Services, grounding and bonding

Grounding and bonding is one of the most heavily examined subjects in the trade and one of the most misunderstood. The vocabulary is precise, the purposes of grounding and bonding are different, and the rules for services, feeders, separately derived systems and special locations differ in important details. Services themselves bring their own rules on the number of services, clearances, disconnecting means and equipment. This lesson reviews service equipment, the purposes of system grounding and equipment bonding, the neutral-to-ground connection, grounding electrodes and their conductors, sizing bonding conductors, bonding of piping and structural metal, separately derived systems and common errors that cause shock and fire hazards.

The service

Article 230 governs services: the conductors and equipment that deliver energy from the serving utility to the premises wiring. In general, a building is supplied by one service, with exceptions for fire pumps, emergency and standby systems, large loads, different characteristics and certain multiple-occupancy buildings. Service conductors must have clearances above grade, roofs and openings: for example, overhead service conductors generally need 3 m (10 ft) above finished grade and sidewalks accessible only to pedestrians, 3.7 m (12 ft) above residential driveways and 5.5 m (18 ft) above public streets and other areas subject to truck traffic, and 900 mm (3 ft) from windows that open. The service disconnecting means must be readily accessible, at a location nearest the point of entrance of the service conductors, and since the 2020 NEC one- and two-family dwellings require an emergency disconnect at a readily accessible outdoor location (230.85).

Grounding versus bonding

Two different ideas sit under this heading, and many errors come from mixing them up.

• System grounding connects one conductor of the supply system, usually the neutral, to earth. It limits the voltage to ground caused by lightning, utility surges and accidental contact with higher-voltage lines, and stabilizes the voltage to ground during normal operation.
• Equipment bonding connects the non-current-carrying metal parts of the installation, such as enclosures, raceways, equipment frames and metal piping, to each other and to the grounded system conductor at the service. It creates a low-impedance fault path, so that if a live conductor touches metal, enough current flows to operate the overcurrent device quickly, and it keeps metal parts at nearly the same potential so people are not exposed to dangerous touch voltages.

The earth itself is a poor conductor. A fault from a live conductor to a ground rod, with no metallic return path, may draw only a few amperes, far too little to trip a breaker, while raising the metal to a dangerous voltage. The metallic fault path through bonding conductors, not the earth, is what clears faults.

The NEC uses "grounding electrode conductor" (GEC) for the conductor to the grounding electrode, "equipment grounding conductor" (EGC) for the conductor that runs with circuit conductors and bonds equipment, and "bonding jumper" for conductors that ensure continuity between metal parts. The "grounded conductor" is the conductor intentionally grounded, usually the neutral. These terms are defined in Article 100 and used throughout Article 250.

The neutral-to-ground connection

At the service, the grounded neutral is connected to the bonding system and grounding electrode through a main or system bonding jumper. Downstream of the service, in sub-panels and equipment, the neutral must be insulated from the enclosure and the bonding conductors. If the neutral and ground are connected at a sub-panel as well, normal neutral current divides between the neutral and the bonding conductors, raceways and metal piping, creating "objectionable current" on metal parts, magnetic fields, interference with electronic equipment and, if the neutral opens, energized enclosures. Exam questions frequently test where the neutral-to-ground bond belongs and the separation required in sub-panels and in detached buildings supplied by feeders.

Grounding electrodes

Section 250.50 requires all grounding electrodes present at a building to be bonded together into a grounding electrode system. Electrodes listed in 250.52 include metal underground water pipe in contact with earth for 3 m (10 ft) or more, which must be supplemented by another electrode; metal building frames; concrete-encased electrodes (Ufer grounds) of at least 6 m (20 ft) of rebar or bare copper in a footing; ground rings; and rod, pipe and plate electrodes. Rods must be at least 2.44 m (8 ft) long; a single rod must be supplemented by a second electrode unless its resistance to earth is 25 Ω or less, and multiple rods must be at least 1.8 m (6 ft) apart. The grounding electrode conductor is sized from Table 250.66 according to the size of the largest service-entrance conductor, but a GEC to a rod need not be larger than 6 AWG copper and to a concrete-encased electrode not larger than 4 AWG copper.

Sizing bonding conductors

Equipment grounding conductors are sized from Table 250.122 according to the rating of the overcurrent device ahead of the circuit: for example 14 AWG copper for 15 A, 12 AWG for 20 A, 10 AWG for 30 to 60 A, 8 AWG for 100 A, 6 AWG for 200 A, 4 AWG for 300 A, 3 AWG for 400 A and 1/0 for 800 A. Where ungrounded conductors are increased in size for voltage drop, 250.122(B) requires the EGC to be increased proportionally according to circular mil area. Metal raceways such as EMT and rigid conduit can serve as the EGC where 250.118 permits.

Bonding piping, structural metal and other systems

Metal piping systems that may become energized, such as water piping, gas piping and sprinkler piping, must be bonded to the grounding system so that a fault to the piping operates protection and does not create a shock hazard. The bonding conductor size and connection point depend on the code and the type of piping. Structural steel that may become energized is also bonded. Communication systems, such as telephone, cable television and antennas, must be bonded to the same grounding system as the power, so that a lightning surge does not create a large voltage between systems inside the building; codes require an accessible means for making this intersystem bonding connection near the service.

Separately derived systems

A separately derived system is a source with no direct electrical connection to the supply conductors of another system, apart from grounding and bonding connections, such as a transformer with separate primary and secondary windings, a generator with a switched neutral, or a solar inverter in some configurations. Because its secondary is isolated, it needs its own system grounding: a system bonding jumper connecting the derived neutral to the bonding system at one point, either at the source or at the first disconnecting means, and a grounding conductor to a grounding electrode. Questions often ask where to put the bonding jumper, what size it must be and which electrode to use for a building transformer supplying a 120/208 V panel.

Ungrounded and impedance-grounded systems

Not every system is solidly grounded. Ungrounded systems, still used in some industrial plants, continue operating after a first ground fault, which can be valuable where unplanned shutdowns are dangerous or costly, but they need ground detection so that the first fault is found and repaired before a second fault on another phase causes a short circuit. High-resistance grounded systems connect the neutral to ground through a resistor that limits ground-fault current to a few amperes, allowing continued operation with an alarm and avoiding the high transient overvoltages of ungrounded systems. These systems have their own code rules for detection, marking and the loads that may be connected, and appear in industrial exam questions.

Common grounding and bonding errors

• Bonding neutral to ground in sub-panels, creating parallel neutral paths.
• Relying on a ground rod as the fault return path.
• Missing or loose bonding jumpers around water meters, flexible connections or concentric knockouts.
• Undersized bonding conductors where circuit conductors were upsized.
• Painted surfaces under bonding connections.
• Using a neutral as an equipment bonding conductor downstream of the service.

Each of these can leave metal parts energized during a fault or allow current on paths not designed for it. Inspectors and exams both focus on them.

Bonding at the service

At the service, bonding receives special attention because the service conductors are protected only by the utility's overcurrent devices, which may not open quickly for a fault on the line side of the main disconnect. Service raceways and enclosures must therefore be bonded by methods that ensure continuity under high fault currents: threaded connections, bonding jumpers around concentric or eccentric knockouts, bonding bushings and locknuts that cut through paint. Service bonding jumpers are sized from the size of the service conductors rather than an overcurrent device rating.

Bonding flexible and isolated sections

Flexible conduit, expansion joints, non-metallic sections of piping and other discontinuities can break the bonding path. Bonding jumpers are installed around them, sized for the circuit, and kept short. Metal water meters and water heaters with dielectric fittings are common places where bonding jumpers are needed to keep metal water piping continuous. Where equipment is mounted on vibration isolators, flexible bonding jumpers maintain the connection. Questions may describe a piping system or a motor on isolation mounts and ask what bonding is required.

Testing grounding and bonding

Grounding and bonding are tested during commissioning and maintenance. Continuity tests with a low-resistance ohmmeter confirm that bonding paths are complete. Ground resistance tests, such as the fall-of-potential method, measure the resistance of electrodes to earth. Ground-fault loop impedance measurements show whether a fault would draw enough current to trip the overcurrent device quickly. Exam questions on testing link this lesson with troubleshooting topics.

Grounding and bonding for generators and inverters

Portable and standby generators raise grounding questions that appear on exams. Whether a generator is a separately derived system depends on whether its neutral is switched by the transfer switch: if the neutral is solidly connected through the transfer switch to the service neutral, the generator is not separately derived and must not have its own neutral-to-ground bond, because that would create a parallel neutral path; if the transfer switch switches the neutral, the generator is separately derived and needs its own system bonding and grounding. Inverters for solar and battery systems have similar considerations, and their manufacturers' instructions state how they must be grounded. Getting this wrong creates either objectionable current or an ungrounded system, both of which inspectors and exam writers look for.