Grounding vs Bonding: What Article 250 Is Really Doing

Article 250 is one of the longest and most misunderstood parts of the NEC. A big reason is that two separate ideas, grounding and bonding, share the same green wire. Pulling them apart makes the whole article easier to follow.

Bonding: a fast path for fault current

Bonding means connecting all the metal parts that could become energized (boxes, panel enclosures, appliance frames, metal piping) to each other and back to the source through a low-resistance path. If a hot wire touches a metal case, that path lets a large current flow back to the source right away. The large current is what makes the breaker or fuse open quickly, which removes the shock hazard. In code terms, this is the effective ground-fault current path.

Grounding: a connection to the earth

Grounding connects the electrical system to the earth, through a grounding electrode such as ground rods, a concrete-encased electrode (often called a Ufer) or a metal water pipe, with a grounding electrode conductor. Its main jobs are to limit voltage from lightning, line surges or accidental contact with higher-voltage lines, and to stabilize voltage to earth during normal operation.

The earth itself has a lot of resistance. A fault that only reaches a ground rod may not draw enough current to trip a breaker at all, which is why the code says the earth can’t be used as the fault-current path.

The one place neutral and ground meet

At the service equipment, the main bonding jumper connects the neutral (grounded conductor) to the equipment grounding system and the grounding electrode. That single connection is what lets fault current on a metal part find its way back to the utility transformer.

Downstream of the service, for example in a subpanel in a garage, the neutral and ground stay separate: the neutral bar is isolated and the ground bar is bonded to the enclosure, fed by a four-wire feeder. If they were tied together again, normal neutral current would flow on grounds, metal conduit and piping, which can create shock and fire risks and confuse GFCIs. (Separately derived systems such as some transformers and generators have their own bonding rules.)

Common problems we find

• Neutrals and grounds tied together in a subpanel.
• Missing or loose bonding jumpers on water and gas piping.
• Three-prong receptacles installed with no ground wire behind them.
• Corroded ground rod clamps, or a grounding electrode conductor that was cut during other work.

For the homeowner view, see our guide on grounding and bonding basics.