Document 65OXgJx7dqam86oJkjYMrMqng

National Electrical Code 1968 Edition A USA Standard NFPA No. 70-1968; USAS Cl-1968 (Rev. of Cl-1965) 70-1 1968 Edition of National Electrical Code This 1968 Edition of the National Electrical Code (N FPA No. 701968; USAS Cl-1968) was adopted by the National Fire Protection As sociation at its Annual Meeting held May 20-24, 1968 in Atlanta, Georgia and approved by the United States of America Standards Insti tute on July 11, 1968. It supersedes all previous editions of the National Electrical Code, the most recent previous edition being 1965. Changes in this 1968 Edition of the National Electrical Code (as compared with the 1965 Edition) are indicated by vertical marginal rules on the affected pages. In certain cases, minor editorial changes are not so indicated. Special attention is called to the inclusion in this 1968 Edition of Tentative Interim Amendments Nos. 146 and 148 appearing in the Ap pendix. Attention is also called to the Appendix for the "Time Schedule for the 1971 National Electrical Code" which is the next regular edition. The "Method of Submitting Proposal to Revise the National Electrical Code" is included in the Appendix to guide anyone concerned with the proper procedures for handling proposed revisions. This Code is purely advisory as far as the NFPA and the USASI are concerned but is offered for use in law and for regulatory purposes in the interest of life and property protection (see copyright statement on Page 70-ii). Anyone noticing any errors should please notify the NFPA Executive Office, the Chairman and the Secretary of the Committee. This edition is also published by the NFPA in the 1968-69 edition of Volume 5 of the National Fire Codes and will appear in subsequent yearly editions of this Volume until superseded by a later edition. Origin and Development of National Electrical Code Consistent with the Rules of Procedure for the National Electrical Code Committee (see Appendix) a "Preprint" of the Proposed A mendments for the 1968 N ational E lectrical Code was pub lished by the National Fire Protection Association in July 1967. This "Preprint" recorded the actions of the various Code-Making Panels and the Correlating Committee of the National Electrical Code Com mittee on each proposal that had been made to revise the 1965 Code. Subsequently, the various Code-Making Panels finalized their recom mendations and their reports were processed through the Correlating Committee of the National Electrical Code Committee. This was pre paratory to the formal presentation of this 1968 Edition of the National Electrical Code in Volume 2 of the 1968 N FPA Technical Com mittee Reports. This latter publication was circulated to all members of the National Electrical Code Committee, to other interested NFPA members, and offered to the public through technical releases in the in- 70-n NATIONAL ELECTRICAL CODE terested trade papers. The action of the 1968 NFPA Annual Meeting as to the adoption of the 1968 Edition is recorded in the Proceedings of that Meeting published by the Association in the September 1968 issue of Fire Journal. Coincident with the above processing procedures, each of the CodeMaking Panels and the Chairman of the Correlating Committee re ported their recommendations to meetings of the Electrical Section at both the 1967 and 1968 NFPA Annual Meetings. The Electrical Section (See Rules of Procedure, Appendix) thus had opportunity to discuss and review the report of the National Electrical Code Committee prior to the adoption of this Edition of the Code by the Association. Other Printings This 1968 Edition of the National Electrical Code is also printed by the National Fire Protection Association in Volume 5 of the N ational Fire C odes. This Volume also contains the N FPA Standard for the Pro tection of Electronic Computer/Data Processing Equipment (NFPA No. 75), the Standard for Essential Electrical Systems for Hospitals (NFPA No. 76), the Electrical Standard for Metalworking Machine Tools (NFPA No. 79), and the Standard for Purged Enclosures for Electrical Equipment in Hazardous Locations (NFPA No. 496). This Volume (one of ten) is issued annually with the other 190 Standards of the Association and provides those interested with a desk and library compendium of all current Standards issued by the Association. A "One- and Two-Family Residential Occupancy Electrical Code" is being prepared under authority granted by the Correlating Committee of the National Electrical Code Committee. This document is an abridged version of this text, edited only as dictated to eliminate extraneous material not of concern to this type occupancy, and to place in the text only the more popular types of wiring methods, not to exclude any other type authorized by the complete Code. The Compson Code Company of Lansing, Michigan offers a sepa rate printing of this Edition of the Code under an agreement with the Association. The N FPA Handbook of the N ational Electrical C ode is pub lished by The McGraw-Hill Book Company, Inc., 330 West 42nd Street, New York, New York 10036. This text is prepared by the Secretary of the National Electrical Code Committee who is the NFPA Electrical Field Service Engineer. Copyright 1968 National Fire Protection Association This publication is copyrighted () by the National Fire Protection Asso ciation. Permission is granted to republish in full the material herein in laws, ordinances, regulations, administrative orders or similar documents issued by public authorities. All others desiring permission to reproduce this ma terial in whole or in part shall consult the National Fire Protection Asso ciation. Printed in U .S .A . 70-82 Chapter 3. Wiring Methods and Materials ARTICLE 300 -- WIRING METHODS -- GENERAL REQUIREMENTS 300-1. Scope. (a) The provisions of Chapter 3 apply to the electrical and mechani cal requirements for the various methods of installing fixed electrical conductors for electric light, heat, power, and certain signal systems. (b) The provisions of this Article shall apply to all wiring installa tions, except for remote-control, including low voltage relay switching, low-energy power and signal systems as provided in Article 725, and communication systems as provided in Article 800. (c) On premises where a continuous underground metallic water piping network system is not available as a grounding electrode, and where it is not practicable otherwise to secure a ground of permanently low resistance, the use of a wiring method which does not employ metal enclosures for the wires is recommended, unless the character or occupancy of the building is such as to require the use of a metal-en closed wiring system. (d) The provisions of this Article are not intended to apply to the conductors which form an integral part of equipment such as motors, motor controllers and the like. 300-2. Voltage Limitations. Wiring methods specified in Chapter 3 may be used for voltages not exceeding 600, unless specifically limited in some Article of Chapter 3. They may be used for voltages over 600 where specifically permitted elsewhere in this code. 300-3. Conductors of Different Systems. (a) Conductors of light and power systems of 600 volts or less may occupy the same enclosure, without regard to whether the individual circuits are alternating-current or direct-current, only where all con ductors are insulated for the maximum voltage of any conductor within the enclosure. (b) Conductors of light and power systems of over 600 volts shall not occupy the same enclosure with conductors of light and power systems of 600 volts or less. (e) Secondary wiring to electric discharge lamps of 1,000 volts or less, insulated for the secondary voltage involved, may occupy the same fixture enclosure as the branch circuit conductors. (d) Primary leads of electric discharge lamp ballasts, insulated for the primary voltage of the ballast, when contained within the individual wiring enclosure may occupy the same fixture enclosure as the branch circuit conductors. ARTICLE 300-W IRIN G METHODS-GENERAL 70-83 (e) Excitation, control, relay and ammeter conductors used in con nection with any individual motor or starter may occupy the same en closure as the motor circuit conductors. (f) Conductors of signal or radio systems shall not occupy the same enclosure with conductors of light or power systems except as permit ted for elevators in Section 620-36; for sound recording in Section 640-6; for remote-control, low-energy power and signal circuits in Sections 725-16 and 725-42; and communication system in Sections 800-3 and 800-21. 300-4. Protection Against Physical Damage. Where subject to physical damage, conductors shall be adequately protected. 300-5. Protection Against Corrosion. Metal raceways, cable armor, boxes, cable sheathing, cabinets, metallic elbows, couplings, fittings, sup ports and support hardware shall be of materials suitable for the en vironment in which they are to be installed. (a) Ferrous raceways, cable armor, boxes, cable sheathing, cabinets, metallic elbows, couplings, fittings, supports and support hardware shall be suitably protected against corrosion inside and outside (except threads at joints) by a coating of approved corrosion resistant material such as zinc, cadmium, or enamel. Where protected from corrosion solely by enamel, they shall not be used out of doors or in wet locations as described in (c) below. When boxes or cabinets have an approved system of organic coatings and are marked "Raintight" or "Outdoor Type" they may be used out of doors. (b) Unless made of materials judged suitable for the condition, or unless corrosion protection approved for the condition is provided, fer rous or non-ferrous metallic raceways, cable armor, boxes, cable sheathing, cabinets, elbows, couplings, fittings, supports and support hardware shall not be installed in concrete or in direct contact with the earth, or in areas subject to severe corrosive influences. (c) In portions of dairies, laundries, canneries, and other indoor wet locations, and in locations where walls are frequently washed or where there are surfaces of absorbent materials, such as damp paper or wood, the entire wiring system, including all boxes, fittings, conduits and cable used therewith, shall be mounted so that there is at least onequarter inch air space between it and the wall or supporting surface. Meat-packing plants, tanneries, hide cellars, casing rooms, glue houses, fertilizer rooms, salt storage, some chemical works, metal refineries, pulp mills, sugar mills, round houses, some stables, and similar locations are judged to be occupancies where severe corrosive conditions are likely to be present. 300-6. Raceways Exposed to Different Temperatures. (a) Sealing. Where portions of an interior raceway system are ex posed to widely different temperatures, as in refrigerating or coldstorage plants, provision shall be made to prevent circulation of air from a warmer to a colder section through the raceway. (b) Expansion Joints. Expansion joints for runs of raceway shall be provided where required to compensate for thermal expansion and contraction. 70-84 NATIONAL ELECTRICAL CODE 300-7. Underground Runs. Conductors run underground shall comply with the provisions of Section 230-32 as far as mechanical protection is concerned. Underground cable run under a building shall be in a raceway that is extended beyond the outside wall of the building. 300-8. Through Studs, Joists and Rafters. (a) Where exposed or concealed wiring conductors in insulating tubes or cables are installed through bored holes in studs, joists or similar wood members, holes shall be bored at the approximate centers of wood members, or at least two inches from the nearest edge where practical. (b) Where there is no objection because of weakening the building structure, metal-clad or nonmetallic sheathed cable, aluminum sheathed cable and Type MI cable may be laid in notches in the studding or joists when the cable at those points is protected against the driving of nails into it by having the notch covered with a steel plate at least 1/16 inch in thickness before building finish is applied. 300-9. Grounding Metal Enclosures. Metal raceways, boxes, cabinets, cable armor and fittings shall be grounded if and as prescribed in Article 250. 300-10. Electrical Continuity of Metal Raceways and Enclosures. Metal raceways, cable armor, and other metal enclosures for conductors, shall be metallically joined together into a continuous electrical con ductor, and shall be so connected to all boxes, fittings and cabinets as to provide effective electrical continuity. Raceways and cable assem blies shall be mechanically secured to boxes, fittings, cabinets and other enclosures, except as provided for nonmetallic boxes in Section 370-7. 300-11. Secured in Place. Raceways, cable assemblies, boxes, cabi nets and fittings shall be securely fastened in place, unless otherwise provided for specific purposes elsewhere in this Code. See Article 318 for Continuous Rigid Cable Supports. 300-12. Mechanical Continuity--Raceways and Cables. Raceways and cable assemblies shall be continuous from outlet to outlet and from fit ting to fitting. 300-13. Mechanical Continuity--Conductors. Conductors shall be contin uous between outlets, devices, etc., and, except as permitted for auxiliary gutters in Section 374-8, and for wireways in Section 362-6, there shall be no splice or tap within a raceway itself. 300-14. Free Length of Conductors at Outlets and Switch Points. A t least six inches of free conductor shall be left at each outlet and switch point for the making up of joints or the connection of fixtures or devices, except where conductors are intended to loop without joints through lampholders, receptacles and similar devices. 300-15. Boxes or Fittings. Except as permitted in Sections 336-11 and 410-60 a box or fitting shall be installed at each outlet, switch point, ARTICLE 300-W IRIN G METHODS-GENERAL 70-85 junction point or conductor splice connection joining sections of con duit, electrical metallic tubing or surface raceway. A box shall be in stalled at each outlet, switch point, or conductor splice connection joining sections of metal-clad cable, aluminum sheathed cable, nonmetallic sheathed cable or Type MI cable, and at each outlet and switch point of concealed knob-and-tube work. Exception: Straight through splice joints for M l cable are permitted without a box provided the splice is accessible and a fitting approved for the purpose is used. 300-16. Raceway or Cable to Open or Concealed Wiring. (a) A box or terminal fitting having a separately bushed hole for each conductor shall be used wherever a change is made from conduit, electrical metallic tubing, nonmetallic sheathed cable, metal-clad ca ble, aluminum sheathed cable, or Type MI cable and surface raceway wiring to open wiring or to concealed knob-and-tube work. A fitting used for this purpose shall contain no taps or spices and shall not be used at fixture outlets. (b) A bushing may be used in lieu of a box or terminal fitting at ends of conduit or electrical metallic tubing where conductors leave the conduit or tubing behind a switchboard, or where more than 4 con ductors leave the conduit or tubing at control apparatus or in similar locations, in which case the conductors shall be bunched, taped and painted with insulating paint. Such a bushing shall be of the insulating type except for lead-covered conductors. 300-17. Number of Conductors in Raceway. In general the percent age of the total interior cross-sectional area of a raceway occupied by conductors shall not be more than will permit a ready installation or withdrawal of the conductors and dissipation of the heat generated without injury to the insulation of the conductors. See the follow ing Sections of this Code: conduit, Section 346-6; electrical metallic tubing, 348-6; flexible metal conduit, 350-3; surface raceways, 352-4 and 352-25; underfloor raceways, 354-5; cellular metal floor raceways, 356-5; structural raceways, 357-6; cellular concrete floor raceways, 358-9; wireways, 362-5; auxiliary gutters, 374-5; theatres, 520-5; signs, 600-21 (d ); elevators, 620-33; and sound recording, 640-3 and 640-4; and Remote-Control, Low-Energy Power, Low-Voltage Power and Signal Circuits, Article 725. 300-18. Inserting Conductors in Raceways. (a) Raceways shall first be installed as a complete raceway system without conductors, except those raceways exposed and having a re movable cover or capping. (b) As far as possible, conductors shall not be inserted until the in terior of the building has been physically protected from the weather, and all mechanical work on the building which is likely to injure the conductors has been completed. (c) Pull wires, if to be used, shall not be installed until the raceway system is in place. (d) Cleaning agents or materials used as lubricants that might have a deleterious effect on conductor coverings shall not be used. 70-86 NATIONAL ELECTRICAL CODE 300-19. Supporting Conductors in Vertical Raceways. (a) Conductors in vertical raceways shall be supported at intervals not greater than those specified in the following Table: Table 300-19{a). Spacings for Conductor Supports No. 18 to No. 8 .......... No. 6 to No. 0 .......... No. 00 to No. 0000. .. 211,601 CM to 350,000 CM 350.001 CM to 500,000 CM 500.001 CM to 750,000 CM Above 750,000 CM Conductors Aluminum Copper Not Greater than. . . . 100 feet " " " ....2 0 0 feet "" " 180feet " " " . . . . 135 feet " " " ....1 2 0 feet " " " . . . . 95 feet "" " 85feet 100 feet 100 feet 80 feet 60 feet 50 feet 40 feet 35 feet (b) One of the following methods of support, or a method of equal effectiveness is recommended: (1) By clamping devices constructed of or employing insulating wedges inserted in the ends of the conduits. With cables having var nished cambric or thermoplastic insulation it may also be necessary to clamp the conductor. (2) By inserting boxes at the required intervals in which insulating supports are installed and secured in a satisfactory manner to with stand the weight of the conductors attached thereto, the boxes being provided with covers. (3) In junction boxes, by deflecting the cables not less than 90 degrees and carrying them horizontally to a distance not less than twice the diameter of the cable, the cables being carried on two or more in sulating supports, and additionally secured thereto by tie wires if de sired. When this method is used cables shall be supported at intervals not greater than 20 per cent of those mentioned in the preceding tabu lation. 300-20. Induced Currents in Metal Enclosures. When conductors car rying alternating current are installed in metal enclosures they shall be so arranged as to avoid heating the surrounding metal by induction. To accomplish this all phase conductors and the neutral, where one is used, shall be grouped together. When a single conductor of a circuit passes through metal with magnetic properties the inductive effect shall be minimized by: (1) Cutting slots in the metal between the individual holes through which the individual conductors pass, or (2) Passing all the conductors in the circuit through an insulating wall sufficiently large for all of the conductors of the circuit. Exception: In the case of circuits supplying vacuum or electric dis charge lighting systems or signs, or X-ray apparatus, the currents car ried by the conductors are so small that the inductive heating effect may be ignored where these conductors are placed in metal enclosures or pass through metal. ARTICLE 300-W IRIN G METHODS--GENERAL 70-87 Aluminum being a nonmagnetic metal, there .will be no heating due to eddy currents, however, induced currents will be present. These are not considered of sufficient magnitude to require grouping of conductors or special treatment in passing conductors through aluminum wall sections. 300-21. Prevention of Spread of Fire. Electrical installations shall be so made that the possible spread of fire through fire-stopped partitions, hollow spaces, fire walls or fire partitions, vertical shafts, ventilating or air-handling ducts is reduced to a minimum. 300-22. Wiring in Ducts, Plenums, and Other Air Handling Spaces. (a) No wiring systems of any type shall be installed in ducts used to transport dust, loose stock, flammable vapors, nor shall any wiring sys tem of any type be installed in any duct, or shaft containing only such ducts, used for vapor removal or ventilation of commercial type cooking equipment. (b) Wiring systems of MI cable, ALS cable, electrical metallic tub ing, or rigid metal conduit may be installed in ducts or plenum chambers used to transport higher velocity environmental air. Flexible metal con duit may be used, in lengths not to exceed 4 feet, to connect physically adjustable equipment and devices permitted to be in these ducts and plenum chambers. The connectors used with flexible metal conduit shall effectively close any openings in the connection. Equipment and devices may only be installed within such ducts or plenum chambers if neces sary for their direct action upon, or sensing of, the contained air. Where equipment or devices are installed and illumination is necessary to fa cilitate maintenance and repair, enclosed gasketed type fixtures may be installed. (c) Hollow spaces which are used as ducts or plenum chambers for lower velocity environmental air, other than those described in Para graph (b) above, may contain MI cable, ALS cable, electrical metallic tubing, rigid metal conduit, flexible metal conduit, and other electrical equipment that is permitted within the concealed spaces of such struc tures, provided: (1) The wiring materials, including fixtures, are suitable for the expected ambient temperature to which they will be subjected. (2) The wiring system, including fixtures, in no way reduces the fire protective rating of the construction in which they are installed. (3) Flexible metal conduit shall be limited to the connection of de vices, equipment, and fixtures in lengths not exceeding six feet. The con nectors used with flexible metal conduit shall effectively close any open ings in the connection. Exception No. /. The above provisions shall not apply to integral fan systems specifically approved for the purpose. Exception No. 2. This section does not include habitable rooms or areas of buildings, the prime purpose of which is not air handling. NFPA Standard for the Installation of Air Conditioning and Ventilating Systems, No. 90A, sets forth requirements of building used for ducts and plenums. (d) The wiring systems used for data processing systems and located 70-88 NATIONAL ELECTRICAL CODE within air handling areas created by raised floors shall conform to Article 645. 300-23. Temporary Wiring. (a) Suitable disconnecting switches or plug connectors shall be in stalled to permit the disconnection of all conductors of the temporary circuit by a single operation. (b) No bare conductors nor earth returns shall be used for the wiring of any temporary circuit. ARTICLE 310 -- CONDUCTORS FOR GENERAL WIRING 310-1. General. (a) The purpose of this Article is to assure that conductors have mechanical strength, insulation, and ampacity adequate for the particu lar conditions under which they are to be used. (b) Conductors shall be insulated, except when covered or bare con ductors are specifically permitted in this Code. (c) The provisions of this Article are not intended to apply to con ductors which form an integral part of equipment such as motors, mo tor controllers, and the like, or which are provided for elsewhere in this Code. 310-2. Application and Construction. (a) Conductor Application. Conductor insulations as specified in the following Table 310-2(a) may be installed for any of the wiring meth ods recognized in this Chapter, except as otherwise provided for in the Table or in Section 310-3, or as otherwise specified in this Code. They are suitable for 600 volts unless otherwise specified. (b) Conductor Construction. Insulated conductors for use at 600 volts or less shall conform to the provisions of Table 310-2(b). (c) Marking. Insulated wires, cables and cords of all kinds except paper-insulated wire shall have a continuous distinctive marking so that their maker may be readily identified. All wires, cables and cords shall also be plainly tagged or marked as follows: (1) The maximum working voltage for which the wire was tested or approved. This may be omitted for asbestos-covered switchboard wires. (2) The words "National Electrical Code Standard," or "NEC Std." (3) The manufacturer's name, trade-mark, or other distinctive marking, by which the organization responsible for the product may be readily identified. (4) Month and year when manufactured. (5) The proper type letter for the particular style of wire or cable as given in the following sections. (d) Surface Marking. A durable marking on the surface shall be provided at intervals not exceeding 24 inches, by which the maker may be identified. Such marking shall be the manufacturer's name, trade- (Continued on page 96) ARTICLE 310-CONDUCTORS Table 310-2(a). Conductor Application 70-89 Trade Name Type Letter Max. Operating Temp. Application Provisions Rubber-Covered Fixture Wire Solid or 7-Strand *RF-1 *RF-2 60C 140F 60C 140F Fixture wiring. Limited to 300 V. Fixture wiring, and as permitted in Section 310-8. Rubbered-Covered Fixture Wire Flexible Stranding *FF-1 *FF-2 Heat-Resistant Rubber-Covered Fixture Wire Solid or 7-Strand * RFH-1 *RFH-2 60C 140F 60C 140F 75C 167F 75C 167F Fixture wiring. Limited to 300 V. Fixture wiring, and as permitted in Section 310-8. Fixture wiring. Limited to 300 V. Fixture wiring, and as permitted in Section 310-8. Heat-Resistant Rubber-Covered Fixture Wire Flexible Stranding *FFH-1 'FFH-2 75C 167F 75C 167F Fixture wiring. Limited to 300 V. Fixture wiring, and as permitted in Section 310-8. ThermoplasticCovered Fixture Wire--Solid or Stranded *TF 60 C Fixture wiring, and as permitted 140F in Section 310-8, and for circuits as permitted in Article 725. ThermoplasticCovered Fixture Wire--Flexible Stranding *TFF 60C 140F Fixture wiring, and as permitted in Section 310-8, and for circuits as permitted in Article 725. Heat Resistant, Thermoplastic-- Covered Fixture Wire--Solid or Stranded *TFN 90C Fixture wiring, and as permitted in Section 310-8. Heat Resistant Thermoplastic-- Covered Fixture Wire-- Flexible Stranding *TFFN 90C \ Fixture wiring, and as permitted in Section 3 10-8. Cotton-Covered, Heat-Resistant, Fixture Wire *CF 90C Fixture wiring. 194F Limited to 300 V. * Fixture wires are not intended for installation as branch circuit conduc tors except as permitted in Article 725. 70-90 NATIONAL ELECTRICAL CODE Table 3 T0-2(a)--Continued ____________________ ....................................................................................................................._.......... ................. Trade Name Type Letter Max. Operating Temp. .. Application Provisions Asbestos-Covered 150C Fixture wiring. Heat-Resistant, *AF 302F Limited to 300 V. and Indoor Fixture Wire Dry Location. Fluorinated Ethylene Propylene Fixture Wire Solid or 7 Strand *PF *PGF 150C 302 F Fixture Wiring and as permitted in Section 310-8. Fluorinated Ethylene Propylene Fixture Wire *PFF *PGFF 150C 302 F Fixture Wiring and as permitted in Section 310-8. Silicone Rubber Insulated Fixture Wire Solid or 7 Strand *SF-1 *SF-2 200C 392F 200 C 392F Fixture wiring. Limited to 300 V. Fixture wiring and as permitted in Section 310-8. Silicone Rubber Insulated Fixture Wire Flexible Stranding *SFF-1 *SFF-2 150C 302F 150C 302F Fixture wiring. Limited to 300 V. Fixture wiring and as permitted in Section 310-8. Heat-Resistant Rubber RH 75C Dry locations. 167F Heat-Resistant Rubber RHH 90C Dry locations. 194F Moisture and Heat-Resistant Rubber RHW 75C 167F Dry and wet locations. For over 2000 volts, insulation shall be ozone-resistant. Heat-Resistant Latex Rubber RUH 75C Dry locations. Moisture Resistant RUW Latex Rubber 60C Dry and wet locations. 140F Thermoplastic T 60C Dry locations. 140F MoistureResistant Thermoplastic TW 60C Dry and wet locations. 140F * Fixture wires are not intended for installation as branch circuit conduc tors except as permitted in Article 725. ARTICLE 310-CONDUCTORS 70-91 Table 3 10-2(a)--Continued Trade Name Heat-Resistant Thermoplastic Type Letter THHN Max. Operating Temp. 90C 194F Application Provisions Dry locations. Moisture and Heat-Resistant Thermoplastic THW 75C Dry and wet locations. 167F Moisture and Heat-Resistant Thermoplastic THWN 75C Dry and wet locations. 167F Moisture and Heat-Resistant Cross-Linked Thermosetting Polyethylene XHHW 90C 194F 75C 167F Dry locations. Wet locations. Moisture-, Heat- and Oil-Resistant Thermoplastic MTW Moisture-, THW-MTW Heat- and Oil-Resistant Thermoplastic 60 C 140F 90C 194F 75C 167F Wet locations, Machine Tool Wir ing. (see Article 670 and NFPA Standard No. 79). Dry locations. Machine Tool Wiring, (see Article 670 and NFPA Standard No. 79). Dry and wet locations. 90 C 194F Special applications within elec tric discharge lighting equipment. Limited to 1000 open-circuit volts or less. (Size 14-8 only.) Thermoplastic and Asbestos TA 90C Switchboard wiring only. 194F Thermoplastic and Fibrous Outer Braid TBS 90C Switchboard wiring only. 194F Synthetic Heat-Resistant Mineral Insulation (Metal Sheathed) SIS 90C Switchboard wiring only. 194F MI 85C Dry and wet locations with Type 185F O termination fittings. 250C For special application. 482F 70-92 NATIONAL ELECTRICAL CODE Table 3 10-2(a)--Continued Trade Name Type Letter Max. Operating Temp. Application Provisions SiliconeAsbestos Fluorinated Ethylene Propylene Varnished Cambric Asbestos and Varnished Cambric Asbestos and Varnished Cambric Asbestos and Varnished Cambric Asbestos Asbestos Asbestos Asbestos Paper SA FEP or FEPB V AVA 90C 194F 125C 257F 90C 194F 200C 392F 85C 185F 110C 230F Dry locations. For special application. Dry locations. Dry locations--special applica tions. Dry locations only. Smaller than No. 6 by special permission. Dry locations only. AVL 110C 230F Dry and wet locations. AVB 90C Dry locations only. 194F A 200C Dry locations only. Only for leads 392F within apparatus or within race ways connected to apparatus. Limited to 300 V. AA 200C Dry locations only. Only for leads 392F within apparatus or within race ways connected to apparatus or as open wiring. Limited to 300 V. AI 125C Dry locations only. Only for leads 257F within apparatus or within race ways connected to apparatus. Limited to 300 V. AIA 125C Dry locations only. Only for leads 257F within apparatus or within race ways connected to apparatus or as open wiring. 85C For underground service conduc 185F tors, or by special permission. ARTICLE 310-CONDUCTORS 70-93 Table 310-2(b). Conductor Insulations Trade Name Type letter Insulation Thickness of Insulation Outer Covering HeatResistant RH HeatRHH Resistant Rubber **14-12 ................. 2/64 Inch '"Moisture- 1 0 .............................3/64 Inch resistant, 8-2 ...........................4/64 Inch flame1-4/0 ....................... 5/64 Inch retardant, 213-500 ................. 6/64 Inch non- 501-1000 ................. 7/64 Inch metallic 1001-2000 ............. 8/64 Inch covering Moisture and HeatResistant HeatResistant Latex Rubber RHW Moisture and Heat Resistant Rubber RUH 90% Unmilled, Grainless Rubber 14-10 ..................... 3/64 Inch "Moisture- 8 - 2 ...........................4/64 Inch resistant, 1-4/0 ..................... 5/64 Inch flame213-500 ................... 6/64 Inch retardant, 501-1000 ................. 7/64 Inch non1001-2000 ............... 8/64 Inch metallic covering 14-10...........................18 Mils 8 -2 .............................. 25 Mils Moistureresistant, flameretardant, non- metallic covering Moisture- Resistant Latex Rubber RUW 90% Un milled, Grainless Rubber 14-10...........................18 Mils 8-2 .............................. 25 Mils Moistureresistant, flameretardant, nonmetallic covering Thermo plastic T Flame- 14-10 .......................2/64 Inch Retardant, 8 ............................ 3/64 Inch Thermo 6-2 ........... ............. 4/64 Inch plastic 1-4/0 .....................5/64 Inch None Compound 213-500 ................... 6/64 Inch 501-1000 ................. 7/64 Inch 1001-2000 ............... 8/64 Inch MoistureResistant Thermo plastic TW Flame- 14-10 ..................... 2/64 Inch Retardant, 8 ............................ 3/64 Inch Moisture- 6-2 ...........................4/64 Inch Resistant 1-4/0 ..................... 5/64 Inch None Thermo 213-500 ................... 6/64 Inch plastic 501-1000 ................. 7/64 Inch 1001-2000 ............... 8/64 Inch * Outer covering is not required over rubber insulations which have been specifically approved for the purpose. ** For 14-12 sizes RHH shall be 3/64 inch thickness insulation. For insulated aluminum conductors, the minimum size is No. 12 AWG. See Tables 310-14 and 310-15. 70-94 NATIONAL ELECTRICAL CODE Table 3 10-2(b)--Continued Trade Name Type Letter Insulation Thickness of Insulation Outer Covering HeatResistant Thermo plastic Moisture and HeatResistant Thermo plastic FlameRetardant THHN Heat Resistant Thermo plastic THW Flame- Retardant, Moisture and HeatResistant Thermo plastic 14-12...........................15 Mils 10 .............................. 20 Mils 8 -6 .............................. 30 Mils Nylon 4-2 .............................. 40 Mils Jacket 1-4/0 ...........................50 Mils 250-500 M C M ............60 Mils 14-10.......................3/64 Inch 8-2 ...........................4/64 Inch 1-4/0 .......................5/64 Inch 213-500 ................... 6/64 Inch None 501-1000 ................. 7/64 Inch 1001-2000 ............... 8/64 Inch Moisture and HeatResistant Thermo plastic Flame- Retardant, Moisture TOWN and HeatResistant Thermo plastic 14-12...........................15 Mils 10 .............................. 20 Mils 8 - 6 .............................. 30 Mils 4-2 .............................. 40 Mils 1-4/0 ...........................50 Mils 250-500 M C M ............60 Mils Nylon Jacket Moisture and Heat- Resistant CrossLinked Thermo setting Polyethylene Flame- 14-10 ...........................30 Mils Retardant 8 - 2 .............................. 45 Mils Cross- 1-4/0 ...........................55 Mils XHHW Linked 213-500 ....................... 65 Mils None Polyethylene 501-1000 ................... 80 Mils 1001-2000 ................. 95 Mils Moisture-, Heat- and Oil-Resistant Thermo plastic Moisture-, Heat, and Oil-Resistant Thermo plastic Thermo plastic and Asbestos MTW (A) (B) Flame- 22-12 ................. 2/64 15 Mils (A) None Retardant, 10 ....................... 2/64 20 Mils Moisture-, 8 ......................... 3/64 30 Mils (B) Nylon Heat- and 6 ......................... 4/64 30 Mils jacket Oil-Resistant 4-2 ..................... 4/64 40 Mils Thermo 1-4/0 ................. 5/64 50 Mils plastic 213-500MCM . . . 6/64 60 Mils 501-1000MCM .. 7/64 70 Mils Flame- 14-10 .............................. 3/64 Retardant, 8-2 .................................. 4/64 Moisture-, 1-4/0 .............................. 5/64 THW- Heat- and 213-500 ........................... 6/64 None MTW Oil-Resistant 501-1000 ......................... 7/64 Thermo 1001-2000 ....................... 8/64 plastic Thermo Th'pl'. Asb. Flame- plastic and 14-8 ............. 20 Mils 20 Mils retardant, T A Asbestos 6 -2 ............... 30 Mils 25 Mils non-metallic 1 -4 /0 ........... 40 Mils 30 Mils covering For insulated aluminum conductors, the minimum size is No. 12 AWG. See Tables 310-14 and 310-15. ARTICLE 310-CONDUCTORS 70-95 Table 310-2(b)--Continued Trade Name Type Letter Insulation Thickness of Insulatio n Outer Covering Thermoplastic and Fibrous Braid TBS Thermoplastic 14-10 ......... 8 ............... 6-2 ........... 1-4/0 .......... . . 2/64 Incll Flame. . 3/64 Inc}l retardant, non-metallic covering Synthetic Heat- Resistant HeatSIS Resistant Rubber 14-10 ........... 8 ................. 6-2 ............... 1-4/0 ............ . . 2/64 Inc} None .. 5/64 Inch MineralInsulated Metal- Sheathed Ml Magnesium 16-4 ............. Oxide 3-250 MCM . ... ... 50 Mils 55 Mils Copper SiliconeAsbestos SA Silicone Rubber 14-10 ........... 8-2 ............... 1-4/0 ........... 213-500 ........ 501-1000 ___ 1001-2000 . . . . 3/64 Inch . 4/64 Inch Asbestos or glass . 7/64 Inch . 8/64 Inch Fluorinated Ethylene Propylene FEP Fluorinated Ethylene Propylene 14-10 ........... 8-2 ............... Fluorinated 14-8 ............. FEPB Ethylene Propylene 6 - 2 ............... ... ... 20 Mils 30 Mils None Glass braid .. . 14 Mils Asbestos braid Varnished Cambric 14-8 ............... 6-2 ................. V Varnished 1-4/0 ........... Cambric 213-500 ........ 500-1000 ........ 1001-2000 . .. . 3/64 Inch . 4/64 Inch . 5/64 Inch Non-metallic . 6/64 Inch covering or 7/64 Inch ead-sheath . 8/64 Inch Asbestos and Varnished Cambric AVA and AVL Impregnated (Dimen. in Mils) AVA- Asbestos and A V A A V L asbestos Varnished Cambric 1st A sb . VC 2nd 2nd Asb. Asb. braid or 14-8 (solid glass only) . . -- 30 20 25 14-8........10 30 15 25 AVL-lead 6 -2 ..........15 30 20 25 sheath 1 -4 /0 ___20 30 30 30 213-500 . . 25 40 40 40 501-1000 . 30 40 40 40 1001-2000 30 50 50 50 For insulated aluminum conductors, the minimum size is No. 12 AWG. See Tables 310-14 and 310-15. 70-96 NATIONAL ELECTRICAL CODE Table 3 10-2{b)--Continued Trade Name Type Letter Insulation Thickness of Insulation Outer Covering Asbestos and Varnished Cambric AVB 1 8 - 8 ........ 6 - 2 ......... 1-4/0 . . . . V C Asb. . . . . 30 20 Flame. . . . 40 30 retardant, . . . . 40 40 cotton braid (switchboard wiring) Asb. Impregnated 14-8 ........ . 10 Asbestos and 6-2 .......... . 15 Varnished 1-4/0 . . . . . 20 Cambric 213-500 .. . 25 501-1000 . . 30 1001-2000 . 30 VC 30 30 30 40 40 50 2nd Asb. 15 20 Flame30 retardant, 40 cotton braid 40 50 Asbestos A Asbestos 14 ........... 12-8 ........ 30 Mils Without as40 Mils bestos braid Asbestos AA Asbestos 14 ........... 12-8 ........ 6-2 .......... 1 - 4 / 0 ........ 30 Mils With asbestos 30 Mils braid or 40 Mils glass 60 Mils Asbestos AI Impregnated 14 ........... Asbestos 12-8 ........ 30 Mils Without as40 Mils bestos braid Asbestos Sol. Str. 14 ........... . 30 Mils 30 Mils 12-8.......... . 30 Mils 40 Mils AIA Impregnated 6 -2 .......... . 40 Mils 60 Mils With asbestos Asbestos 1-4/0 . .. . 60 Mils 75 Mils braid or 213-500 .. 90 Mils glass 501-1000 . 105 Mils Paper Paper Lead sheath The nonmetallic covering over individual rubber-covered conductors of aluminum sheathed cable and of lead-sheathed or multiple-conductor cable is not required to be flame retardant. For metal-clad cable, see Section 334-2. For nonmetallic-sheathed cable, see Section 336-2. For Type UF cable, see Section 339-1. For aluminum sheathed cable, see Section 331-9. For insulated aluminum conductors, the minimum size is No. 12 AWG. See Tables 310-14 and 310-15. (Continued jrotn page 88) mark, or other distinctive marking by which the organization responsi ble for the product may be identified. The marking shall also include the type letter or letters as specified in Article 310, Table 310-2(b) and Articles 336, 338 and 339, the size AWG or circular mils and the maxi mum voltage on the following types of wire and cable rated 600 volts or less: ARTICLE 310-CONDUCTORS 70-97 (1) Single conductor only, rubber insulated and thermoplastic in sulated for General Wiring. (2) Nonmetallic Sheathed Cable. (3) Service-Entrance Cable. (4) Underground Feeder and Branch Circuit Cable. All other types of wire and cable, flexible cords, fixture wires, switch board wires, and any of the above types with a metallic covering are not required to have a marking on the surface. (e) Classification. In addition to the type letters specified in Table 310-2(b). the following letters shall apply: (1) A type letter or letters used alone indicates a single insulated conductor. (2) The letter "D " used as a suffix indicates a twin wire with two insulated conductors laid parallel under an outer nonmetallic covering. (3) The letter "M " used as a suffix indicates an assembly of two or more insulated conductors twisted together under an outer nonme tallic covering. (4) The letter "L" used as a suffix indicates an outer covering of lead. (5) Voltage. Type letters, when used alone, indicate conductors for use at not more than 600 volts. Conductors for use at higher volt ages shall be indicated by adding numerical suffixes to the type letters as follows: 10-- for use at not more than 1000 volts. 20-- for use at not more than 2000 volts. 30-- for use at not more than 3000 volts. 40-- for use at not more than 4000 volts. 50-- for use at not more than 5000 volts. The working voltages referred to in the Table are the operating voltages between phases of single and two-phase systems, and three-phase systems with grounded or ungrounded neutral. (6) All NEC standard conductors shall be examined and tested at the factory and, if approved, shall be labeled before shipment. (f) Identification of Insulation. All rubber-insulated conductors and all thermoplastic-insulated conductors, No. 18 and larger, shall have a readily recognizable durable marking to indicate the type of insulation; except that single-conductor, code-rubber insulated conductors having a lead sheath; and multi-conductor cables, metal-clad cable, and nonmetallic-sheathed cable having code-rubber or nonmoisture-resistant thermoplastic-insulated conductors, need not be so marked. (g) Identified Conductors. Single insulated conductors of No. 6 or smaller, intended for use as identified conductors of circuits shall have an outer identification of a white or natural gray color. Twin and twisted-pair conductors and three-conductor cables shall have one con ductor, and four-conductor cables shall have at least one conductor identified in this manner. Multiple conductor flat cable No. 4 AWG or larger may employ an external ridge on the identified conductor. 70-98 NATIONAL ELECTRICAL CODE Exception No. 1-- Multiple-conductor varnished cloth insulated cables. Exception No. 2 -- Fixture wires as outlined below. Exception No. 3 -- Mineral insulated-metal sheathed cable. For fixture wires the identification shall be as above, or by means of (1) stripes, or (2) by the means described in Sections 400-13 (a ), (b ), (c), (d) and (e). For aerial cable the identification shall be as above, or by means of a ridge so located on the exterior of the cable as to identify it. Wires having their outer covering finished to show a white or natural gray color but having colored tracer threads in the braid, identifying the source of manufacture, are considered as meeting the provisions of this Section. (h) Unidentified Conductors. Single conductors, intended for use as unidentified conductors, and conductors other than the identified con ductor in multi-conductor cables, shall be finished to show a color or combination of colors other than, and contrasting with, white or natural gray. The colors contrasting with white or natural gray, may be pro vided by means of an approved stripe or stripes on black conductors. For identification requirements for conductors larger than No. 6 see Section 200-6 (b). (i) Insulation Thickness--Over 600 Volts. The thickness of insula tion for conductors for use at over 600 volts shall conform to Tables 310-2 (i-1 thru i-4 ): 310-3. Insulating Materials. (a) The rubber insulations include those made from natural and synthetic rubber, neoprene and other vulcanizable materials. (b) Thermoplastic insulation may stiffen at temperatures below mi nus 10C (14F) and care should be used in its installation at such tem peratures. It may be deformed when subject to pressure; care should be taken in its installation, as for example, at bushings, or points of support. See Section 373-6( b ) . 310-4. Temperature Limitations. N o conductor shall be used under such conditions that its temperature, even when carrying current, will exceed the temperature specified in Table 310-2(a) for the type of in sulation involved. 310-5 Wet Locations. Insulated conductors used underground, in con crete slabs or other masonry in direct contact with earth, in wet loca tions, or where condensation or accumulation of moisture within the raceway is likely to occur, shall be moisture- and heat-resistant, rubbercovered (Type RHW ); moisture-resistant latex rubber (Type RUW ); moisture-resistant, thermoplastic-covered (Type TW ); moisture- and (Continued on page 101) ARTICLE 310-CONDUCTORS Table 310-2(i) (1) Thickness of Rubber Insulation for Rubber-Covered Wire and Cable, in 64ths of an Inch 70-99 Conductor Size AWG or MCM Classification RH 10 RH 20 RHW 3 0 f RHW 4 0 f RHW 50 f ' 14- 12* 10- 8* 6- 2 1- 4/0 213-500 501-1000 1001-2000 45 45 7 9 io* 5 6 8 9 10 67 8 9 10 78 9 10 l i 89 9 10 l i 9 9 10 11 12 *No. 12 AWG is the minimum conductor size for aluminum and No. 8 AWG is the minimum conductor size for 5000 volts operation. t Shall be of approved ozone-resistant type for operation at voltages over 2000. Table 310-2(i)(2) Thickness of Varnished-Cambric Insulation for Single-Conductor Cable, in 64ths of an Inch Conductor Size AWG or MCM 14 12 10 8-2 1- 4/0 213-500 501-1000 1001-2000 1000 4 4 4 4 5 6 7 8 For Voltages Not Exceeding 2000 3000 4000 5 56 5 67 667 678 778 889 5000 9 9 10 10 10