1, Exception No. 2.

70-128 NATIONAL ELECTRICAL CODE (b) Metal Frame of the Building. The metal frame of the building, where effectively grounded. (FPN): Effectively grounded means intentionally connected to earth through a ground connection or connections of sufficiently low impedance and having sufficient current-carrying capacity to prevent the buildup of voltages that may result in undue hazard to connected equipment or to persons. (c) Concrete-Encased Electrode. An electrode encased by at least 2 inches (50.8 mm) of concrete, located within and near the bottom of a concrete foundation or footing that is in direct contact with the earth, consisting of at least 20 feet (6.1 m) of one or more bare or zinc galvanized or other electrically conductive coated steel reinforcing bars or rods of not less than ½ inch (12.7 mm) diameter, or consisting of at least 20 feet (6.1 m) of bare copper conductor not smaller than No. 4. (d) Ground Ring. A ground ring encircling the building or structure, in direct contact with the earth at a depth below earth surface not less than 2½ feet (762 mm), consisting of at least 20 feet (6.1 m) of bare copper conductor not smaller than No. 2. 250-83. Made and Other Electrodes. Where none of the electrodes specified in Section 250-81 is available, one or more of the electrodes specified in (b) through (d) below shall be used. Where practicable, made electrodes shall be embedded below permanent moisture level. Made electrodes shall be free from nonconductive coatings, such as paint or enamel. Where more than one electrode is used, each electrode of one grounding system (including that used for lightning rods) shall not be less than 6 feet (1.83 m) from any other electrode of another grounding system. (FPN): Two or more electrodes that are effectively bonded together are to be treated as a single electrode system in this sense. (a) Metal Underground Gas Piping System. A metal underground gas piping system shall not be used as a grounding electrode. (b) Other Local Metal Underground Systems or Structures. Other local metal underground systems or structures, such as piping systems and underground tanks. (c) Rod and Pipe Electrodes. Rod and pipe electrodes shall not be less than 8 feet (2.44 m) in length and shall consist of the following materials, and shall be installed in the following manner: (1) Electrodes of pipe or conduit shall not be smaller than ³/₄-inch trade size and, where of iron or steel, shall have the outer surface galvanized or otherwise metal-coated for corrosion protection. (2) Electrodes of rods of iron or steel shall be at least 5/8 inch (15.87 mm) in diameter. Stainless steel rods less than 5/8 inch (15.87 mm) in diameter, nonferrous rods, or their equivalent shall be listed and shall not be less than 1/2 inch (12.7 mm) in diameter. (3) The electrode shall be installed such that at least 8 feet (2.44 m) of length is in contact with the soil. It shall be driven to a depth of not less than 8 feet (2.44 m) except that, where rock bottom is encountered, the electrode shall be driven at an oblique angle not to exceed 45 degrees from the vertical or shall be buried in a trench that is at least 2½ feet (762 mm) deep. The upper end of the electrode shall be flush with or below ground ARTICLE 250 - GROUNDING 70-129 level unless the aboveground end and the grounding electrode conductor attachment are protected against physical damage as specified in Section 250-117. (d) Plate Electrodes. Each plate electrode shall expose not less than 2 square feet (0.186 sq m) of surface to exterior soil. Electrodes of iron or steel plates shall be at least ¼ inch (6.35 mm) in thickness. Electrodes of nonferrous metal shall be at least 0.06 inch (1.52 mm) in thickness. (e) Aluminum Electrodes. Aluminum electrodes shall not be permitted. 250-84. Resistance of Made Electrodes. A single electrode consisting of a rod, pipe, or plate that does not have a resistance to ground of 25 ohms or less shall be augmented by one additional electrode of any of the types specified in Section 250-81 or 250-83. Where multiple rod, pipe, or plate electrodes are installed to meet the requirements of this section, they shall be not less than 6 feet (1.83 m) apart. (FPN): The paralleling efficiency of rods longer than 8 feet (2.44 m) is improved by spacing greater than 6 feet (1.83 m). 250-86. Use of Lightning Rods. Lightning rod conductors and driven pipes, rods, or other made electrodes used for grounding lightning rods shall not be used in lieu of the made grounding electrodes required by Section 250-83 for grounding wiring systems and equipment. This provision shall not prohibit the required bonding together of grounding electrodes of different systems. (FPN No. 1): See Sections 250-46, 800-40(d), 810-21(j), and 820-40(d) for bonding of electrodes. (FPN No. 2): Bonding together of all separate grounding electrodes will limit potential differences between them and between their associated wiring systems. J. Grounding Conductors 250-91. Material. The material for grounding conductors shall be as specified in (a), (b), and (c) below. (a) Grounding Electrode Conductor. The grounding electrode conductor shall be of copper, aluminum, or copper-clad aluminum. The material selected shall be resistant to any corrosive condition existing at the installation or shall be suitably protected against corrosion. The conductor shall be solid or stranded, insulated, covered, or bare and shall be installed in one continuous length without a splice or joint. Exception No. 1: Splices in busbars shall be permitted. Exception No. 2: Where a service consists of more than a single enclosure as permitted in Section 230-40, Exception No. 2, it shall be permissible to connect taps to the grounding electrode conductor. Each such tap conductor shall extend to the inside of each such enclosure. The grounding electrode conductor shall be sized in accordance with Section 250-94, but the tap conductors shall be permitted to be sized in accordance with the grounding electrode conductors specified in Section 250-94 for the largest conductor serving the respective enclosures. The tap conductors shall be connected to the grounding electrode conductor in such a manner that the grounding electrode conductor remains without a splice or joint. 70-130 NATIONAL ELECTRICAL CODE Exception No. 3: It shall be permitted to splice the grounding electrode conductor by means of irreversible compression-type connectors listed for the purpose or the exothermic welding process. (b) Types of Equipment Grounding Conductors. The equipment grounding conductor run with or enclosing the circuit conductors shall be one or more or a combination of the following: (1) a copper or other corrosion-resistant conductor. This conductor shall be solid or stranded; insulated, covered, or bare; and in the form of a wire or a busbar of any shape; (2) rigid metal conduit; (3) intermediate metal conduit; (4) electrical metallic tubing; (5) flexible metal conduit where both the conduit and fittings are listed for grounding; (6) armor of Type AC cable; (7) the copper sheath of mineral-insulated, metal-sheathed cable; (8) the metallic sheath or the combined metallic sheath and grounding conductors of Type MC cable; (9) cable trays as permitted in Sections 318-3(c) and 318-7; (10) cablebus framework as permitted in Section 365-2(a); (11) other electrically continuous metal raceways listed for grounding. Exception No. 1: Listed flexible metal conduit and listed flexible metallic tubing shall be permitted for grounding if all the following conditions are met: a. The total combined length of flexible metal conduit and flexible metallic tubing and liquidtight flexible metal conduit in the same ground return path does not exceed 6 ft (1.83 m). b. The circuit conductors contained therein are protected by overcurrent devices rated at 20 amperes or less. c. The conduit or tubing is terminated in fittings listed for grounding. Exception No. 2: Listed liquidtight flexible metal conduit shall be permitted as a grounding means in the 1¹/4-inch and smaller trade sizes if the total combined length of liquidtight flexible metal conduit and flexible metal conduit and flexible metallic tubing in the same ground return path does not exceed 6 ft (1.83 m), the conduit is terminated in fittings listed for grounding, and the circuit conductors contained therein are protected by overcurrent devices rated at 20 amperes or less for ³/₈-inch and ¹/₂-inch trade sizes and 60 amperes or less for 3/4-inch through 1¹/4-inch trade sizes. Exception No. 3: For direct-current circuits only, the equipment grounding conductor shall be permitted to be run separately from the circuit conductors. (c) Supplementary Grounding. Supplementary grounding electrodes shall be permitted to augment the equipment grounding conductors specified in Section 250-91(b), but the earth shall not be used as the sole equipment grounding conductor. 250-92. Installation. Grounding conductors shall be installed as specified in (a), (b), and (c) below. (a) Grounding Electrode Conductor. A grounding electrode conductor or its enclosure shall be securely fastened to the surface on which it is carried. A No. 4, copper or aluminum, or larger conductor shall be protected ARTICLE 250 - GROUNDING 70-131 if exposed to severe physical damage. A No. 6 grounding conductor that is free from exposure to physical damage shall be permitted to be run along the surface of the building construction without metal covering or protection where it is securely fastened to the construction; otherwise, it shall be in rigid metal conduit, intermediate metal conduit, rigid nonmetallic conduit, electrical metallic tubing, or cable armor. Grounding conductors smaller than No. 6 shall be in rigid metal conduit, intermediate metal conduit, rigid nonmetallic conduit, electrical metallic tubing, or cable armor. Insulated or bare aluminum or copper-clad aluminum grounding conductors shall not be used where in direct contact with masonry or the earth or where subject to corrosive conditions. Where used outside, aluminum or copper-clad aluminum grounding conductors shall not be installed within 18 inches (457 mm) of the earth. (b) Enclosures for Grounding Electrode Conductors. Metal enclosures for grounding electrode conductors shall be electrically continuous from the point of attachment to cabinets or equipment to the grounding electrode, and shall be securely fastened to the ground clamp or fitting. Metal enclosures that are not physically continuous from cabinet or equipment to the grounding electrode shall be made electrically continuous by bonding each end to the grounding conductor. Where a raceway is used as protection for a grounding conductor, the installation shall comply with the requirements of the appropriate raceway article. (c) Equipment Grounding Conductor. An equipment grounding conductor shall be installed as follows. (1) Where it consists of a raceway, cable tray, cable armor, or cable sheath or where it is a wire within a raceway or cable, it shall be installed in accordance with the applicable provisions in this Code using fittings for joints and terminations approved for use with the type raceway or cable used. All connections, joints, and fittings shall be made tight using suitable tools. (2) Where it is a separate equipment grounding conductor as provided in the exception for Sections 250-50(a) and (b), it shall be installed in accordance with (a) above in regard to restrictions for aluminum and also in regard to protection from physical damage. Exception: Sizes smaller than No. 6 shall not be required to be enclosed in a raceway or armor where run in the hollow spaces of a wall or partition or where otherwise installed so as not to be subject to physical damage. 250-93. Size of Direct-Current System Grounding Conductor. The size of the grounding conductor for a dc system shall be as specified in (a) through (c) below. (a) Not Be Smaller than the Neutral Conductor. Where the dc system consists of a 3-wire balancer set or a balancer winding with overcurrent protection as provided in Section 445-4(d), the grounding conductor shall not be smaller than the neutral conductor. (b) Not Be Smaller than the Largest Conductor. Where the dc system is other than as in (a) above, the grounding conductor shall not be smaller than the largest conductor supplied by the system. 70-132 NATIONAL ELECTRICAL CODE (c) Not Be Smaller than No. 8. In no case shall the grounding conductor be smaller than No. 8 copper or No. 6 aluminum. 250-94. Size of Alternating-Current Grounding Electrode Conductor. The size of the grounding electrode conductor of a grounded or ungrounded ac system shall not be less than given in Table 250-94. Table 250-94. Grounding Electrode Conductor for AC Systems Size of Largest Service-Entrance Size of Grounding Conductor or Equivalent Area for Electrode Parallel Conductors Conductor Aluminum or Aluminum or Copper-Clad Copper-Clad Copper Aluminum Copper Aluminum* 2 or smaller 1/0 or smaller 8 6 1 or 1/0 2/0 or 3/0 6 4 2/0 or 3/0 4/0 or 250 kcmil 4 2 Over 3/0 thru Over 250 kcmil 2 1/0 350 kcmil thru 500 kcmil Over 350 kcmil Over 500 kcmil 1/0 3/0 thru 600 kcmil thru 900 kcmil Over 600 kcmil Over 900 kcmil 2/0 4/0 thru 1100 kcmil thru 1750 kcmil Over 1100 kcmil Over 1750 kcmil 3/0 250 kcmil Where multiple sets of service-entrance conductors are used as permitted in Section 230-40, Exception No. 2, the equivalent size of the largest service-entrance conductor shall be determined by the largest sum of the areas of the corresponding conductors of each set. Where there are no service-entrance conductors, the grounding electrode conductor size shall be determined by the equivalent size of the largest service-entrance conductor required for the load to be served. *See installation restrictions in Section 250-92(a). (FPN): See Section 250-23(b) for size of alternating-current system grounded conductor brought to service equipment. Exception No. 1: Grounded Systems. a. Where connected to made electrodes as in Section 250-83 (c) or (d), that portion of the grounding electrode conductor that is the sole connection to the grounding electrode shall not be required to be larger than No. 6 copper wire or No. 4 aluminum wire. b. Where connected to a concrete-encased electrode as in Section 250- 81(c), that portion of the grounding electrode conductor that is the sole connection to the grounding electrode shall not be required to be larger than No. 4 copper wire. C. Where connected to a ground ring as in Section 250-81(d), that portion of the grounding electrode conductor that is the sole connection to the grounding electrode shall not be required to be larger than the conductor used for the ground ring. ARTICLE 250 - GROUNDING 70-133 Exception No. 2: Ungrounded Systems. a. Where connected to made electrodes as in Section 250-83 (c) or (d), that portion of the grounding electrode conductor that is the sole connection to the grounding electrode shall not be required to be larger than No. 6 copper wire or No. 4 aluminum wire. b. Where connected to a concrete-encased electrode as in Section 250-81(c), that portion of the grounding electrode conductor that is the sole connection to the grounding electrode shall not be required to be larger than No. 4 copper wire. C. Where connected to a ground ring as in Section 250-81(d), that portion of the grounding electrode conductor that is the sole connection to the grounding electrode shall not be required to be larger than the conductor used for the ground ring. 250-95. Size of Equipment Grounding Conductors. Copper, aluminum, or copper-clad aluminum equipment grounding conductors shall not be less than shown in Table 250-95. Where conductors are run in parallel in multiple raceways or cables, as permitted in Section 310-4, the equipment grounding conductor, where used, shall be run in parallel. Each parallel equipment grounding conductor shall be sized on the basis of the ampere rating of the overcurrent device protecting the circuit conductors in the raceway in accordance with Table 250-95. When conductors are adjusted in size to compensate for voltage drop, equipment grounding conductors, where required, shall be adjusted proportionately according to circular mil area. Where a single equipment grounding conductor is run with multiple circuits in the same raceway, it shall be sized for the largest overcurrent device protecting conductors in the raceway or cable. Where the overcurrent device consists of an instantaneous trip circuit breaker or a motor short-circuit protector, as allowed in Section 430-52, the equipment grounding conductor size shall be permitted to be based on the rating of the motor overload protective device but not less than the size shown in Table 250-95. Exception No. 1: An equipment grounding conductor not smaller than No. 18 copper and not smaller than the circuit conductors and part of fixture wires or cords in accordance with Section 240-4. Exception No. 2: The equipment grounding conductor shall not be required to be larger than the circuit conductors supplying the equipment. Exception No. 3: Where a raceway or a cable armor or sheath is used as the equipment grounding conductor, as provided in Sections 250-51, 250-57(a), 250-73, and 250-91(b). 250-97. Outline Lighting. Isolated noncurrent-carrying metal parts of outline lighting systems shall be permitted to be bonded together by a No. 14 copper or No. 12 aluminum conductor protected from physical damage, where a conductor complying with Section 250-95 is used to ground the group. 70-134 NATIONAL ELECTRICAL CODE Table 250-95. Minimum Size Equipment Grounding Conductors for Grounding Raceway and Equipment Size Rating or Setting of Automatic Overcurrent Device in Circuit Ahead Aluminum or of Equipment, Conduit, Copper-Clad etc., Not Exceeding Copper Aluminum (Amperes) Wire No. Wire No.* 15 14 12 20 12 10 30 10 8 40 10 8 60 10 8 100 8 6 200 6 4 300 4 2 400 3 1 500 2 1/0 600 1 2/0 800 1/0 3/0 1000 2/0 4/0 1200 3/0 250 kcmil 1600 4/0 350 " 2000 250 kcmil 400 " 2500 350 " 600 " 3000 400 " 600 " 4000 500 " 800 " 5000 700 " 1200 " 6000 800 " 1200 " * See installation restrictions in Section 250-92(a). 250-99. Equipment Grounding Conductor Continuity. (a) Separable Connections. Separable connections such as those provided in draw-out equipment or attachment plugs and mating connectors and receptacles shall provide for first-make, last-break of the equipment grounding conductor. Exception: Interlocked equipment, plugs, receptacles, and connectors that preclude energization without grounding continuity. (b) Switches. No automatic cutout or switch shall be placed in the equipment grounding conductor of a premises wiring system. Exception: Where the opening of the cutout or switch disconnects all sources of energy. ARTICLE 250 - GROUNDING 70-135 K. Grounding Conductor Connections 250-112. To Grounding Electrode. The connection of a grounding electrode conductor to a grounding electrode shall be accessible and made in a manner that will assure a permanent and effective ground. Where necessary to assure this for a metal piping system used as a grounding electrode, effective bonding shall be provided around insulated joints and sections and around any equipment that is likely to be disconnected for repairs or replacement. Bonding conductors shall be of sufficient length to permit removal of such equipment while retaining the integrity of the bond. Exception: An encased or buried connection to a concrete-encased, driven, or buried grounding electrode shall not be required to be accessible. 250-113. To Conductors and Equipment. Grounding conductors and bonding jumpers shall be connected by exothermic welding, listed pressure connectors, listed clamps, or other listed means. Connection devices or fittings that depend solely on solder shall not be used. 250-114. Continuity and Attachment of Branch-Circuit Equipment Grounding Conductors to Boxes. Where more than one equipment grounding conductor enters a box, all such conductors shall be spliced or joined within the box or to the box with devices suitable for the use. Connections depending solely on solder shall not be used. Splices shall be made in accordance with Section 110-14(b) except that insulation shall not be required. The arrangement of grounding connections shall be such that the disconnection or the removal of a receptacle, fixture, or other device fed from the box will not interfere with or interrupt the grounding continuity. Exception: The equipment grounding conductor permitted in Section 250-74, Exception No. 4 shall not be required to be connected to the other equipment grounding conductors or to the box. (a) Metal Boxes. A connection shall be made between the one or more equipment grounding conductors and a metal box by means of a grounding screw that shall be used for no other purpose, or a listed grounding device. (b) Nonmetallic Boxes. One or more equipment grounding conductors brought into a nonmetallic outlet box shall be so arranged that a connection can be made to any fitting or device in that box requiring grounding. 250-115. Connection to Electrodes. The grounding conductor shall be connected to the grounding electrode by exothermic welding, listed lugs, listed pressure connectors, listed clamps, or other listed means. Connections depending on solder shall not be used. Ground clamps shall be listed for the materials of the grounding electrode and the grounding electrode conductor and, where used on pipe, rod, or other buried electrodes, shall also be listed for direct soil burial. Not more than one conductor shall be connected to the grounding electrode by a single clamp or fitting unless the 70-136 NATIONAL ELECTRICAL CODE clamp or fitting is listed for multiple conductors. One of the methods indicated in (a), (b), (c), or (d) below shall be used. (a) Bolted Clamp. A listed bolted clamp of cast bronze or brass, or plain or malleable iron. (b) Pipe Fitting, Pipe Plug, etc. A pipe fitting, pipe plug, or other approved device screwed into a pipe or pipe fitting. (c) Sheet-Metal-Strap Type Ground Clamp. A listed sheet-metal-strap type ground clamp having a rigid metal base that seats on the electrode and having a strap of such material and dimensions that it is not likely to stretch during or after installation. (d) Other Means. An equally substantial approved means. 250-117. Protection of Attachment. Ground clamps or other fittings shall be approved for general use without protection or shall be protected from ordinary physical damage as indicated in (a) or (b) below. (a) Not Likely to Be Damaged. Installations where they are not likely to be damaged. (b) Protective Covering. Enclosing in metal, wood, or equivalent protective covering. 250-118. Clean Surfaces. Nonconductive coatings (such as paint, lacquer, and enamel) on equipment to be grounded shall be removed from threads and other contact surfaces to assure good electrical continuity or be connected by means of fittings so designed as to make such removal unnecessary. 250-119. Identification of Wiring Device Terminals. The terminal for the connection of the equipment grounding conductor shall be identified by (1) a green-colored, not readily removable terminal screw with a hexagonal head; (2) a green-colored, hexagonal, not readily removable terminal nut; or (3) a green-colored pressure wire connector. If the terminal for the grounding conductor is not visible, the conductor entrance hole shall be marked with the word "green" or otherwise identified by a distinctive green color. L. Instrument Transformers, Relays, Etc. 250-121. Instrument Transformer Circuits. Secondary circuits of current and potential instrument transformers shall be grounded where the primary windings are connected to circuits of 300 volts or more to ground and, where on switchboards, shall be grounded irrespective of voltage. Exception: Circuits where the primary windings are connected to circuits of less than 1000 volts with no live parts or wiring exposed or accessible to other than qualified persons. 250-122. Instrument Transformer Cases. Cases or frames of instrument transformers shall be grounded where accessible to other than qualified persons. ARTICLE 250 - GROUNDING 70-137 Exception: Cases or frames of current transformers, the primaries of which are not over 150 volts to ground and that are used exclusively to supply current to meters. 250-123. Cases of Instruments, Meters, and Relays - Operating at Less than 1000 Volts. Instruments, meters, and relays operating with windings or working parts at less than 1000 volts shall be grounded as specified in (a), (b), or (c) below. (a) Not on Switchboards. Instruments, meters, and relays not located on switchboards, operating with windings or working parts at 300 volts or more to ground, and accessible to other than qualified persons, shall have the cases and other exposed metal parts grounded. (b) On Dead-Front Switchboards. Instruments, meters, and relays (whether operated from current and potential transformers or connected directly in the circuit) on switchboards having no live parts on the front of the panels shall have the cases grounded. (c) On Live-Front Switchboards. Instruments, meters, and relays (whether operated from current and potential transformers or connected directly in the circuit) on switchboards having exposed live parts on the front of panels shall not have their cases grounded. Mats of insulating rubber or other suitable floor insulation shall be provided for the operator where the voltage to ground exceeds 150. 250-124. Cases of Instruments, Meters, and Relays - Operating Voltage 1 kV and Over. Where instruments, meters, and relays have currentcarrying parts of 1 kV and over to ground, they shall be isolated by elevation or protected by suitable barriers, grounded metal, or insulating covers or guards. Their cases shall not be grounded. Exception: Cases of electrostatic ground detectors where the internal ground segments of the instrument are connected to the instrument case and grounded and the ground detector is isolated by elevation. 250-125. Instrument Grounding Conductor. The grounding conductor for secondary circuits of instrument transformers and for instrument cases shall not be smaller than No. 12 copper or No. 10 aluminum. Cases of instrument transformers, instruments, meters, and relays that are mounted directly on grounded metal surfaces of enclosures or grounded metal switchboard panels shall be considered to be grounded, and no additional grounding conductor shall be required. M. Grounding of Systems and Circuits of 1 kV and Over (High Voltage) 250-150. General. Where high-voltage systems are grounded, they shall comply with all applicable provisions of the preceding sections of this article and with the following sections, which supplement and modify the preceding sections. 70-138 NATIONAL ELECTRICAL CODE 250-151. Derived Neutral Systems. A system neutral derived from a grounding transformer shall be permitted to be used for grounding highvoltage systems. 250-152. Solidly Grounded Neutral Systems. (a) Neutral Conductor. The minimum insulation level for neutral conductors of solidly grounded systems shall be 600 volts. Exception No. 1: Bare copper conductors shall be permitted to be used for the neutral of service entrances and the neutral of direct buried portions of feeders. Exception No. 2: Bare conductors shall be permitted for the neutral of overhead portions installed outdoors. (FPN): See Section 225-4 for conductor covering where within 10 feet (3.05 m) of any building or other structure. (b) Multiple Grounding. The neutral of a solidly grounded neutral system shall be permitted to be grounded at more than one point for: (1) Services. (2) Direct buried portions of feeders employing a bare copper neutral. (3) Overhead portion installed outdoors. (c) Neutral Grounding Conductor. The neutral grounding conductor shall be permitted to be a bare conductor if isolated from phase conductors and protected from physical damage. 250-153. Impedance Grounded Neutral Systems. Impedance grounded neutral systems shall comply with the provisions of (a) through (d) below. (a) Location. The grounding impedance shall be inserted in the grounding conductor between the grounding electrode of the supply system and the neutral point of the supply transformer or generator. (b) Identified and Insulated. Where the neutral conductor of an impedance grounded neutral system is used, it shall be identified, as well as fully insulated with the same insulation as the phase conductors. (c) System Neutral Connection. The system neutral shall not be connected to ground, except through the neutral grounding impedance. (d) Equipment Grounding Conductors. Equipment grounding conductors shall be permitted to be bare and shall be connected to the ground bus and grounding electrode conductor at the service-entrance equipment and extended to the system ground. 250-154. Grounding of Systems Supplying Portable or Mobile Equipment. Systems supplying portable or mobile high-voltage equipment, other than substations installed on a temporary basis, shall comply with (a) through (f) below. (a) Portable or Mobile Equipment. Portable or mobile high-voltage equipment shall be supplied from a system having its neutral grounded through an impedance. Where a delta-connected high-voltage system is used to supply portable or mobile equipment, a system neutral shall be derived.