– Commonly used around the base of tall structures and towers.
Plate electrode
– Copper or galvanized plate buried at least 30 inches deep.
– Less common alternative to rods or rings.
Other NEC approved grounding electrodes may be permitted after verifying suitability with the local Authority Having Jurisdiction (AHJ).
Chemical ground electrodes
Chemical ground electrodes provide another option for achieving a low resistance ground connection without having to drive rods into problematic soils. They work by dissipating current across a larger soil surface area:
Ground rod box
– Box filled with conductive bentonite clay or gypsum cement
– Buries around a standard ground rod
– Creates a low resistance area surrounding the rod
Ground plate box
– Larger rectangular box with conductive material
– Creates an artificial ground plate
– Can achieve under 5 ohm ground resistance
Chemical ground electrodes must be approved for use as main or supplemental grounding electrodes within the local jurisdiction.
Ground rod clamps
There are specific code requirements for connectors used to bond the grounding electrode conductor to ground rods or other buried elements like rebar within concrete. Key rules include:
- Irreversible compression connectors must be used in most cases.
- Listed clamps suitable for direct soil burial are required.
- Connectors for bonding to rebar must be sized appropriately for the diameter of the rebar.
- Exothermic welding provides the optimal permanent connection that does not loosen over time.
Using the proper, code-approved ground rod clamps helps maintain the integrity of the vital grounding electrode system.
Ground rod maintenance
Ground rods provide an important safety function for electrical systems, so periodic inspection and maintenance is essential. Recommended ground rod maintenance activities include:
- Visually check clamps securing conductors to rods for tightness and corrosion.
- Measure resistance to determine if additional rods are needed to lower the overall system resistance.
- Check for physical damage to rods that may impair effectiveness.
- Ensure buried portions of rods remain covered with soil; backfill if needed.
- Repair damaged rods or replace corroded rods as required.
A preventative ground rod maintenance program is recommended to verify the electrodes continue meeting code requirements over the life of the electrical installation.
Ground rod testing
Testing ground rod resistance is important both at the time of installation and periodically thereafter. Testing methods include:
Fall-of-potential
– Involves taking measurements at specific distances away from rods
– Provides most accurate resistance reading
Clamp-on ground tester
– Clamps directly onto the rod
– Simple go/no go indication of resistance
Multimeter
– Measures resistance between rod and reference probe
– Best for measuring individual rods not system as a whole
Ideally, a fall-of-potential test should be conducted annually as part of an electrical preventive maintenance program to verify ground resistance does not exceed maximum permitted levels.
Ground rod certification
For certain installations, the local permitting agency may require documentation certifying that the ground rod system meets NEC requirements. This formal certification should include:
- Record of resistance testing indicating overall ground resistance achieved.
- Description of the ground rods used including size, spacing, and depth.
- Photos documenting the installation.
- Statement that the installation complies with NEC Article 250 grounding requirements.
The certification should be completed by a qualified electrician or inspector and submitted for approval prior to the electrical system being placed into service.
Conclusion
Ground rods installed per NEC guidelines provide critical protection against electric shock hazards and damage from electrical surges. They safely direct stray currents into the earth to prevent risks to people and property. All electrical installations from services to generators require properly driven and connected ground rods. Periodic inspection, maintenance, and testing help ensure optimal safety and function over time.