Grounding and bonding

Grounding and bonding are the parts of the code most often misunderstood, by homeowners, by apprentices and sometimes by experienced electricians. They are also among the most important for safety, because they decide what happens when something goes wrong: when a hot conductor touches a metal box, when lightning strikes a line, or when a transformer fails. A properly grounded and bonded system makes a fault trip the breaker in a fraction of a second. A poorly bonded one can leave a metal appliance or water pipe energized at a lethal voltage, silently, until someone touches it.

In the CEC, grounding and bonding are covered in Section 10, which was reorganized in recent editions to separate system grounding, grounding electrodes, equipment bonding and special cases more clearly. Bonding conductor sizes are in Table 16. This chapter explains the principles and the main rules; always confirm the details in the edition your province has adopted.

Two different jobs

Grounding and bonding are related but different, and keeping them separate in your mind solves most confusion:

• Grounding connects the electrical system to the earth. Its purposes are to limit the voltage imposed by lightning, line surges and accidental contact with higher-voltage lines, and to stabilize the system voltage relative to earth during normal operation.
• Bonding connects all the non-current-carrying metal parts (enclosures, raceways, equipment frames, metal piping) to each other and back to the source, through a low-impedance path. Its purpose is to carry fault current back to the source quickly so that the overcurrent device trips, and to keep all metal parts at the same potential so that a person touching two of them is not shocked.

The single most important fact in this chapter: the earth is not a good path for fault current. Clearing a fault depends on bonding, a conductor path back to the source, not on the ground rod.

System grounding at the service

In a typical North American system, the utility transformer's secondary has a centre point (neutral) that is grounded at the transformer. At the building's service, the neutral is grounded again and connected to the equipment bonding system at one point. This connection is what lets fault current return to the source: a fault from a hot conductor to a metal enclosure flows through the bonding path to the service, across the neutral-to-ground connection, and back to the transformer on the neutral, a low-impedance loop that trips the breaker.

At the service, the grounded (identified) conductor is connected to the service equipment enclosure through a main bonding connection, and to the grounding electrode through the grounding conductor. Downstream of the service, the neutral and the bonding conductors must be kept separate: in subpanels, the neutral bar is isolated from the enclosure, and the bonding conductor is connected to the enclosure.

Why only one neutral-to-ground connection?

If the neutral were bonded to ground again at a subpanel, normal neutral current would divide: some would flow on the neutral and some on the bonding conductors, metal raceways, cable armour and even water pipes. Metal parts would then carry current all the time, creating shock risk, electrical noise, overheating of small bonding paths, and false trips of ground fault protection. This is why the code insists on a single bond at the service (or at each separately derived system), and why a four-wire feeder (two hots, neutral and bonding conductor) is required to subpanels.

Grounding electrodes

The grounding electrode is the physical connection to the earth. A building usually uses several electrodes bonded together into a grounding electrode system.

The CEC recognizes in-situ electrodes, which are already part of the building, such as metal water piping in contact with the earth (with conditions) and a concrete-encased electrode formed by reinforcing steel or a bare copper conductor in the footing, and manufactured electrodes, such as ground rods and plates, which must meet minimum dimensions and, in the case of rods, are typically installed as more than one rod with a minimum spacing between them. The grounding conductor that connects the service to the electrode is sized according to the size of the service conductors. Check the electrode and grounding conductor rules of Section 10 for the exact requirements in your edition.

The concrete-encased electrode, often called a Ufer ground after the engineer who developed it during the Second World War, is usually the best electrode available, because concrete in contact with the earth holds moisture and presents a large contact area. Designers and electricians should arrange to connect to the footing reinforcement before the concrete is poured; afterwards the opportunity is lost.

Grounding electrodes have a resistance to earth of a few ohms at best, and often 10 to 100 Ω. That is fine for their job (limiting voltages from lightning and surges, and stabilizing the system) but far too high to carry fault current that will trip a breaker, as the next example shows.

The effective fault current path

Bonding conductors are sized from Table 16 according to the rating of the overcurrent device protecting the circuit. A metal raceway or armoured cable can serve as the bonding path where the code allows, provided every joint and connector is tight; in non-metallic raceways and cables, a separate bonding conductor is installed with the circuit conductors.

Bonding other metal systems

Any metal system that could become energized must be bonded so that it cannot rise to a dangerous voltage. This includes metal water piping, gas piping (bonded, but never used as an electrode), structural steel, metal ducts in some cases, and, in pools and spas, the entire area around the water (an equipotential bonding grid covered in later courses). At the service, extra care is taken because the fault currents are highest and the service conductors have no overcurrent protection on the line side: service raceways are bonded with bonding bushings or jumpers, not relying on locknuts alone.

The 2024 CEC added an option to bond gas piping through the circuit of the gas appliance (Rule 10-702), reflecting how gas piping is typically connected to appliances that also have electrical supplies.

Separately derived systems

A transformer, generator or other source whose output conductors have no direct electrical connection to the supply conductors creates a new system. That system needs its own neutral-to-ground connection (a system bonding jumper) and its own connection to a grounding electrode, at the source or at its first disconnecting means. A dry-type transformer in a commercial building that steps 600 V or 480 V down to 120/208 V is the most common example. Treating its secondary like a subpanel, with no neutral-to-ground bond, leaves the secondary ungrounded and faults on it would not trip the breakers.

GFCI, AFCI and isolated grounds

Ground fault circuit interrupters (GFCIs) protect people, not equipment. A GFCI compares the current on the hot and neutral conductors; if they differ by about 5 mA (Class A devices trip at 4 to 6 mA), current is leaking somewhere, possibly through a person, and the device opens the circuit within a fraction of a second. A GFCI works even without a bonding conductor, which is why it is accepted as a way to replace old two-wire receptacles in some cases, but it does not replace bonding for clearing faults to metal. Arc fault circuit interrupters (AFCIs) detect the electrical signature of arcing in damaged cords and wiring and are required for many dwelling circuits; ELEC-130 covers where each is required.

An isolated ground receptacle (marked with an orange triangle) has its grounding terminal insulated from its mounting strap, so that a separate insulated grounding conductor can be run back to the panel for sensitive electronic equipment, reducing electrical noise. The metal box must still be bonded in the normal way.

Lesson 12 applies these rules to a complete installation, including the grounding electrode and isolated neutral at a detached building.