A metal water pipe used as the grounding electrode for a home’s electrical system. The copper wire connects to the ground bus in the main service panel or service disconnect. Image via z22
Do You Have To Ground A Generator
Of all the generator questions I get asked every week, whether you have to ground your generator comes in second place. It’s a good question, and it’s usually asked about portable generators. It can be confusing, and the answer could be yes, no, or maybe. In the right context, all three would be correct.
That’s why it’s important to know how the factory configured your generator and how that affects the ways you use it.
Hey MJ, I just want to plug in my drill!
Let’s clear up the easy question. You just bought a new portable generator and want to plug your tool, appliance, or light into it. No problem. Plug it into the generator and don’t worry about the ground. Out of the box, portable generators come ready to use for this exact purpose. There’s one caveat. If the appliance or tool relies on ground to operate or checks for ground, and no ground is present, it simply won’t work. No harm done—it’s just not going to work.
Generator Grounding Terms to Know
Ground: The earth. The Earth is a great conductor of electricity. Compared to the amount of electricity we produce, the Earth has a nearly infinite capacity to act as a return path for electrical current. A ground wire is called the grounding conductor and connects to a grounding electrode.
Grounding Electrode: A connection to the earth. Buried metal water pipes, ground rods, concrete-encased electrodes, and qualified structural metal, are examples of grounding electrodes. The NEC recognizes other types of electrode.
Grounding Electrode Conductor: The wire that connects the electrical service to the grounding electrode.
Grounding Electrode System: When more than one grounding electrode is present for a building’s electrical system, the NEC requires bonding all the electrodes together to form a grounding electrode system.
Equipment Grounding Conductor: A wire connecting the non-current carrying metal parts of electrical equipment to the ground, providing a safe path for fault current to prevent electric shock hazards.
Line 1, Line 2: The two lines (L1, L2) that carry current from the transformer to the house. L1 and L2 are the hot wires in a 240-volt circuit. Either L1 or L2 can be the hot wire in a 120-volt circuit. In North American house wiring, L1 and L2 are 120 volts each. L1 + L2 = 240 volts.
Neutral: In North America, Neutral (N) is the return path in a 120-volt circuit, and carries the imbalance between the two hot wires in a 120/240 volt split single-phase system. In North American house wiring, Neutral is properly called the grounded conductor. (see Neutral-Ground Bond)
Neutral-Ground Bond: Provides additional protection against electric shock and ensures that a short circuit will trip the circuit breaker or other overcurrent protection device. Required in one location, usually the main disconnect or main circuit breaker panel in home wiring. Also required at the generator when used with a 3-pole transfer switch.
NEC: The National Electrical Code is a set of rules put out by the National Fire Protection Association and widely adopted nationwide as the gold standard for electrical wiring safety in homes, commercial applications, and industry. The NFPA updates the NEC every three years.
Portable Inverter Generator: A generator that converts DC to AC electronically. Most have variable-speed engines that only turn fast enough to generate the power required. Modern inverters output clean, almost distortion-free power. The electronics determine the output frequency.
Traditional Portable Generator: A generator with an engine that turns an AC alternator. The output frequency is dependent on the engine speed. In North America, most portable generator engines operate at 3600 RPM to output a 60 Hertz signal. The rest of the world operates on 50 Hertz power, so the engine speed is 3000 RPM. RPM / 60 seconds per minute = Frequency in Hertz. 3600 / 60 = 60 Hertz.
Why Ground a Generator
The history of grounding dates back to at least 1746, when experiments proved that the earth and rivers conduct electricity. Early telegraph systems used a single wire for communication, with the earth as the second conductor in the circuit. In 1882, Thomas Edison built the first commercial power station in New York City and used the same technique as the telegraph stations—one wire carried the current to the customer and the return path was through the earth.
That worked fine until laborers and horses started receiving electric shocks.
The National Fire Protection Association (NFPA) started in March 1896 at a conference of insurance organizations. Out of that came a little hand-held book, 183 pages long. It had a section on “Wiring and Apparatus for Electric Light, Heat and Power.” Under rule 46 in that section, any building must test free of grounds including the branch circuits and main line.
In 1911, the NFPA began sponsoring the NEC. They defined the purpose as “The purpose of this Code is the practical safeguarding of persons and of buildings and their contents from hazards arising from the use of electricity for light, heat, power, radio, signaling and for other purposes.”
Today, we know a lot more about electricity than they did back in the 1800s and early 1900s. Simply put, connecting a wire from the metal enclosure of any device to a grounding electrode allows current to travel easily from the metal enclosure into the earth. So if a hot wire inside the appliance comes loose and touches the cabinet, the ground ensures that a person touching the cabinet does not become the path of electricity. That the purpose of a ground.
In 1947, the NEC required at least one grounding receptacle for laundry equipment. The NEC updated its requirements for homes to use grounding receptacles throughout starting in 1962.
Some generators use their metal frame as ground. Others don’t have a ground at all. One thing they all have is a ground terminal, which allows you to connect the generator to a grounding electrode.
When Do You Have To Ground A Generator?
For most electrical systems, the National Electrical Code (NEC) requires a neutral-ground bond (connection) in exactly one location. At this single point in an electrical system, the Neutral wire and Ground wire connect. The Ground attaches to a grounding electrode, very often a rod driven into the Earth, but there are other methods. The neutral-ground bond ensures a zero or near-zero difference in potential between the two conductors. That means if you try to measure voltage between ground and neutral, it will be 0 volts, or almost 0 volts.
This arrangement ensures that any current that escapes the wires and other electrical parts has a path directly into the Earth. It’s not uncommon to refer to ground as Earth, and grounding as Earthing. They mean the same thing. If the hot wire touches the metal case, ground carries the current into the Earth, removing any possibility of a shock. If it’s a short, the circuit breaker will trip. A broken neutral could cause the same hazard, but with the neutral-ground bond, the potential is 0 volts.
That’s why you need a ground in the first place, and why most traditional (not inverter) portable generators ship from the factory with a neutral-ground bond.
If you don’t attach a grounding rod to the generator frame, the frame becomes the ground. That’s fine for a couple of tools or lights, or even your fridge during a power outage.
The most common reason you have to ground your generator is NEC rules for home electrical systems and when you use your generator to power your home, such as during a power outage. It doesn’t apply to the times you plug your tool or appliance directly into the generator.
Backup Power from a Portable Generator
The safest, easiest, most convenient way to use a portable generator for backup power is through a manual transfer switch. The transfer switch handles several important functions and prevents you from making serious and possibly life-threatening mistakes.
- Isolates the generator from the utility.
- Distributes power to the circuits served by the transfer switch.
- Helps balance the electrical load.
- Helps prevent overloads by limiting the circuits powered by the generator.
If the generator has a neutral-ground bond, and your house has a neutral-ground bond, that’s two bonds and they are not equal. By NEC rules, that is not allowed.
A manual transfer switch for a generator with a neutral-ground bond is a 3-pole switch—meaning it transfers 3 poles, or 3 wires—L1, L2, and N (hot, hot, and neutral).
Transferring 3 wires restores the system to a single neutral-ground bond and brings it back into compliance with the NEC, and it makes the system safe by avoiding a large potential difference between the generator neutral and house neutral.
But now, there’s no ground rod leading stray currents into the Earth, and because the generator is powering the house circuits with its own neutral-ground bond, we don’t have the ground required by the NEC for safety.
How To Ground Generator
Ground Rod with Grounding Electrode Conductor
So we must add a ground to the generator. You can drive a ground rod for your generator. They are readily available as an eight-foot copper rod. Any method you use requires some physical effort. But, there’s another way—use the existing ground rod. Attach a 10-gauge* wire to the ground lug on the generator and the other end to the ground rod. Make sure you keep your generator at least 20 feet from any windows or doors. The ground lug may have another wire connected to it. Don’t remove it. It is the bond point for the neutral and ground.
Now you are in compliance with the NEC. Just make sure it complies with local codes who have the final say in how you do it.
Problem solved.
*Note: The 10-gauge ground wire is adequate for generators with a 30-Amp or 50-amp outlet. Large portables may connect to two transfer switches. In this case, the generator requires a larger ground wire.
When adding a grounding electrode after the home is built, the two easiest options are a new ground rod or using existing the water pipes where they come into the house through the concrete slab or basement floor or wall.
How to Ground a Portable Generator for an RV
When you’re not at a campground or RV park with a shore power connection, you might use a portable generator to power your RV. Here again, it’s important to know your generator.
A generator with a bonded-neutral will work just fine. Connect your shore power cable to the TT-30R or L5-30R outlet. The generator frame is the ground, and your RV power works the way you expect it to.
Small to medium portable inverter generators have a floating neutral—it’s not bonded to ground.
The surge protectors and energy management systems found in many RVs require a ground. They work fine with a bonded-neutral generator because generator frame is the ground. A generator with a floating neutral probably won’t work. The solution is simple. Bond the neutral to ground.
Neutral-ground bonding plugs are very inexpensive. Plug one into your generator’s regular household outlet to bond neutral to ground. Internally, the plug’s neutral blade connects directly to the ground blade. Now the generator’s frame is ground, and your RV electrical system will work the way you expect it to.
When Not to Ground a Portable Generator
As mentioned earlier, it’s unnecessary to ground a generator to use it with a tool or appliance.
Some homeowners opt for a generator interlock instead of a manual transfer switch. A double-pole circuit breaker installed in the main circuit breaker panel connects to the power inlet box. A mechanical interlock keeps the generator breaker in the OFF position unless the main breaker is in the OFF position. Both can be OFF, but only one can be ON.
When the main breaker is OFF and the generator breaker is ON, the generator supplies power to the panel. The main circuit breaker panel already has neutral-ground bond, so the generator must not.
Earlier, we mentioned the wire on the ground lug of the generator. Most likely, that wire connects to neutral to make the bond. Check your owner’s manual first. Remove the neutral-ground bond for use with a generator interlock breaker.
If you’re not using the generator power your house, and just want to plug in a tool, reconnect the neutral-ground bond.
- “History Of Grounding/earthing Practices In The United States” by Donald W. Zipse – Electrical Forensics, LLC
- “When were grounded three-slot receptacle outlets first required?” by McGarry and Madsen Inspection
- “Portable Generator Grounding” Norwall Power Expert
About MJ Logan
MJ Logan began his writing career in third grade creating small booklets with a construction paper cover and two or three sheets of white typing paper, carefully folded in half and glued to the construction paper. His career led from electrical engineering and electronic test to network management, independent software development, and finally back to writing.
