OVER 600 VOLTS, NOMINAL GENERAL

A. General 710-1. Scope. This article covers the general requirements for all circuits and equipment operating at more than 600 volts, nominal. 710-2. Definition: For the purposes of this article, "high voltage" shall be defined as more than 600 volts, nominal. 710-3. Other Articles. Provisions applicable to specific types of installations are included in the following articles: Article Busways 364 Cable Trays 318 Capacitors 460 Conductors for General Wiring 310 Definitions 100 Electric Signs and Outline Lighting 600 Flexible Cords and Cables 400 Grounding 250 Lighting Fixtures, Lampholders, Lamps, and Receptacles 410 Motors, Motor Circuits, and Controllers 430 Outlet, Device, Pull and Junction Boxes, Conduit Bodies, and Fittings 370 Outside Branch Circuits and Feeders 225 Overcurrent Protection 240 Requirements for Electrical Installations 110 Resistors and Reactors 470 Services 230 Surge Arresters 280 Transformers and Transformer Vaults 450 Wiring Methods 300 710-4. Wiring Methods. (a) Aboveground Conductors. Aboveground conductors shall be installed in rigid metal conduit, in intermediate metal conduit, in rigid nonmetallic conduit, in cable trays, as busways, as cablebus, in other identified raceways, or as open runs of metal-clad cable suitable for the use and purpose. In locations accessible to qualified persons only, open runs of Type MV cables, bare conductors, and bare busbars shall also be permitted. (b) Underground Conductors. Underground conductors shall be identified for the voltage and conditions under which they are installed. Direct burial cables shall comply with the provisions of Section 310-7. Underground cables shall be permitted to be direct buried or installed in raceways identified for the use and shall meet the depth requirements of Table 710-4(b). ARTICLE 710 - OVER 600 VOLTS, NOMINAL GENERAL 70-739 Nonshielded cables shall be installed in rigid metal conduit, in intermediate metal conduit, or in rigid nonmetallic conduit encased in not less than 3 inches (76 mm) of concrete. Exception No. 1: Type MC cable with nonshielded conductor where the metallic sheath is grounded through an effective grounding path meeting the requirements of Section 250-51. Exception No. 2: Lead-sheath cable with nonshielded conductor where the lead sheath is grounded through an effective grounding path meeting the requirements of Section 250-51. Table 710-4(b). Minimum Cover Requirements Cover is defined as the shortest distance in inches measured between a point on the top surface of any direct buried conductor, cable, conduit, or other raceway and the top surface of finished grade, concrete, or similar cover. Direct Rigid Nonmetallic Rigid Metal Conduit Circuit Buried Conduit Approved and Intermediate Voltage Cables for Direct Burial* Metal Conduit Over 600-22kV 30 18 6 Over 22kV-40kV 36 24 6 Over 40kV 42 30 6 For SI units: one inch = 25.4 millimeters. * Listed by a qualified testing agency as suitable for direct burial without encasement. All other nonmetallic systems shall require 2 inches (50.8 mm) of concrete or equivalent above conduit in addition to above depth. Exception No. 1: Areas subject to vehicular traffic, such as thoroughfares or commercial parking areas, shall have a minimum cover of 24 inches (610 mm). Exception No. 2: The minimum cover requirements for other than rigid metal conduit and intermediate metal conduit shall be permitted to be reduced 6 inches (152 mm) for each 2 inches (50.8 mm) of concrete or equivalent protection placed in the trench over the underground installation. Exception No. 3: The minimum cover requirements shall not apply to conduits or other raceways that are located under a building or exterior concrete slab not less than 4 inches (102 mm) in thickness and extending not less than 6 inches (152 mm) beyond the underground installation. A warning ribbon or other effective means suitable for the conditions shall be placed above the underground installation. Exception No. 4: Lesser depths shall be permitted where cables and conductors rise for terminations or splices or where access is otherwise required. Exception No. 5: In airport runways, including adjacent defined areas where trespass is prohibited, cable shall be permitted to be buried not less than 18 inches (457 mm) deep and without raceways, concrete enclosement, or equivalent. 70-740 NATIONAL ELECTRICAL CODE Exception No. 6: Raceways installed in solid rock shall be permitted to be buried at lesser depth where covered by 2 inches (50.8 mm) of concrete, which shall be permitted to extend to the rock surface. (1) Protection from Damage. Conductors emerging from the ground shall be enclosed in approved raceway. Raceways installed on poles shall be of rigid metal conduit, intermediate metal conduit, PVC Schedule 80, or equivalent extending from the ground line up to a point 8 feet (2.44 m) above finished grade. Conductors entering a building shall be protected by an approved enclosure from the ground line to the point of entrance. Metallic enclosures shall be grounded. (2) Splices. Direct burial cables shall be permitted to be spliced or tapped without the use of splice boxes, provided they are installed using materials suitable for the application. The taps and splices shall be watertight and protected from mechanical damage. Where cables are shielded, the shielding shall be continuous across the splice or tap. (3) Backfill. Backfill containing large rock, paving materials, cinders, large or sharply angular substance, or corrosive materials shall not be placed in an excavation where materials can damage raceways, cables, or other substructures or prevent adequate compaction of fill or contribute to corrosion of raceways, cables, or other substructures. Protection in the form of granular or selected material or suitable sleeves shall be provided to prevent physical damage to the raceway or cable. (4) Raceway Seal. Where a raceway enters from an underground system, the end within the building shall be sealed with an identified compound so as to prevent the entrance of moisture or gases, or it shall be so arranged to prevent moisture from contacting live parts. (c) Busbars. Busbars shall be permitted to be either copper or aluminum. 710-5. Braid-Covered Insulated Conductors - Open Installation. Open runs of braid-covered insulated conductors shall have a flame-retardant braid. If the conductors used do not have this protection, a flame-retardant saturant shall be applied to the braid covering after installation. This treated braid covering shall be stripped back a safe distance at conductor terminals, according to the operating voltage. This distance shall not be less than 1 inch (25.4 mm) for each kilovolt of the conductor-to-ground voltage of the circuit, where practicable. 710-6. Insulation Shielding. Metallic and semiconducting insulation shielding components of shielded cables shall be removed for a distance dependent on the circuit voltage and insulation. Stress reduction means shall be provided at all terminations of factory-applied shielding. Metallic shielding components such as tapes, wires or braids, or combinations thereof and their associated conducting or semiconducting components shall be grounded. 710-7. Grounding. Wiring and equipment installations shall be grounded in accordance with the applicable provisions of Article 250. 710-8. Moisture or Mechanical Protection for Metal-Sheathed Cables. Where cable conductors emerge from a metal sheath and where protection ARTICLE 710 - OVER 600 VOLTS, NOMINAL GENERAL 70-741 against moisture or physical damage is necessary, the insulation of the conductors shall be protected by a cable sheath terminating device. 710-9. Protection of Service Equipment, Metal-Enclosed Power Switchgear, and Industrial Control Assemblies. Pipes or ducts foreign to the electrical installation that require periodic maintenance or whose malfunction would endanger the operation of the electrical system shall not be located in the vicinity of the service equipment, metal-enclosed power switchgear, or industrial control assemblies. Protection shall be provided where necessary to avoid damage from condensation leaks and breaks in such foreign systems. Piping and other facilities shall not be considered foreign if provided for fire protection of the electrical installation. B. Equipment - General Provisions 710-11. Indoor Installations. See Section 110-31(a). 710-12. Outdoor Installations. See Section 110-31(b). 710-13. Metal-Enclosed Equipment. See Section 110-31(c). 710-14. Oil-Filled Equipment. Installation of electrical equipment, other than transformers, covered in Article 450, containing more than 10 gallons (37.85 L) of flammable oil per unit shall meet the requirements of Parts B and C of Article 450. C. Equipment - Specific Provisions (FPN): See also references to specific types of installations in Section 710-3. 710-20. Overcurrent Protection. Overcurrent protection shall be provided for each ungrounded conductor by one of the following: (a) Overcurrent Relays and Current Transformers. Circuit breakers used for overcurrent protection of ac 3-phase circuits shall have a minimum of three overcurrent relays operated from three current transformers. Exception No. 1: On 3-phase, 3-wire circuits, an overcurrent relay in the residual circuit of the current transformers shall be permitted to replace one of the phase relays. Exception No. 2: An overcurrent relay, operated from a current transformer that links all phases of a 3-phase, 3-wire circuit, shall be permitted to replace the residual relay and one of the phase conductor current transformers. Where the neutral is not regrounded on the load side of the circuit as permitted in Section 250-152(b), the current transformer shall be permitted to link all three phase conductors and the grounded circuit conductor (neutral). (b) Fuses. A fuse shall be connected in series with each ungrounded conductor. 70-742 NATIONAL ELECTRICAL CODE 710-21. Circuit-Interrupting Devices. (a) Circuit Breakers. (1) Indoor installations shall consist of metal-enclosed units or fireresistant cell-mounted units. Exception: Open mounting of circuit breakers shall be permitted in locations accessible to qualified persons only. (2) Circuit breakers used to control oil-filled transformers shall be either located outside the transformer vault or be capable of operation from outside the vault. (3) Oil circuit breakers shall be so arranged or located that adjacent readily combustible structures or materials are safeguarded in an approved manner. $ (4) Circuit breakers shall have the following equipment or operating characteristics: a. An accessible mechanical or other approved means for manual tripping, independent of control power. b. Be release free (trip free). c. If capable of being opened or closed manually while energized, the main contacts shall operate independently of the speed of the manual operation. d. A mechanical position indicator at the circuit breaker to show the open or closed position of the main contacts. e. A means of indicating the open and closed position of the breaker at the point(s) from which they may be operated. f. A permanent and legible nameplate showing manufacturer's name or trademark, manufacturer's type or identification number, continuous current rating, interrupting rating in mVA or amperes, and maximum voltage rating. Modification of a circuit breaker affecting its rating(s) shall be accompanied by an appropriate change of nameplate information. (5) The continuous current rating of a circuit breaker shall be not less than the maximum continuous current through the circuit breaker. (6) The interrupting rating of a circuit breaker shall not be less than the maximum fault current the circuit breaker will be required to interrupt, including contributions from all connected sources of energy. (7) The closing rating of a circuit breaker shall not be less than the maximum asymmetrical fault current into which the circuit breaker can be closed. (8) The momentary rating of a circuit breaker shall not be less than the maximum asymmetrical fault current at the point of installation. (9) The rated maximum voltage of a circuit breaker shall not be less than the maximum circuit voltage. (b) Power Fuses and Fuseholders. (1) Use. Where fuses are used to protect conductors and equipment, a fuse shall be placed in each ungrounded conductor. Two power fuses shall be permitted to be used in parallel to protect the same load, if both fuses have identical ratings, and both fuses are installed in an identified common mounting with electrical connections that will divide the current equally. ARTICLE 710 - OVER 600 VOLTS, NOMINAL GENERAL 70-743 Power fuses of the vented type shall not be used indoors, underground, or in metal enclosures unless identified for the use. (2) Interrupting Rating. The interrupting rating of power fuses shall not be less than the maximum fault current the fuse will be required to interrupt, including contributions from all connected sources of energy. (3) Voltage Rating. The maximum voltage rating of power fuses shall not be less than the maximum circuit voltage. Fuses having a minimum recommended operating voltage shall not be applied below this voltage. (4) Identification of Fuse Mountings and Fuse Units. Fuse mountings and fuse units shall have permanent and legible nameplates showing the manufacturer's type or designation, continuous current rating, interrupting current rating, and maximum voltage rating. (5) Fuses. Fuses that expel flame in opening the circuit shall be so designed or arranged that they will function properly without hazard to persons or property. (6) Fuseholders. Fuseholders shall be designed or installed so that they will be deenergized while replacing a fuse. Exception: Fuse and fuseholder designed to permit fuse replacement by qualified persons using equipment designed for the purpose without deenergizing the fuseholder. (7) High-Voltage Fuses. Metal-enclosed switchgear and substations that utilize high-voltage fuses shall be provided with a gang-operated disconnecting switch. Isolation of the fuses from the circuit shall be provided by either connecting a switch between the source and the fuses or providing roll-out switch and fuse-type of construction. The switch shall be of the load-interrupter type, unless mechanically or electrically interlocked with a load-interrupting device arranged to reduce the load to the interrupting capability of the switch. Exception: More than one switch shall be permitted as the disconnecting means for one set of fuses where the switches are installed to provide connection to more than one set of supply conductors. The switches shall be mechanically or electrically interlocked to permit access to the fuses only when all switches are open. A conspicuous sign shall be placed at the fuses reading: "WARNING -FUSES MAY BE ENERGIZED FROM MORE THAN ONE SOURCE. (c) Distribution Cutouts and Fuse Links - Expulsion Type. (1) Installation. Cutouts shall be so located that they may be readily and safely operated and re-fused, and so that the exhaust of the fuses will not endanger persons. Distribution cutouts shall not be used indoors, underground, or in metal enclosures. (2) Operation. Where fused cutouts are not suitable to interrupt the circuit manually while carrying full load, an approved means shall be installed to interrupt the entire load. Unless the fused cutouts are interlocked with the switch to prevent opening of the cutouts under load, a conspicuous sign shall be placed at such cutouts reading, "WARNING - DO NOT OPEN UNDER LOAD." 70-744 NATIONAL ELECTRICAL CODE (3) Interrupting Rating. The interrupting rating of distribution cutouts shall not be less than the maximum fault current the cutout will be required to interrupt, including contributions from all connected sources of energy. (4) Voltage Rating. The maximum voltage rating of cutouts shall not be less than the maximum circuit voltage. (5) Identification. Distribution cutouts shall have on their body, door, or fuse tube a permanent and legible nameplate or identification showing the manufacturer's type or designation, continuous current rating, maximum voltage rating, and interrupting rating. (6) Fuse Links. Fuse links shall have a permanent and legible identification showing continuous current rating and type. (7) Structure Mounted Outdoors. The height of cutouts mounted outdoors on structures shall provide safe clearance between lowest energized parts (open or closed position) and standing surfaces, in accordance with