Components and requirements for properly grounding and bonding electrical equipment, plumbing, and appliances in the home.
Electricity flows into the home through the ungrounded wires connected to your local utility. The electricity comes in through the service entrance wires and into the service equipment or electric meter. From there, it flows through feeder wires you’re your panelboard, through circuit breakers or fuses and then into branch circuit wires. The electricity goes to the load, the appliance or electrical device connected to it or plugged in, and returns to its source through the home’s grounded (neutral) wires and the utility’s grounded wire.
Electricity flows without the home’s grounding system because it is not required for normal daily operation of the home’s electrical system.
One reason for a grounding system is to deal with voltage surges that may be caused by lightning, power line problems, or a switching surge. Some of that energy may enter the home’s electrical system and the grounding system provides a safe path to ground. Without it, surges may find an unsafe path by arcing through wire insulation to metal water or gas pipes, or welding the wires in a motor, or electronic device.
Damaged equipment, electrocution and fires all are possible when voltage surges have no safe path to ground. If the voltage surge is large enough, for example from a lightning strike, even the home’s grounding system may not prevent equipment damage and fires due to the size or the wire. The grounding system gets more attention in the home, but bonding is as important for electrical safety.
Bonding is necessary for situations where metal that could carry electricity, even though it is not supposed to, is intentionally connected together to provide a permanent low-resistance return path that is capable of conducting all electricity accidentally carried by the metal back to its source.
In most homes, the electricity’s source is the utility’s transformer and the low-resistance return path is the utility’s grounded wire. That’s why you should find metal water and gas pipe bonding wires and the equipment grounding wires in electrical cable connected to the grounded terminal bar at or before the service equipment. This low-resistance return path is critical to helping overcurrent devices clear ground faults.
A ground fault occurs when electricity flows in metal, or other conductive material, that is not normally energized. Ground faults usually occur when a hot wire comes in contact with conductive material.
Suppose the wire insulation on an electrical cable continuously rubs against the side of a hot water heater until it is worn through. If the pipe is bonded, as it should be, the pipe returns the electricity to its source. The current in the pipe should quickly increase past the circuit breaker’s limit and trips and clears the fault. But what if the pipe were not bonded? Without bonding, the pipe is still energized even when no current flows. This will change when someone touches a metal fixture connected to the pipe. If the person is grounded, which may be standing on a concrete slab, the person becomes the conductive material and the return path. Current flows and injury or death may occur by electrocution.
A low-resistance return path is critical for proper bonding because a high-resistance return path may not allow electricity to flow and trip the circuit breaker. Suppose the water pipe is bonded but the bonding clamp is loose or is clamped to a painted or rusted surface creating 10 ohms of resistance at the clamp. From Ohm’s Law: I = V/R
I (current in amps) = V (voltage in volts) / R (resistance in ohms).
If V =120 volts and R =10 ohms, then 120/10 = 12 amps. This poorly bonded circuit will carry 12 amps. Twelve amps will not trip a 15-amp circuit breaker, but it’s more than enough to kill someone.
There are two reasons why the home’s grounding system has a minimal impact on clearing ground faults.
Electricity wants to return to its source, which, in the case of most home electrical systems, is the utility’s transformer, not necessarily to ground. If all bonded metal is connected by a low-resistance path to the utility’s grounded wire, that’s the lowest resistance return path to the electricity’s source. The home’s grounding system is usually a higher-resistance return path and thus has less impact on returning electricity to its source.
Electricity doesn’t take the path of least resistance back to its source, it takes all available paths. The current carried by each path is determined by the resistance in each path. Assume that the resistance in an effectively bonded water supply pipe is 0.5 ohms and the resistance through a person touching a metal water supply fixture connected to the pipe is 10 ohms. The current in the bonded water supply pipe is 120/0.5=240 amps and the current through the person is 120/10=12 amps. The person will get shocked, but only for a fraction of a second because the current flowing through the bonded path will quickly trip the circuit breaker.
Equipment and Installation
A home’s grounding system consists of grounding electrodes and grounding electrode wire. Common grounding electrodes include: (1) at least 20 feet of #4 rebar or #4 AWG bare copper wire encased in concrete (a “Ufer” ground), (2) a galvanized iron or steel rod or pipe or a copper rod or pipe driven at least eight feet into the ground, or (3) metal water pipe, including metal well casings that come in contact with the ground for at least ten feet. Only one grounding electrode is usually required, however all accessible grounding electrodes around a home must be bonded together using the same size wire as the grounding electrode wire. In newer homes, metal water pipe can’t be the only grounding electrode, but if it is present, it must be bonded to all other grounding electrodes.
The grounding electrode wire is at least #6 AWG copper for 150-amp and greater service. Number 4 AWG copper wire is required for Ufer grounding electrodes with 200-amp and greater service. A number 8 AWG copper wire may be used for 125-amp and smaller services. An aluminum wire may be used, but restrictions make it impractical in most cases. In newer homes, the connection to a water pipe grounding electrode may not occur more than five feet from where the pipe enters the home.
The grounding electrode wire must be connected to the utility’s grounded wire at an accessible location between the load side of the service drop or lateral and the service equipment. No other grounding connections are permitted downstream from this connection except for detached buildings supplied from service at another building. This is because additional grounding connections could create unintentional return paths to the electricity’s source.
If it’s metal and it is or could be near electrical wires, then it probably needs to be bonded to the utility’s grounded wire. Components that may require bonding include metal water and gas distribution pipes, metal electrical conduit, electrical equipment cabinets and cases, metal framing and sheathing, metal HVAC ducts, and all metal parts of the electrical service and distribution systems. Typically satellite and cable TV coax cable must also be bonded. Bonding connections at electrical fittings and boxes must be physically secure and provide a low-resistance electrical connection.
Any non-conductive contaminants such as paint and rust must be removed at the bonding connection point. Bonding jumpers must be installed around interruptions such as plastic boxes, water meters and sometimes components such as water softeners and filters. Bonding wires must usually be the same size as the grounding electrode wire.
Water meters usually have a bonding jumper to cross the meter.
Remember, if you are not sure whether or not your home is properly bonded or grounded, hire a home inspector to look at your system, or contact your local building inspector and discuss your situation.