Ask a group of electricians what grounding and bonding mean, and you will get several different answers. Yet in the National Electrical Code (NEC), Article 250, they are two different jobs. Grounding connects the electrical system to the earth. Bonding connects metal parts together so fault current can get back to its source.
Grounding: Connecting to the Earth
Grounding is the intentional connection of an electrical system to the earth. In a typical US home, the neutral conductor from the utility transformer is connected to the earth at the service through the grounding electrode system. The electrodes can be a metal underground water pipe, a ground rod driven into the soil, the metal frame of the building, or a concrete-encased electrode, often called a Ufer ground. The wire that makes this connection is the grounding electrode conductor (GEC). Grounding stabilises the system voltage and gives lightning and surges a path into the earth. It is a connection to the planet, not a path for normal current.
A grounding electrode conductor clamped to a metal water pipe, one of the grounding electrodes.
Bonding: Connecting Metal Parts Together
Bonding is the permanent joining of normally non-current-carrying metal parts, such as panel enclosures, conduit, junction boxes and appliance frames, so they form one continuous, low-impedance path. The wire that does this job is the equipment grounding conductor (EGC), the bare or green wire in your cables. When a hot wire accidentally touches a metal case, the fault current must have a fast, low-resistance road back to the source, so the breaker trips in a fraction of a second. Bonding also keeps all metal parts at the same voltage, so you cannot get a shock by touching two of them at once. The NEC calls this the effective ground-fault current path, in Article 250.4(A)(4).
Diagram: in US split-phase service, the centre tap of the transformer secondary is grounded (GND), giving 120 volts from each hot wire to neutral and 240 volts between the two hot wires.
The Key Point: Earth Is Not the Fault Path
Here is the fact that surprises most students. The NEC states that the earth itself is not an effective ground-fault current path, in Article 250.4(A)(5). Soil has far too much resistance: if a fault had to travel through dirt, the breaker would never trip and the metal case would stay live at 120 volts. The fault current must return to the transformer through metal: the EGC, the panel, and the main bonding jumper. The ground rod is for lightning and voltage stability, not for clearing faults.
The Main Bonding Jumper and the One-Bond Rule
Grounding and bonding meet at one place in a home: the service equipment. The main bonding jumper (MBJ), sized under NEC 250.28, connects the neutral bus to the equipment grounding bus at the main panel, completing the fault-current path back to the transformer. But it must happen only once. NEC 250.24(A)(5) forbids grounding the neutral again on the load side of the service disconnect, so a subpanel must have separate neutral and ground bars. Bonding the neutral again in a subpanel makes metal cases carry current all the time: a shock hazard and a common inspection failure.
Grounding vs Bonding: A Simple Comparison
- Grounding connects the system to the earth, through electrodes and the GEC, for voltage stability and a path for lightning and surges.
- Bonding connects metal parts to each other, through the EGC, giving a low-impedance fault-current path that trips the breaker fast.
- Grounding uses the grounding electrode conductor (GEC) and electrodes such as a ground rod, water pipe or Ufer ground.
- Bonding uses the equipment grounding conductor (EGC), the bare or green wire, and the main bonding jumper at the service.
- Without bonding, a ground fault may not trip the breaker. Without grounding, surges have nowhere to go.
Key Takeaways
- Grounding = system to earth. Bonding = metal parts to each other and back to the source.
- The earth is not an effective ground-fault current path (NEC 250.4(A)(5)); the bonded metal path clears faults.
- The main bonding jumper connects neutral to ground at the service only, never again in a subpanel.
Objective Questions
1. In the NEC, which article covers grounding and bonding?
a) Article 90
b) Article 210
c) Article 250
d) Article 430
Answer: (c) Article 250
Explanation: In the National Electrical Code (NEC), Article 250 covers grounding and bonding.
2. What is grounding?
a) Connecting metal parts to each other
b) The intentional connection of an electrical system to the earth
c) Connecting the hot wire to the neutral wire
d) Driving a rod to trip the breaker
Answer: (b) The intentional connection of an electrical system to the earth
Explanation: Grounding connects the electrical system to the earth, which stabilises the system voltage and gives lightning and surges a path into the earth.
3. Which wire bonds the metal parts of an installation together?
a) The grounding electrode conductor (GEC)
b) The equipment grounding conductor (EGC), the bare or green wire in the cables
c) The neutral conductor
d) The service conductor
Answer: (b) The equipment grounding conductor (EGC), the bare or green wire in the cables
Explanation: The equipment grounding conductor (EGC), the bare or green wire, bonds metal parts together and gives fault current a fast, low-resistance road back to the source.
4. Is the earth itself an effective ground-fault current path under the NEC?
a) Yes, the ground rod trips the breaker
b) No, NEC 250.4(A)(5) says the earth is not an effective ground-fault current path
c) Yes, but only in wet soil
d) No, because the earth is not connected
Answer: (b) No, NEC 250.4(A)(5) says the earth is not an effective ground-fault current path
Explanation: Soil has far too much resistance: if a fault had to travel through dirt, the breaker would never trip and the metal case would stay live at 120 volts.
5. Where must the neutral be bonded to ground in a home?
a) At every outlet
b) Only once, at the service equipment
c) In every subpanel
d) At the meter only
Answer: (b) Only once, at the service equipment
Explanation: Grounding and bonding meet at one place in a home, the service equipment, and the main bonding jumper must be installed only once.
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