GROUNDING

70-103 H. Overcurrent Protection Over 600 Volts, Nominal 240-100. Feeders. Feeders shall have a short-circuit protective device in each ungrounded conductor or comply with Section 230-208(d)(2) or (d)(3). The protective device(s) shall be capable of detecting and interrupting all values of current that can occur at their location in excess of their trip setting or melting point. In no case shall the fuse rating in continuous amperes exceed three times, or the long-time trip element setting of a breaker, six times, the ampacity of the conductor. Conductors tapped to a feeder shall be permitted to be protected by the feeder overcurrent device where that overcurrent device also protects the tap conductors. (FPN): The operating time of the protective device, the available short-circuit current, and the conductor used will need to be coordinated to prevent damaging or dangerous temperatures in conductors or conductor insulation under short-circuit conditions. 240-101. Branch Circuits. Branch circuits shall have a short-circuit protective device in each ungrounded conductor or comply with Section 230- 208(d)(2) or (d)(3). The protective device(s) shall be capable of detecting and interrupting all values of current that can occur at their location in excess of their trip setting or melting point. ARTICLE 250 - GROUNDING NOTICE Following the issuance of this edition an appeal was filed with respect to Section 250-45 of this article - refer to page 70-iii. A. General 250-1. Scope. This article covers general requirements for grounding and bonding of electrical installations, and specific requirements in (a) through (f) below. (a) Systems, circuits, and equipment required, permitted, or not permitted to be grounded. (b) Circuit conductor to be grounded on grounded systems. (c) Location of grounding connections. (d) Types and sizes of grounding and bonding conductors and electrodes. (e) Methods of grounding and bonding. (f) Conditions under which guards, isolation, or insulation may be substituted for grounding. (FPN No. 1): Systems and circuit conductors are grounded to limit voltages due to lightning, line surges, or unintentional contact with higher voltage lines, and to stabilize the voltage to ground during normal operation. Equipment grounding conductors are bonded to the system grounded conductor to provide a low impedance path for fault current that will facilitate the operation of overcurrent devices under ground-fault conditions. (FPN No. 2): Conductive materials enclosing electrical conductors or equipment, or forming part of such equipment, are grounded to limit the voltage to ground 70-104 NATIONAL ELECTRICAL CODE on these materials and bonded to facilitate the operation of overcurrent devices under ground-fault conditions. See Section 110-10. 250-2. Application of Other Articles. In other articles applying to particular cases of installation of conductors and equipment, there are requirements that are in addition to those of this article or are modifications of them: Article Section Agricultural Buildings 547-8 Appliances 422-16 Branch Circuits 210-5 210-6 210-7 Cablebus 365-9 Circuits and Equipment Operating at Less Than 50 Volts 720 Class 1, Class 2, and Class 3 Remote-Control, Signaling, and Power-Limited Circuits 725-20 725-43 Closed-Loop and Programmed Power Distribution 780-3 Communications Circuits 800 Community Antenna Television and Radio Distribution Systems 820-33 820-40 820-41 Conductors for General Wiring 310 Cranes and Hoists 610 Electronic Computer/Data Processing Equipment 645-15 Electrically Driven or Controlled Irrigation Machines 675-11(c) 675-12 675-13 675-14 675-15 Electric Signs and Outline Lighting 600 Electrolytic Cells 668 Elevators, Dumbwaiters, Escalators, Moving Walks, Wheelchair Lifts, and Stairway Chair Lifts 620 Fire Protective Signaling Systems 760-6 Fixed Electric Heating Equipment for Pipelines and Vessels 427-21 427-29 427-48 Fixed Electric Space Heating Equipment 424-14 Fixed Equipment Outdoor Electric Deicing and Snow-Melting 426-27 Fixtures and Lighting Equipment 410-17 410-18 410-19 410-21 410-105(b) Flexible Cords and Cables 400-22 400-23 ARTICLE 250 - GROUNDING 70-105 Floating Buildings 553-8 553-10 553-11 Grounding-type Receptacles, Adapters, Cord Connectors, and Attachment Plugs 410-58 Hazardous (Classified) Locations 500-517 Health Care Facilities 517 Induction and Dielectric Heating Equipment 665 Industrial Machinery 670 Intrinsically Safe Systems 504-50 I Lighting Fixtures, Lampholders, Lamps, and Receptacles 410 Marinas and Boatyards 555-7 Mobile Homes and Mobile Home Parks 550 Motion Picture and Television Studios and Similar Locations 530-20 530-66 Motors, Motor Circuits, and Controllers 430 Outlet, Device, Pull and Junction Boxes, Conduit Bodies and Fittings 370-4 370-25 Over 600 Volts, Nominal-General 710-4(b)(1) Panelboards 384-20 Pipe Organs 650 Radio and Television Equipment 810 Receptacles and Cord Connectors 210-7 Recreational Vehicles and Recreational Vehicle Parks 551 Services 230 Service Equipment 230-63 Solar Photovoltaic Systems 690-41 690-42 690-43 690-44 Sound-Recording and Similar Equipment 640-4 Swimming Pools, Fountains, and Similar Installations 680 Switchboards and Panelboards 384-3(d) 384-11 Switches 380-12 Theaters, Audience Areas of Motion Picture and Television Studios, and Similar Locations 520-81 Transformers and Transformer Vaults 450-10 Use and Identification of Grounded Conductors 200 X-Ray Equipment 660 517-77 B. Circuit and System Grounding 250-3. Direct-Current Systems. (a) Two-Wire Direct-Current Systems. Two-wire dc systems supplying premises wiring shall be grounded. 70-106 NATIONAL ELECTRICAL CODE Exception No. 1: A system equipped with a ground detector and supplying only industrial equipment in limited areas. Exception No. 2: A system operating at 50 volts or less between conductors. Exception No. 3: A system operating at over 300 volts between conductors. Exception No. 4: A rectifier-derived dc system supplied from an ac system complying with Section 250-5. Exception No. 5: DC fire protective signaling circuits having a maximum current of 0.030 amperes as specified in Article 760, Part C. (b) Three-Wire Direct-Current Systems. The neutral conductor of all 3-wire dc systems supplying premises wiring shall be grounded. 250-5. Alternating-Current Circuits and Systems to Be Grounded. AC circuits and systems shall be grounded as provided for in (a), (b), (c), or (d) below. Other circuits and systems shall be permitted to be grounded. (FPN): An example of a system permitted to be grounded is a corner-grounded delta transformer connection. See Section 250-25(4) for conductor to be grounded. (a) Alternating-Current Circuits of Less than 50 Volts. AC circuits of less than 50 volts shall be grounded under any of the following conditions: (1) Where supplied by transformers if the transformer supply system exceeds 150 volts to ground. (2) Where supplied by transformers if the transformer supply system is ungrounded. (3) Where installed as overhead conductors outside of buildings. (b) Alternating-Current Systems of 50 Volts to 1000 Volts. AC systems of 50 volts to 1000 volts supplying premises wiring and premises wiring systems shall be grounded under any of the following conditions: (1) Where the system can be so grounded that the maximum voltage to ground on the ungrounded conductors does not exceed 150 volts. (2) Where the system is 3-phase, 4-wire wye connected in which the neutral is used as a circuit conductor. (3) Where the system is 3-phase, 4-wire delta connected in which the midpoint of one phase winding is used as a circuit conductor. (4) Where a grounded service conductor is uninsulated in accordance with the exceptions to Sections 230-22, 230-30, and 230-41. Exception No. 1: Electric systems used exclusively to supply industrial electric furnaces for melting, refining, tempering, and the like. Exception No. 2: Separately derived systems used exclusively for rectifiers supplying only adjustable speed industrial drives. Exception No. 3: Separately derived systems supplied by transformers that have a primary voltage rating less than 1000 volts, provided that all of the following conditions are met: a. The system is used exclusively for control circuits. b. The conditions of maintenance and supervision assure that only qualified persons will service the installation. ARTICLE 250 - GROUNDING 70-107 c. Continuity of control power is required. d. Ground detectors are installed on the control system. Exception No. 4: Isolated systems as permitted or required in Articles 517 and 668. (FPN): The proper use of suitable ground detectors on ungrounded systems can provide additional protection. Exception No. 5: High-impedance grounded neutral systems in which a grounding impedance, usually a resistor, limits the ground-fault current to a low value. High-impedance grounded neutral systems shall be permitted for 3-phase ac systems of 480 volts to 1000 volts where all of the following conditions are met: a. The conditions of maintenance and supervision assure that only qualified persons will service the installation. b. Continuity of power is required. c. Ground detectors are installed on the system. d. Line-to-neutral loads are not served. (c) Alternating-Current Systems of 1 kV and Over. AC systems supplying mobile or portable equipment shall be grounded as specified in Section 250-154. Where supplying other than portable equipment, such systems shall be permitted to be grounded. Where such systems are grounded, they shall comply with the applicable provisions of this article. (d) Separately Derived Systems. A premises wiring system whose power is derived from generator, transformer, or converter windings and has no direct electrical connection, including a solidly connected grounded circuit conductor, to supply conductors originating in another system, if required to be grounded as in (a) or (b) above, shall be grounded as specified in Section 250-26. (FPN No. 1): An alternate alternating-current power source such as an on-site generator is not a separately derived system if the neutral is solidly interconnected to a service-supplied system neutral. (FPN No. 2): For systems that are not separately derived and are not required to be grounded as specified in Section 250-26, see Section 445-5 for minimum size of conductors that must carry fault current. 250-6. Portable and Vehicle-Mounted Generators. (a) Portable Generators. Under the following conditions, the frame of a portable generator shall not be required to be grounded and shall be permitted to serve as the grounding electrode for a system supplied by the generator: (1) The generator supplies only equipment mounted on the generator or cord- and plug-connected equipment through receptacles mounted on the generator, or both, and (2) The noncurrent-carrying metal parts of equipment and the equipment grounding conductor terminals of the receptacles are bonded to the generator frame. (b) Vehicle-Mounted Generators. Under the following conditions, the frame of a vehicle shall be permitted to serve as the grounding electrode for a system supplied by a generator located on the vehicle: 70-108 NATIONAL ELECTRICAL CODE (1) The frame of the generator is bonded to the vehicle frame, and (2) The generator supplies only equipment located on the vehicle or cord- and plug-connected equipment through receptacles mounted on the vehicle or both equipment located on the vehicle and cord- and plugconnected equipment through receptacles mounted on the vehicle or on the generator, and (3) The noncurrent-carrying metal parts of equipment and the equipment grounding conductor terminals of the receptacles are bonded to the generator frame, and (4) The system complies with all other provisions of this article. (c) Neutral Conductor Bonding. A neutral conductor shall be bonded to the generator frame where the generator is a component of a separately derived system. The bonding of any conductor other than a neutral within the generator to its frame shall not be required. (FPN): For grounding of portable generators supplying fixed wiring systems, see Section 250-5(d). 250-7. Circuits Not to Be Grounded. The following circuits shall not be grounded: (a) Cranes. Circuits for electric cranes operating over combustible fibers in Class III locations, as provided in Section 503-13. (b) Health Care Facilities. Circuits as provided in Article 517. (c) Electrolytic Cells. Circuits as provided in Article 668. C. Location of System Grounding Connections 250-21. Objectionable Current over Grounding Conductors. (a) Arrangement to Prevent Objectionable Current. The grounding of electric systems, circuit conductors, surge arresters, and conductive noncurrent-carrying materials and equipment shall be installed and arranged in a manner that will prevent an objectionable flow of current over the grounding conductors or grounding paths. (b) Alterations to Stop Objectionable Current. If the use of multiple grounding connections results in an objectionable flow of current, one or more of the following alterations shall be permitted to be made, provided that the requirements of Section 250-51 are met: (1) Discontinue one or more but not all of such grounding connections. (2) Change the locations of the grounding connections. (3) Interrupt the continuity of the conductor or conductive path interconnecting the grounding connections. (4) Take other suitable remedial action satisfactory to the authority having jurisdiction. ARTICLE 250 - GROUNDING 70-109 (c) Temporary Currents Not Classified as Objectionable Currents. Temporary currents resulting from accidental conditions, such as groundfault currents, that occur only while the grounding conductors are performing their intended protective functions shall not be classified as objectionable current for the purposes specified in (a) and (b) above. (d) Limitations to Permissible Alterations. The provisions of this section shall not be considered as permitting electronic equipment being operated on ac systems or branch circuits that are not grounded as required by this article. Currents that introduce noise or data errors in electronic equipment shall not be considered the objectionable currents addressed in this section. 250-22. Point of Connection for Direct-Current Systems. DC systems to be grounded shall have the grounding connection made at one or more supply stations. A grounding connection shall not be made at individual services nor at any point on premises wiring. Exception: Where the dc system source is located on the premises, a grounding connection shall be made either (1) at the source or the first system disconnecting means or overcurrent device, or (2) by another means that accomplishes equivalent system protection and that utilizes equipment listed and identified for the use. 250-23. Grounding Service-Supplied Alternating-Current Systems. (a) System Grounding Connections. A premises wiring system that is supplied by an ac service and is required to be grounded by Section 250-5 shall have at each service a grounding electrode conductor connected to a grounding electrode that complies with Part H of Article 250. The grounding electrode conductor shall be connected to the grounded service conductor at any accessible point from the load end of the service drop or service lateral to and including the terminal or bus to which the grounded service conductor is connected at the service disconnecting means. Where the transformer supplying the service is located outside the building, at least one additional grounding connection shall be made from the grounded service conductor to a grounding electrode, either at the transformer or elsewhere outside the building. A grounding connection shall not be made to any grounded circuit conductor on the load side of the service disconnecting means. (FPN): See definition of "Service Drop" and "Service Lateral"; also Section 230-21. Exception No. 1: A grounding electrode conductor shall be connected to the grounded conductor of a separately derived system in accordance with the provisions of Section 250-26(b). Exception No. 2: A grounding conductor connection shall be made at each separate building where required by Section 250-24. Exception No. 3: For ranges, counter-mounted cooking units, wallmounted ovens, clothes dryers, and meter enclosures as permitted by Section 250-61. 70-110 NATIONAL ELECTRICAL CODE Exception No. 4: For services that are dual fed (double ended) in a common enclosure or grouped together in separate enclosures and employing a secondary tie, a single grounding electrode connection to the tie point of the grounded circuit conductors from each power source shall be permitted. Exception No. 5: Where the main bonding jumper specified in Sections 250-53(b) and 250-79 is a wire or busbar, and is installed from the neutral bar or bus to the equipment grounding terminal bar or bus in the service equipment, the grounding electrode conductor shall be permitted to be connected to the equipment grounding terminal bar or bus to which the main bonding jumper is connected. Exception No. 6: As covered in Section 250-27 for high-impedance grounded neutral systems grounding connection requirements. (b) Grounded Conductor Brought to Service Equipment. Where an ac system operating at less than 1000 volts is grounded at any point, the grounded conductor shall be run to each service disconnecting means and shall be bonded to each disconnecting means enclosure. This conductor shall be routed with the phase conductors and shall not be smaller than the required grounding electrode conductor specified in Table 250-94, and, in addition, for service-entrance phase conductors larger than 1100 kcmil copper or 1750 kcmil aluminum, the grounded conductor shall not be smaller than 12½ percent of the area of the largest service-entrance phase conductor. Where the service-entrance phase conductors are paralleled, the size of the grounded conductor shall be based on the equivalent area for parallel conductors as indicated in this section. (FPN): See Section 310-4 for grounded conductors connected in parallel. Exception No. 1: The grounded conductor shall not be required to be larger than the largest ungrounded service-entrance phase conductor. Exception No. 2: As covered in Section 250-27 for high-impedance grounded neutral systems grounding connection requirements. Exception No. 3: Where more than one service disconnecting means are located in an assembly listed for use as service equipment, one grounded conductor shall be required to be run to the assembly, and it shall be bonded to the assembly enclosure. 250-24. Two or More Buildings or Structures Supplied from a Common Service. (a) Grounded Systems. Where two or more buildings or structures are supplied from a common service, the grounded system in each building or structure shall have a grounding electrode as described in Part H connected to the metal enclosure of the building disconnecting means and to the ac system grounded circuit conductor on the supply side of the building or structure disconnecting means. Exception No. 1: A grounding electrode at separate buildings or structures shall not be required where only one branch circuit is supplied and there is no equipment in the building or structure that requires grounding. ARTICLE 250 - GROUNDING 70-111 Exception No. 2: A grounded circuit conductor connection to the grounding electrode shall not be required at a separate building or structure if an equipment grounding conductor is run with the circuit conductors for grounding any noncurrent-carrying equipment, interior metal piping systems, and building or structural metal frames, and the equipment grounding conductor is bonded at a separate building or structure disconnecting means to existing grounding electrodes described in Part H. Where there are no existing electrodes, a grounding electrode meeting the requirements of Part H shall be installed where the building or structure supplies more than one branch circuit. Where livestock is housed, that portion of the equipment grounding conductor run underground to the disconnecting means shall be insulated or covered copper. (FPN): See Section 547-8(a), Exception for special grounding requirements for agricultural buildings. (b) Ungrounded Systems. Where two or more buildings or structures are supplied by a common service from an ungrounded system, each building or structure shall have a grounding electrode as described in Part H connected to the metal enclosure of the building or structure disconnecting means. Exception No. 1: A grounding electrode at separate buildings or structures shall not be required where only one branch circuit is supplied and there is no equipment in the building or structure that requires grounding. Exception No. 2: A grounding electrode and grounding electrode conductor connection to the metal enclosure of the building or structure disconnecting means shall not be required, provided all of the following conditions are met: a. An equipment grounding conductor is run with the circuit conductors to the building or structure disconnecting means for grounding any noncurrent-carrying equipment, interior metal piping systems, and building or structural metal frames. b. There are no existing grounding electrodes as described in Part H. c. The building or structure supplies only one branch circuit. d. Where livestock is housed, that portion of the equipment grounding conductor run underground to the disconnecting means shall be insulated or covered copper. (FPN): See Section 547-8(a), Exception for special grounding requirements for agricultural buildings. (c) Disconnecting Means Located in Separate Building or Structure on the Same Premises. Where one or more disconnecting means supply one or more additional buildings or structures under single management, and where these disconnecting means are located remote from those buildings or structures in accordance with the provisions of Section 225-8(b), Exception Nos. 1 and 2 or Section 230-205(a), Exception, all of the following conditions shall be met: 70-112 NATIONAL ELECTRICAL CODE (1) The connection of the grounded circuit conductor to the grounding electrode at a separate building or structure shall not be made; (2) An equipment grounding conductor for grounding any noncurrentcarrying equipment, interior metal piping systems, and building or structural metal frames is run with the circuit conductors to a separate building or structure and bonded to existing grounding electrodes described in Part H, or, where there are no existing electrodes, a grounding electrode meeting the requirements of Part H shall be installed where a separate building or structure supplies more than one branch circuit; (3) Bonding the equipment grounding conductor to the grounding electrode at a separate building or structure shall be made in a junction box, panelboard, or similar enclosure located immediately inside or outside the separate building or structure. Exception No. 1: A grounding electrode at a separate building or structure shall not be required where only one branch circuit is supplied and there is no equipment in the building or structure that requires grounding. Exception No. 2: Where livestock is housed, that portion of the equipment grounding conductor run underground to the disconnecting means shall be insulated or covered copper. (d) Grounding Conductor. The size of the grounding conductor to the grounding electrode(s) shall not be less than given in Table 250-95, and the I installation shall be in accordance with Sections 250-92(a) and (b). Exception No. 1: The grounding conductor shall not be required to be larger than the largest ungrounded supply conductor. Exception No. 2: Where connected to made electrodes as in Section 250-83(c) or (d), that portion of the grounding conductor that is the sole connection between the electrode(s) and the grounded or grounding conductor or the metal enclosure of the building disconnecting means shall not be required to be larger than No. 6 copper or No. 4 aluminum. 250-25. Conductor to Be Grounded - Alternating-Current Systems. For ac premises wiring systems, the conductor to be grounded shall be as specified in (1) through (5) below. (1) Single-phase, 2-wire: one conductor. (2) Single-phase, 3-wire: the neutral conductor. (3) Multiphase systems having one wire common to all phases: the common conductor. (4) Multiphase systems requiring one grounded phase: one phase conductor. (5) Multiphase systems in which one phase is used as in (2) above: the neutral conductor. Grounded conductors shall be identified by the means specified in