Article 250 Grounding and Bonding

I. General

Generally gives requirements for grounding and bonding.

The following other articles have special grounding requirements:

(A) Grounded Systems

(1) Electrical System Grounding. If the system is to be grounded, it has to be connected to earth so that it will limit voltages due to lightning, unintentional contact with higher voltages, and line surges. It must also stabilize line voltages during normal operation.

(2) Grounding of Electrical Equipment. Any non-current-carrying conductive material enclosing electrical conductors or equipment, or part of a piece of equipment must be connected to earth.

(3) Bonding of Electrical Equipment. All non-current carrying conductive material which encloses conductors or equipment must be connected together and to the electrical supply source.

(4) Bonding of Electrically Conductive Materials and Other Equipment. Any electrically conductive material that may become energized has to be bonded together and to the supply system at the source.

(5) Effective Ground Fault Current Path. The fault current path has to carry the maximum fault current, have low impedance and be continuous and permanent in nature.

The earth is not to be considered an effective ground-fault current path.

(B) Ungrounded Systems

(1) Grounding of Electrical Equipment. Non-current carrying conductive materials that enclose conductors or equipment must be connected to earth so that it will limit voltages due to lightning, unintentional contact and will limit the voltage to ground.

(2) Bonding of Electrical Equipment. All materials that enclose conductors or equipment must be connected together and to the supply system grounded equipment so that the fault current path carries maximum fault current, has low impedance and is continuous and permanent in nature. It must be capable of carrying the maximum fault current that can occur.

(3) Bonding of Electrically Conductive Materials and Other Equipment. Any electrically conductive material that can become energized has to be bonded together and to the supply system grounding equipment so that the fault current path carries maximum fault current, has low impedance, and is continuous and permanent in nature. It must be capable of carrying the maximum fault current that can occur.

(4) Path for Fault Current. The fault current path has to carry the maximum fault current path and have low impedance from any place on the system so that the overcurrent devices will operate on a second fault. The earth is not to be considered an effective fault current path.

There are two fine print notes.

The entire electrical system is to be installed to prevent an objectionable flow of current over the grounding system. There is a discussion of alterations to stop objectionable current, limitations to permissible alterations, isolation of objectionable direct- current ground currents, and the fact that temporary currents are not classified as objectionable currents.

II. System Grounding

The following alternating-current systems must be grounded. Others may be grounded if desired.

(A) Alternating-current systems of less than 50 V must be grounded if supplied by transformer systems of more than 150 V to ground by ungrounded transformer systems and where the conductors are outdoors and overhead.

(B) Alternating-current circuits between 50 and 1000 V of the following types must be grounded:

1. Systems of up to 150 V to ground from an ungrounded conductor
2. Systems that are three-phase, four-wire with a neutral circuit conductor
3. Three-phase, four-wire delta systems with the midpoint of one phase used as a circuit conductor

(C) Alternating-Current Systems 1 kV and Over

(D) Separately Derived Systems. If the source is separately derived, and covered in (A) or (B), it must be grounded as required by Section 250.30.

(E) Impedance Grounded Neutral Systems

There are a number of systems that are permitted to be grounded but are not required to be grounded. They are: electric systems used to supply industrial electric furnaces for melting, etc.; separately derived systems used only to supply rectifiers for adjustable speed industrial drives; other systems in accordance with 250.20(B) and separately derived systems supplied by transformers less than 1000 volts provided that special conditions are met. Where an alternating current system is not grounded as permitted above, ground detectors must be installed.

Those specified in Article 517, circuits for electric cranes used in areas of Class III over combustible fibers as indicated in Section 503.13, circuits for equipment in electrolytic cell working zones noted in Article 668 and lighting systems secondary circuits as noted in Section 411.5(A).

As required, each service must have a grounding electrode conductor connected to the grounded service conductor. The reader should be familiar with the requirements of this article in the NEC®. There is a discussion of system grounding connections in general and specifically from outdoor transformers, dual-fed services, using the main bonding jumper as wire or busbar, and loadside grounding connections. There is also a discussion of systems where the grounded conductor is brought to the service equipment, ungrounded system grounding connections, as well as the grounding electrode conductor.

One conductor must be grounded in a two-wire, single-phase system. The neutral conductor is grounded in a three-wire, single-phase system and a multiphase system where one phase is used as a three-wire, single-phase system. The common conductor is grounded in a multiphase system with one wire common to all phases. A phase conductor is grounded in a multiphase system where one phase is grounded and the neutral conductor is grounded in a multiphase system where one phase is used as a single phase 3 wire system.

Each building or structure must have a grounding electrode connected to the building grounding means. There are methods noted for grounded systems, ungrounded systems, where the disconnecting means is located in a separate building or structure on the same premises, and the size of the grounding electrode conductor. The size of grounding electrode conductor to the grounding electrode is given in 250.66 based on the largest ungrounded supply conductor. The reader should refer to this section in the NEC®.

III. Grounding Electrode System and Grounding Electrode Conductor

All grounding electrodes delineated in 250.52(A)(1) through (A)(6) have to be bonded together to form the grounding electrode system. If none exists, then at least one of the electrodes in 252(A)(4) through (A)(7) must be used.

(A) Electrodes Permitted for Grounding

(1) Metal Underground Water Pipe. It must be at least in contact with the earth for 3 m (10) or more feet. It must be electrically continuous to the point of connection to the grounding electrode system and bonding conductors. Interior water piping can not be used if it is more than 1.52 m (5 feet) from the point of entrance to the building. There is an exception for industrial and commercial buildings under certain conditions.

(2) Metal Frame of the Building or Structure. The metal frame of the building or structure can be used with any of the following methods:

(a) At least 3.0 m (10 ft.) of a structural member is in direct contact with the earth or it is encased in concrete that is in direct contact with the earth

(b) The structural metal frame is bonded to at least one grounding electrode as in 250.52(A)(1), (3), or (4)

(c) The structural metal frame is bonded to at least one or more grounding electrodes as noted in 250.52(A)(5) or (6) and complies with 250.56 or

(d) Any other approved method of establishing a connection to the earth.

(3) Concrete-Encased Electrode. The electrode must be at least 6 m (20 ft.) of 4 AWG bare copper conductor or 6 m (20 ft.) of steel or zinc galvanized reinforcing bars or rods at least 13 mm (½ in.) in diameter. It must be encased in at least 50 mm (2 in.) of concrete within or near the bottom of the foundation.

(4) Ground Ring

(5) Rod and Pipe Electrodes

(6) Plate Electrodes

(7) Other Local Metal Underground Systems or Structures

(B) Electrodes Not Permitted for Grounding. Metal underground gas piping and aluminum electrodes are not permitted. Refer to 250.104(B) for bonding requirements for gas piping.

(A) Rod, Pipe, and Plate Electrodes

(B) Electrode Spacing

(C) Bonding Jumper

(D) Metal Underground Water Pipe

(E) Supplemental Electrode Bonding Site

(F) Ground Ring

(G) Rod and Pipe Electrodes

(H) Plate Electrode

They are permitted to be connected to the equipment grounding conductors specified in Section 250.118. The earth can not be used as an effective ground-fault current path as noted in 250.4(A)(5) and 250.4(B)(4). They are not required to comply with bonding requirements of 250.50 or 250.53(C) or resistance requirements of 250.56.

Where an ac system is grounded, the system and equipment must be grounded to the same electrode or electrodes bonded together.

You cannot use air terminals conductors and driven pipes, rods, or plate electrodes used for grounding air terminals in lieu of grounding electrodes as noted in Section 250.50. You can bond grounding electrodes of different systems.

The material is to be solid or stranded copper, aluminum, or copper-clad aluminum. It can be bare, stranded, covered, or insulated. It must be corrosion resistant.

Install grounding electrode conductors as follows:

(A) Aluminum or Copper-Clad Aluminum Conductors. Where in direct contact with masonry or the earth where there are corrosive conditions, insulated or bare aluminum or copper-clad aluminum grounding conductors are not permitted. When used outside, these types of conductors cannot be used within 450 mm (18 in.) of the earth. The installation must be as noted below.

(B) Securing and Protection Against Physical Damage. If it is 4 AWG copper or aluminum or larger, it must be protected if it is subject to physical damage. 6 AWG wires, if fastened to the building and free from physical damage, do not need protection; if not, they must be protected in the same manner as wires smaller than 6 AWG. Wires smaller than 6 AWG must be enclosed in either rigid metal conduit, intermediate metal conduit, rigid nonmetallic conduit, electrical metallic tubing, or cable armor. In all cases above, the wire or enclosure must be fastened to the surface it is running on.

(C) Continuous. The grounding electrode conductors cannot have a splice or joint. A splice will be permitted only as noted below in (1) through (4).

(1) An irreversible compression-type fitting is used which is listed as grounding and bonding equipment or by an exothermic welding process.

(2) Sections of busbars can be connected together to form a grounding electrode conductor.

(3) Bonding jumpers from grounding electrodes and grounding electrode conductors can be connected to either an aluminum or copper busbar when the busbar is not less than 6 mm x 50 mm (¼ in. x 2 in.). Connections must be made by using a listed connector or the exothermic welding process. The busbar must be accessible and securely fastened.

(4) The installation must comply with 250.64(A) when aluminum busbars are used.

(D) Grounding Electrode Conductor Taps. Taps to the grounding electrode conductor are permitted where a service has more than one enclosure as noted in Section 230.40, Exception No. 2. The taps must go inside of the enclosure. The common grounding electrode conductor has to be sized in accordance with Section 250.66 based on the sum of the circular mil area of the largest ungrounded service entrance conductor. When, as permitted in 240.40 Exception No. 2, more than one set of service entrance conductors connect directly to a service drop or lateral, then the common grounding electrode conductor must be sized as per Table 250.66 Note 1. The tap conductors can be sized for the largest conductor. The tap conductor has to be connected to the grounding electrode conductor in such a way as to maintain the grounding electrode conductor without a splice.

[1] This table applies to the derived conductors of separately derived ac systems.

[2] See installation restrictions in Section 250.64(A).

(E) Enclosures for Grounding Electrode Conductors. A complete electrical path must be ensured for metal enclosures of grounding electrode conductors. If a raceway is used as protection for a grounding conductor, compliance is required with the appropriate article concerning the raceway.

(F) To Electrodes. A grounding conductor can be run to any convenient grounding electrode in the grounding electrode system or to one or more grounding electrodes or to aluminum or copper busbar as noted in 250.64(C). It must be sized for the largest grounding electrode required by the electrodes connected to it.

These minimum sizes are given in Table 250.66. There are three special cases.

It must be accessible and provide an effective and permanent ground. There are two exeptions.

IV. Enclosure, Raceway, and Service Cable Grounding

These metal enclosures and raceways must be grounded. There is an exception for a metal elbow installed in an underground installation under certain conditions.

(A) Underground Service Cable. A continuous underground armored or metal sheathed service cable bonded to the grounded underground system does not have to have the sheath or armor grounded at the building or structure. The armor or sheath can be insulated from the interior metal raceway conduit or piping.

(B) Underground Service Raceway Containing Cable. If an underground metal service raceway contains a metal sheathed or armored cable that is bonded to the grounded underground system, then it does not have to be grounded at the building or structure. The sheath or armor can be insulated from the interior metal raceway or piping.

These metal enclosures and raceways must be grounded except as permitted in Section 250.112(I). Note three exceptions: for open wire, knob and tubing, and nonmetallic-sheathed cable; enclosures used to physically protect cable assemblies; and metal elbows under certain conditions.

V. Bonding

Bonding must be provided to make sure there is proper electrical continuity in the grounding system.

(A) Bonding at Services

All service equipment must be bonded, items are listed. A means must be provided for intersystem bonding.

(B) Method of Bonding at the Service

There are four approved methods:

(1) For the grounded service conductor use a method noted in Section 250.8

(2) Threaded couplings or threaded bosses made up wrench tight

(3) Threadless couplings and connectors for where made tight for metal raceways and metal-clad cables

(4) Other listed devices

All items that will be used as grounding conductors must be bonded. There is a provision for isolated grounding circuits.

In a hazardous location as defined in Article 500, the electrical continuity of non-current carrying metal parts must be accomplished by using any of the methods described in 250.92(B)(2) through (4). This must be done even if supplementary equipment grounding conductors are used.

These ground terminals have to be bonded to the building ground system.

VI. Equipment Grounding and Equipment Grounding Conductors

Metal parts of fixed equipment that are likely to become energized must be grounded if located in a wet or damp location and not isolated, if they can be touched by people and are 2.5 m (8 ft.) or less vertically and 1.5 m (5 ft.) or less horizontally to ground or a grounded object, if in contact with metal, if in a hazardous location, if metal-clad type of wiring is used under certain conditions, and if the voltage is greater than 150 V to ground. There are three exceptions.

All exposed metal parts that do not conduct current of the following kinds of equipment and enclosures must be grounded: switchboard frames and structures except two-wire dc where insulated from ground: pipe organs; motor frames; enclosures for motor controllers; elevators and cranes; all electric equipment in commercial garages, theaters, and motion picture studios except pendant lampholders of 150 V or less: electric signs, outline lighting and associated equipment as noted in Article 600; motion picture equipment; power-limited remote-control, signaling, and fire-alarm signaling equipment supplied by Class 1, 2, and 3 circuits where required; luminaires (lighting fixtures) as per Article 410; skid-mounted equipment, motor-operated water pumps, and metal well casings.

There are areas where exposed metal parts (noncurrent-carrying) must be grounded even though connected by cord and plug. This is true of hazardous locations and circuits over 150 V to ground; two exceptions are motors when guarded and electrically heated appliances with special exemptions and insulation to ground. In a residence the following items are required to have equipment grounding: refrigerators, freezers, and air conditioners; clothes washing and clothes drying equipment, dishwashing machines, kitchen waste disposers, information technology equipment, sump pumps, and electrical aquarium equipment; hand-held motor-operated tools, stationary and fixed motor-operated tools, light industrial motor-operated tools; motor-operated hedge clippers, lawn mowers, snow blowers, wet scrubbers, etc.; and portable handlamps. In other than residences, the list is the same as residences, with the addition of cord- and plug-connected appliances in damp or wet locations and tools used in wet or conductive locations. There is an exception for tools and portable handlamps in wet or conductive locations if supplied through an isolating transformer with a secondary ungrounded and not over 50 V.

The metal parts of this equipment must be grounded: cranes and hoists; elevator cars of nonelectric type; and electric elevators.

The following may be used for equipment grounding conductors.

1. Copper or other corrosion-resistant conductor. It can be solid or stranded, and bare, covered, or insulated.
2. Rigid metal conduit.
3. Intermediate metal conduit.
4. Electrical metallic tubing.
5. Listed flexible metal conduit that meets the following requirements: the terminations are listed for grounding; the conductors in the conduit are protected by devices that are rated 20 A or less; and the total length of flexible metal conduit, flexible metallic tubing, and liquidtight flexible metal conduit in the same ground return path is not more than 1.8 m (6 ft.); if flexibility is required an equipment grounding conductor is used.
6. Listed liquidtight flexible metal conduit meeting certain requirements.
7. Flexible metal tubing under certain conditions.
8. Armor of Type AC cable.
9. The copper sheath of MI metal sheathed cable.
10. Type MC cable under certain conditions.
11. Cable trays as noted in Sections 318.3(c) and 318.7.
12. Cablebus framework and noted in Section 365.2(A).
13. Any other continuous metal raceways and auxiliary gutters that are continuous and listed for grounding.
14. Listed surface metal raceways

The equipment grounding conductor can either be bare, insulated, or covered unless there is a specific required by the code. There are identification requirements for conductors. Special requirements are listed for conductors larger than 6 AWG, multi-conductor cable, and flexible cord.

Where it is a raceway, cable tray, cable armor, cable bus, or cable sheath, or a wire in a raceway it must be installed in accordance with the NEC®. Aluminum and copper-clad aluminum can be used under certain circumstances. If the equipment grounding conductor is smaller than 6 AWG, it has to be protected from physical damage except under certain conditions where it is not subject to physical damage.

These minimum sizes are given in Table 250.122. When circuit conductors are run in parallel, the equipment grounding conductor must be run in parallel and sized according to the over-current device rating shown in the table. The conductor never has to be larger than the overcurrent device protecting the circuit conductor to the equipment. There are other requirements and exceptions, including where conductors are adjusted in size to compensate for voltage drop.

[1] See installation restrictions in Section 250.120.

VII. Methods of Equipment Grounding

(A) This is true when it is electrically connected to a supporting metal rack or structure that is properly grounded. The building's structural steel is not considered an effective grounding method.

(B) Metal car frames

(A) Generally, a grounded circuit conductor can be used to ground equipment on the supply side of the service disconnect, supply side of main disconnects for a separate building [Section 250.32(B)], and in a separate, derived system the supply side of the disconnecting or overcurrent devices.

(B) Generally, a grounded circuit conductor cannot be used to ground equipment on the load side of the service disconnect. There are four exceptions to this:

1. As indicated in Section 250.140 for ranges, wall-mounted ovens, etc.
2. Grounding meter enclosures if no service ground-fault protection is used, the meter is immediately adjacent to the service disconnecting means, and the grounded circuit conductor is not smaller than noted in Table 250.122 for equipment grounding conductors.
3. Dc systems in accordance with Section 250.22, Exception.
4. Ground electrode-type boilers over 600 V in accordance with 490.72(E)(1) and 490.74.

Generally, an equipment bonding jumper must be used unless grounded as follows:

A. Surface Mounted Box. If the box is surface mounted and the yoke makes good electrical contact with the box or a contact yoke or device complying with 250.146(B). There are restrictions on cover-mounted receptacles.

B. Contact Devices or Yokes. Where the receptacle is listed as such with special yokes and screws.

C. Floor Boxes. Floor boxes so listed.

D. Isolated Receptacles. Where there is a requirement for the reduction of electrical noise, an insulated grounding conductor can be used. It is run with the circuit conductors. It can pass through panelboards without connection to the grounding terminal. The grounding terminal in the receptacle can be insulated from the mounting.

VIII. Direct Current Systems

It cannot be smaller than the neutral conductor for a three-wire balancer set or balancer winding. It cannot be smaller than the largest conductor if other than as stated in the preceding sentence. It can never be smaller than 8 AWG copper or 6 AWG aluminum. There are requirements for connections to make electrodes, concrete-encased electrodes, and ground rings.

IX. Instruments, Meters, and Relays

X. Grounding of Systems and Circuits of 1 kV and over (High Voltage)

If these systems are grounded, they must follow all previous sections as well as this section.