Two words that get used as one
Walk any job site and you'll hear ground and bond traded like synonyms. "Ground the box." "Bond the pipe." "Make sure it's grounded." The words blur together because for years nobody slowed down to separate them, and the work usually passes inspection anyway. But the two ideas do different jobs, and the moment a fault actually happens—a hot conductor touching a metal enclosure—the difference decides whether a breaker trips in a fraction of a second or whether a refrigerator door sits energized at 120 volts waiting for someone's hand.
This is one of those topics where the names actively work against understanding. So set the vocabulary aside for a moment and follow the current.
What a fault current is actually looking for
Imagine a hot conductor inside a metal junction box chafes through its insulation and lands against the steel. The box is now energized. What you want to happen next is simple and violent: a huge surge of current, hundreds or thousands of amps, racing back to its source so fast that the breaker senses the overload and opens the circuit. Clearing the fault means giving that current an easy, low-resistance road home.
Here is the part that surprises people. The road home is not through the earth. The dirt under your feet is a terrible conductor—often tens or hundreds of ohms between two ground rods. Push 120 volts through 25 ohms of soil and you get less than five amps. That won't trip a 20-amp breaker. It won't trip anything. The metal stays hot, quietly, indefinitely.
So the current's real path home is metallic: a continuous run of copper or steel back to the point where the system is bonded to the neutral, and from there back to the transformer. That metallic path is what does the lifesaving. The earth connection does something else entirely, and confusing the two is the root of nearly every grounding-and-bonding mistake.
Bonding: tying the metal together
Bonding is the act of connecting all the metal that isn't supposed to carry current—boxes, enclosures, conduit, equipment frames, the metal water pipe—into one continuous, electrically connected mass. The equipment grounding conductor, the green or bare wire, is a bonding conductor. So is the metal raceway when it's used as the fault path. So is the little screw or jumper that ties the device yoke to the box.
The goal of bonding is twofold. First, it keeps every piece of accessible metal at the same potential, so there's no voltage difference between the dishwasher and the sink for someone touching both. Second, and this is the one that saves lives, it creates that low-impedance metallic highway for fault current. When the hot touches the bonded enclosure, the fault current rips through the bonding path back to the source, the breaker sees a massive overcurrent, and the circuit opens. The metal is dangerous for only milliseconds.
That's why an equipment grounding conductor has to be sized for the job. The NEC sizes it from Table 250.122 against the rating of the overcurrent device protecting the circuit, because it has to survive carrying fault current long enough to trip the breaker. It is not a token wire. It is a fuse-blower.
Grounding: the connection to the earth
Grounding, in the strict sense, is connecting the electrical system to the physical earth—through ground rods, a concrete-encased electrode (the "Ufer"), metal underground water pipe, and the rest of the grounding electrode system described in NEC Article 250, Part III.
If the earth can't clear a fault, why bother? Because grounding the system does jobs the bonding path can't. It stabilizes the voltage of the system relative to earth, so the whole installation sits at a known reference rather than floating. It gives lightning and high-voltage surges—say, a primary line falling onto a secondary—a path to dissipate into the ground instead of through your wiring. It limits the voltage that can build up on the system from those outside events. These are real, important functions. They are just not the same function as tripping a breaker, and they happen on a completely different timescale.
A useful way to hold it: bonding clears faults; grounding references the system to earth. One is about the road home for current that's already loose. The other is about keeping the whole system at a sane voltage relative to the world.
Where the two worlds meet: the main bonding jumper
Now the piece that ties it together, literally. In your service equipment—the first panel where the utility's conductors land—there is a single, deliberate connection between the grounded neutral and the equipment grounding system. It might be a green screw threaded through the neutral bar into the can, or a strap, or a wire. That's the main bonding jumper, and it is one of the most important connections in the whole building.
That jumper is what gives fault current its path back to the source. Remember, the breaker trips because current returns to the transformer. The equipment grounding conductors carry the fault to the service; the main bonding jumper passes it onto the neutral; the neutral carries it back to the utility transformer where the circuit completes. Without that one bond, all your beautifully bonded enclosures would have nowhere to send the fault current. The metal would be tied together and energized—the worst of both worlds.
And this is exactly why neutral and ground are bonded only at the service, and never again downstream. Past that point, the neutral is a current-carrying conductor doing its normal job, and the equipment grounding conductor is supposed to be carrying nothing. If you bond them together again in a subpanel, you've created a parallel path: now normal neutral current splits and travels back partly on the ground wire, partly on the conduit, partly on the water pipe. The grounding system starts carrying current it was never meant to carry, and accessible metal can sit at a few volts above earth all day. That's why the neutral floats on insulated bars in every subpanel, and the equipment grounds land on their own bar bonded to the metal can.
The separately derived system—a transformer, or a generator with switched neutral—follows the same logic with its own main bonding jumper, established once at the source. Same rule, new origin point.
How to keep them straight on the job
When you're standing at a panel trying to remember what connects where, ask two separate questions. How does fault current get home?—that's bonding, that's the green wire and the jumper, that's what trips the breaker. How is this system referenced to earth?—that's grounding, that's the rods and the Ufer and the electrode conductor.
Keep asking them separately and the confusing cases resolve themselves. A detached structure with its own grounding electrode still needs an equipment grounding conductor run back with the feeder—because the rod at the outbuilding can't clear a fault, only the metallic path back to the service can. A metal water pipe gets bonded so it can't become an energized surprise, separately from its old role as a grounding electrode. Each piece of metal, each rod, each jumper is answering one of those two questions, never both.
Get the vocabulary straight and the code stops feeling arbitrary. Article 250 isn't a pile of rules to memorize; it's the story of two different jobs—one fast and lethal, one slow and stabilizing—sharing a single careful connection at the service.
Where this lives on the truck
This is the kind of distinction that's easy to nod along to and hard to apply at 4 p.m. with an inspector waiting—sizing that equipment grounding conductor off Table 250.122, confirming the bonding jumper, checking that the subpanel neutral really is isolated. Voltly keeps those tables and calculations in your pocket, offline, so the answer is the code's answer and not a guess: ampacity, box fill, conduit fill, voltage drop, and the grounding and bonding references you reach for when the theory has to become a wire size. If you want the reasoning and the number in the same place, give it a look.