Electrical Grounding vs. Bonding: What Is the Difference?

The words grounding and bonding are often used as if they mean the same thing. They are two related systems with different jobs. Both are required by the National Electrical Code (NEC), and both contribute to protecting people and equipment from shock and fire.

This guide defines each term, explains how they are installed in a house, and describes what to look for in older Twin Cities homes. A licensed electrician should do the actual work, and work on a grounding electrode system or bonding connection at the panel is part of grounding and bonding services.

What is the difference between grounding and bonding?

Grounding connects the electrical system to the earth through a grounding electrode, such as a ground rod or a metal underground water pipe. This connection limits the voltage on the system from lightning, line surges, or contact with higher-voltage lines. It also stabilizes the system voltage during normal operation.

Bonding joins metal parts together so they are electrically connected. Bonding the metal enclosures, conduit, and equipment frames to the panel ensures that a fault, such as a hot wire touching a metal box, has a low-resistance path back to the source. That path allows a large current to flow so the circuit breaker trips quickly.

A common misunderstanding is that the earth carries fault current back to the panel and trips the breaker. Soil is a poor conductor, so it does not carry enough current to open a breaker. The bonded metal path back to the panel does that job.

What is a grounding electrode system?

The grounding electrode system is the set of connections between the service panel and the earth. A grounding electrode conductor, a wire usually run from the main service equipment, connects to the electrodes. Common electrodes are:

• Ground rods, which are metal rods driven into the soil outside the building. Where a single rod does not meet the required resistance, the NEC calls for a second rod.
• A metal underground water pipe, where the pipe is in direct contact with the earth for the length the NEC specifies. The NEC requires a supplemental electrode when a water pipe is used.
• A concrete-encased electrode, which is a steel reinforcing bar or conductor embedded in the foundation, when it is available in new construction.

Many houses built before the 1990s rely on the water pipe alone. If the water service has been replaced with plastic pipe, as is now common, the pipe no longer works as an electrode. A house in that situation may need ground rods installed.

What must be bonded in a house?

The NEC requires bonding of metal systems that could become energized or that could carry current in a fault. In a typical house, this includes the metal water piping system, the metal enclosure of the service equipment, and metal gas piping.

Water piping

Interior metal water piping is bonded to the service equipment or grounding electrode system with a bonding jumper. This keeps pipes at the same voltage as other grounded metal if an energized wire touches a pipe. A clamp on the pipe near where it enters the house is a common sign of this connection.

Gas piping

Metal gas piping is bonded as part of the grounding path of the circuit that may energize it, such as the circuit for a gas furnace or water heater. In some cases a local utility or gas code adds requirements. Do not move, remove, or cut a bond near a gas line; call a licensed electrician or plumber.

Service equipment and panels

At the main panel, the neutral bar is connected to the equipment grounding bar through a main bonding jumper or screw. This is the one place in the system where neutral and ground are connected. In a subpanel, the neutral and ground are kept separate, and combining them there creates a safety problem.

Why do grounding and bonding both matter?

Without proper bonding, a fault may leave metal parts energized without tripping the breaker. A person who touches that part and another grounded object, such as a water faucet, can receive a shock. With bonding in place, the fault current is large enough that the breaker opens quickly.

Without proper grounding, surges from lightning or the utility system have no controlled path to the earth. Surge protection devices rely on that connection to work as intended. A service upgrade is a common time to install a new grounding electrode system.

What are signs of grounding and bonding problems in older homes?

• Two-prong outlets, or three-prong outlets with no ground wire behind them.
• A water pipe clamp that is loose, corroded, or missing, or a ground wire that is cut or disconnected at the panel.
• No ground rod outside, in a house that has had its metal water service replaced with plastic.
• A mild tingle when touching a metal appliance, faucet, or a metal panel cover. Stop and call an electrician if this occurs.
• Flickering lights or frequent surge damage to electronics, which can have several causes and are worth having checked.
• A neutral and ground connected together in a subpanel.

Cold weather affects grounding in Minnesota. Frozen ground is a poor conductor, and ground rods need to be driven deep enough to reach soil that stays in contact through the seasons. An electrician can test and install the electrodes to meet the code.

When does service work bring grounding up to code?

When a panel is replaced or a service is upgraded, the installation is inspected under the current adopted code. The inspector checks the main bonding connection, the grounding electrode conductor, the electrodes, and the bonding of water and gas piping. If the existing grounding electrode system does not meet the code, the electrician installs the missing parts as part of the project.

Electrical inspections and corrections can also find problems without a service change. A grounding and bonding check typically examines the connection at the panel, the electrode system, and the pipe bonds, and reports any missing or damaged parts.

Next steps if you suspect a grounding or bonding problem

Look for the signs listed above, and note any two-prong outlets or panel issues from a home inspection report. Prime Time Electric MN, LLC (Minnesota license #EA807704) provides grounding and bonding, panel upgrades and replacements, and electrical service upgrades for homes throughout the Twin Cities metro. To describe what you see, request a quote or call 612-445-2794.