Understanding Grounding and Bonding Basics

Article 250 was entitled Grounding through the 2002 edition of the NEC®.  It was renamed Grounding and Bonding in 2005 to help users understand that grounding and bonding are separate — but closely related — electrical safety concepts.

Purposes of Grounding

Grounding is the intentional connection of a current-carrying conductor to the earth.  For ac premises wiring systems in buildings and similar structures, this ground connection is made on the line side of the service equipment [250.24(A)(1)] and the supply source, such as a utility transformer [250.24(A)(2)]. There are three basic reasons for grounding.

1. To limit the voltage surges caused by lightning, utility system operations, or accidental contact with higher-voltage lines [250.4(A)(1)]
2. To provide a ground reference that stabilizes the voltage under normal operating conditions [250.4(A)(1)] 
3. To facilitate the operation of overcurrent devices such as circuit breakers, fuses, and relays under ground-fault conditions [250.4(A)(5)]

Purposes of Bonding

Bonding is the permanent joining together of metal parts that aren’t intended to carry current during normal operation, which creates an electrically conductive path that can safely carry current under ground-fault conditions.  There are two basic reasons for bonding.

1.   To establish an effective path for fault current that facilitates the operation of overcurrent protective devices [250.4(A)(3) and (4)]
2. To minimize shock hazard to people by providing a low-impedance path to ground.  Bonding limits the touch voltage when non–current-carrying metal parts are inadvertently energized by a ground fault.

Grounded Conductors versus Grounding Conductors

Some Code users are confused by these two different types of conductors with similar names.

• Grounded conductor — A system or circuit conductor that is intentionally grounded.  Neutrals are one type of grounded conductor.  Grounded conductors are identified by either a white or gray outer finish [200.6(E) FPN].
• Grounding conductor — A conductor used to connect equipment or the grounded circuit of a wiring system to a grounding electrode or electrodes. “Grounding conductors” is a general category that includes both grounding electrode conductors and equipment grounding conductors (see definitions at the end of this article). Grounding conductors can be bare, covered, or insulated.

The grounded conductor system of a building or structure is connected to the grounding conductor system in one place only, at the service [250.24(A)].  They are connected together using a main bonding jumper [250.25(A)(5), 250.28].

Ground Faults versus Short Circuits

Section 250.2 defines ground fault as an unintentional connection between an ungrounded (phase) conductor and items such as metal enclosures, metal raceways, or metal equipment frames.  Short circuit isn’t defined in the Code, but is an unintentional connection between two conductors, either phase-to-phase or phase-to-neutral.

Short circuits are less common, since two insulation failures typically must take place for the unintentional connection to occur.  The overcurrent device protecting the circuit usually opens quickly when a short circuit happens.

Ground faults are more common, because they require only a single insulation failure. They can be much more destructive than short circuits, especially when grounding and bonding have not been properly performed.  A poor ground-fault current return path may result in a high-impedance arcing fault that lasts a considerable length of time without causing a circuit breaker to trip or a fuse to blow.  This problem is one reason why arc-fault circuit interrupters (AFCIs) were invented (see 210.12).

Effective Ground-Fault Path

The Code contains definitions of both ground-fault current path and effective ground-fault current path.  The two concepts are different.

• A ground-fault current path consists of any conductive materials over which fault current can flow.  This may include water or gas piping, air ducts, communications wiring, metal structural members, fencing, rain gutters, and the like..
• An effective ground-fault current path is an “intentionally constructed, permanent, low-impedance . . . path designed and intended to carry ground-fault current and facilitate the operation of the overcurrent protective device.”

In other words, a ground-fault current path can be accidental, but an effective ground-fault current path is always an intentional and critical element of an electrical grounding and bonding system.  Article 250 contains many rules to ensure the integrity and proper functioning of an effective ground-fault current path.

Importance of an Effective Ground Fault Path

Section 250.4(A)(5) requires that an effective ground-fault current path meet three conditions.  It must be

• Permanent
• Low-impedance
• Capable of safely carrying the maximum ground-fault current likely to be imposed on it from any point on the wiring system

The last sentence of 250.4(A)(5) states a very important NEC rule: The earth shall not be considered as an effective ground-fault current path.  The resistance of earth is so high that very little fault current returns to the electrical supply source through the earth.  For this reason, equipment grounding conductors must be run with circuit conductors.

Types of Equipment Grounding Conductors

Section 250.118 lists 14 types of equipment grounding conductors that are run with or enclose the conductors of power supply circuits.  These include wire-type conductors, busbars, metal raceways, the armor or sheath of certain metal cables, and cable trays.  Cable assemblies and flexible cords also include equipment grounding conductors [250.138].

Key Definitions from Article 100 and 250.2

Bonded (Bonding).  Connected to establish electrical continuity and conductivity. [Article 100]

Bonding Jumper, Main. The connection between the grounded circuit conductor and the equipment grounding conductor at the service. [Article 100]

Effective Ground-Fault Current Path. An intentionally constructed, permanent, low-impedance electrically conductive path designed and intended to carry current under ground-fault conditions from the point of a ground fault on a wiring system to the electrical supply source and that facilitates the operation of the overcurrent protective device or ground-fault detectors on high-impedance grounded systems. [250.2]

Ground.  The earth. [Article 100]

Grounded (Grounding). Connected (connecting) to ground or to a conductive body that extends the ground connection. [Article 100]

Grounded Conductor.  A system or circuit conductor that is intentionally grounded. [Article 100]

Grounding Conductor. A conductor used to connect equipment or the grounded circuit of a wiring system to a grounding electrode or electrodes. [Article 100]

Grounding Conductor, Equipment (EGC). The conductive path installed to connect the normally non−–current-carrying metal parts of equipment together and to the system grounded conductor or to the grounding electrode conductor, or both. [Article 100]

Grounding Electrode Conductor. A conductor used to connect the system grounded conductor the equipment to the grounding electrode or to a point on the grounding electrode system. [Article 100]

Ground Fault.  An unintentional, electrically conducting connection between an ungrounded conductor of an electrical circuit and the normally non–current-carrying conductors, metallic enclosures, metallic raceways, metallic equipment, or earth. [250.2]

Ground-Fault Current Path. An electrically conductive path from the point of a ground-fault on a wiring system through normally non-current-carrying conductors, equipment, or the earth to the electrical supply source. [250.2]