Grounding and Bonding

Grounding and bonding are the parts of an electrical system you never see working until something goes wrong. When a hot conductor touches a metal case, these connections decide whether a breaker trips in a fraction of a second, or the case stays live until someone touches it. They are also the most misunderstood topic in the trade, because people use the word "ground" for several different things. This guide separates them.

What You Will Be Able to Do

• Explain the different jobs of grounding and bonding.
• Name the neutral, equipment grounding conductor, grounding electrode conductor and bonding jumper, and say what each one does.
• Explain why the earth cannot be relied on to trip a breaker.
• State why the neutral and ground connect at the service and nowhere after it.
• Identify the electrodes a building must use, and how they are connected.

Article 250 of the National Electrical Code (NEC) covers this subject. Code numbers below match the recent editions, but your state or city may enforce a different edition, with local changes. The local rule wins.

Two Words, Two Jobs

Grounding connects the electrical system to the earth. It limits voltage from lightning, utility surges and accidental contact with higher voltage lines, and it keeps the system steady relative to earth.

Bonding connects metal parts together so they stay at the same voltage and so that fault current has an easy, low-resistance path back to the source.

Bonding is what clears faults and protects people from shock in daily life. Grounding handles events from outside the building. You need both.

The Parts and Their Names

The grounded conductor and the grounding conductor sound alike and do different jobs. The neutral carries load current. The EGC should carry nothing, unless something has failed.

Why the Earth Will Not Trip a Breaker

A common belief is that electricity "wants to go to ground." It does not. Current wants to return to its source. Earth has far too much resistance to carry fault current back in any useful amount.

Worked Example: A Fault to a Ground Rod

A 120-volt hot conductor touches a metal pole that connects only to a ground rod. The rod measures 25 ohms to earth.

1. Current: I = V / R = 120 / 25 = 4.8 amps.
2. A 20-amp breaker does not trip on 4.8 amps.
3. The pole stays energized, and anyone touching it may be shocked.

Worked Example: A Fault With a Proper EGC

Now the same pole has a proper equipment grounding conductor back to the panel, and the whole fault path measures 0.2 ohm.

1. Current: I = 120 / 0.2 = 600 amps.
2. The 20-amp breaker trips almost instantly.

That second path is called the effective ground-fault current path. It must be permanent, continuous and low in impedance. A ground rod can never replace it. The NEC says the earth must not be used as the only equipment grounding path.

The Main Bonding Jumper

At the service disconnect, and only there, the neutral is connected to the enclosure and to the EGCs. The main bonding jumper makes that connection. It is usually a green screw or a strap supplied with the panel.

Here is what happens during a fault:

1. A hot wire touches a grounded metal case.
2. Fault current flows on the EGC back to the panel.
3. It crosses the main bonding jumper to the neutral bus.
4. It returns on the neutral to the transformer.
5. The huge current trips the breaker.

Without the jumper, step 3 cannot happen. Current has no good way back, the breaker may not trip, and every grounded metal part in the building can become live.

A transformer inside a building creates a new system, called a separately derived system. It needs its own system bonding jumper and grounding connection, following the same logic.

Why Neutral and Ground Stay Apart After the Service

Past the service disconnect, the neutral and the EGC must stay separate. The NEC says so in 250.24(A)(5) and 250.142.

If they are connected in a subpanel, the neutral and the grounding path are now in parallel. Normal return current splits between them, and it flows on EGCs, conduit and metal cases. Metal that people touch now carries current. If the neutral opens, that metal can rise to line voltage.

In every subpanel:

1. Remove the bonding screw or strap that came with the panel.
2. Land all neutrals on an isolated neutral bar, so it is floating from the enclosure.
3. Land all EGCs on a ground bar that is bonded to the enclosure.

Detached Buildings

A subpanel in a detached garage or shop follows the same rule and adds one more thing. Under 250.32:

• The feeder is 4-wire: two hots, a neutral and a separate EGC.
• The building needs its own grounding electrode system, connected to the ground bar, not the neutral bar.

The electrode at the detached building adds to the EGC. It does not replace it. Older 3-wire feeders, with the neutral bonded at the outbuilding, were permitted in earlier codes. They can remain as existing work in some cases, but new work needs four wires.

The Grounding Electrode System

NEC 250.50 says every electrode that is present at a building must be bonded together into one system. Section 250.52 lists what counts:

Plastic water service is not an electrode. Neither are gas pipes, which must never be used as electrodes.

Concrete-encased electrodes, often called Ufer grounds, must be used in new construction where the footing has suitable rebar. Because they cannot be reached after the pour, coordinate with the concrete crew before the footing goes in.

Ground rods. A single rod must be supplemented by a second electrode, unless it measures 25 ohms or less to earth. Few people test, so two rods are the normal practice. Keep them at least 6 feet apart, drive them to full depth with the top at or below grade unless protected, and connect them with the bonding jumper.

Water pipes. A metal water pipe electrode must also be supplemented, usually with rods. The connection to the pipe is made within the first 5 feet of where the pipe enters the building, because anything further in could be replaced with plastic and cut off the path. The 5-foot rule is in 250.68(C).

The Grounding Electrode Conductor

The GEC connects the service to the electrode system. Its size comes from Table 250.66, based on the size of the service conductors, with two useful limits:

• A GEC run only to rods need not be larger than 6 AWG copper.
• A GEC run only to a concrete-encased electrode need not be larger than 4 AWG copper.

A 200-amp service on 2/0 copper or 4/0 aluminum conductors typically needs 4 AWG copper to a water pipe, and 6 AWG copper is enough for a rod.

Section 250.64(C) requires the GEC to be one continuous length with no splices. The exceptions are specific: irreversible compression connectors, exothermic welds and certain busbars.

Warning: If a GEC runs through ferrous metal conduit, that conduit must be bonded to the conductor at both ends. An unbonded steel raceway acts like a choke coil, which adds impedance and can block fast fault or lightning current.

Equipment Grounding Conductors

An EGC runs with every circuit. Section 250.118 lists what qualifies:

• A copper or aluminum wire, bare or green.
• Rigid metal conduit, intermediate metal conduit and EMT, when every fitting is tight.
• The armor of Type AC cable and some Type MC cable.
• Flexible metal conduit, only within limits on length and size.

Metal conduit works as an EGC because tight, continuous joints give a low-impedance path. In PVC conduit, you must pull a separate wire.

EGC size comes from Table 250.122, based on the overcurrent device ahead of the circuit, not on the load:

If you increase the size of the hot conductors, such as for voltage drop, the EGC goes up in proportion.

Bonding Other Metal Systems

Section 250.104 requires metal water piping to be bonded, and metal gas piping and structural steel that could become energized. Corrugated stainless steel gas tubing may have its own manufacturer bonding instructions. These bonds keep metal at the same voltage, so a fault cannot put a dangerous difference between a faucet and an appliance.

Communications systems such as cable, phone and data connect to the same grounding system through the intersystem bonding termination near the service.

Common Mistakes

Check Your Understanding

1. A pump in a yard is connected to a local ground rod but has no EGC back to the panel. Its case becomes energized at 240 volts. Will the breaker trip?

Probably not. The rod's resistance limits the current to a level that may not trip the breaker, so the case can stay live. It needs an EGC back to the source.

2. An inspector finds a green bonding screw installed in a panel fed from the main panel in the same building. Why is that a failure?

It connects the neutral to the enclosure past the service, so normal current can flow on grounds and metal. Remove the screw and separate the neutrals and grounds.

3. A new house has rebar in its footing, a plastic water line and no ground rods yet. What electrodes must be used?

The concrete-encased electrode is required if the rebar qualifies. The plastic pipe is not an electrode. The jurisdiction may also expect rods.

4. A 100-amp feeder is run in PVC. What must be pulled with it?

A separate EGC, at least 8 AWG copper according to Table 250.122, because PVC cannot serve as a fault path.

Key Takeaways

• Grounding ties the system to earth. Bonding ties metal together and provides the fault path.
• Fault current returns to the source. It trips breakers through bonding, not through the earth.
• The neutral and ground are joined at the service and separated everywhere after it.
• All electrodes present must be bonded into one system.
• EGCs are sized by the overcurrent device, and GECs by the service conductors.
• Tight connections are part of the safety system. A loose fitting can break the fault path.