CLASS I LOCATIONS

501-1. General. The general rules of this Code shall apply to the electric wiring and equipment in locations classified as Class I in Section 500-5. Exception: As modified by this article. ARTICLE 501 - CLASS I LOCATIONS 70-473 501-2. Transformers and Capacitors. (a) Class I, Division 1. In Class I, Division 1 locations, transformers and capacitors shall comply with the following: (1) Containing Liquid that Will Burn. Transformers and capacitors containing a liquid that will burn shall be installed only in approved vaults that comply with Sections 450-41 through 450-48, and, in addition, (1) there shall be no door or other communicating opening between the vault and the Division 1 location; and (2) ample ventilation shall be provided for the continuous removal of flammable gases or vapors; and (3) vent openings or ducts shall lead to a safe location outside of buildings; and (4) vent ducts and openings shall be of sufficient area to relieve explosion pressures within the vault, and all portions of vent ducts within the buildings shall be of reinforced concrete construction. (2) Not Containing Liquid that Will Burn. Transformers and capacitors that do not contain a liquid that will burn shall (1) be installed in vaults complying with (a)(1) above or (2) be approved for Class I locations. (b) Class I, Division 2. In Class I, Division 2 locations, transformers and capacitors shall comply with Sections 450-21 through 450-27. 501-3. Meters, Instruments, and Relays. (a) Class I, Division 1. In Class I, Division 1 locations, meters, instruments, and relays, including kilowatt-hour meters, instrument transformers, resistors, rectifiers, and thermionic tubes, shall be provided with enclosures approved for Class I, Division 1 locations. Enclosures approved for Class I, Division 1 locations include (1) explosionproof enclosures and (2) purged and pressurized enclosures. (FPN): See Purged and Pressurized Enclosures for Electrical Equipment, NFPA 496-1989 (ANSI). (b) Class I, Division 2. In Class I, Division 2 locations, meters, instruments, and relays shall comply with the following: (1) Contacts. Switches, circuit breakers, and make-and-break contacts of pushbuttons, relays, alarm bells, and horns shall have enclosures approved for Class I, Division 1 locations in accordance with (a) above. Exception: General-purpose enclosures shall be permitted, if currentinterrupting contacts are: a. Immersed in oil; or b. Enclosed within a chamber hermetically sealed against the entrance of gases or vapors; or c. In nonincendive circuits that under normal conditions do not release sufficient energy to ignite a specific ignitible atmospheric mixture. (2) Resistors and Similar Equipment. Resistors, resistance devices, thermionic tubes, rectifiers, and similar equipment that are used in or in connection with meters, instruments, and relays shall comply with (a) above. Exception: General-purpose-type enclosures shall be permitted if such equipment is without make-and-break or sliding contacts [other than as provided in (b)(1) above] and if the maximum operating temperature of any 70-474 NATIONAL ELECTRICAL CODE exposed surface will not exceed 80 percent of the ignition temperature in degrees Celsius of the gas or vapor involved or has been tested and found incapable of igniting the gas or vapor. (3) Without Make-or-Break Contacts. Transformer windings, impedance coils, solenoids, and other windings that do not incorporate sliding or make-or-break contacts shall be provided with enclosures. Generalpurpose-type enclosures shall be permitted. (4) General-Purpose Assemblies. Where an assembly is made up of components for which general-purpose enclosures are acceptable as provided in (b)(1), (b)(2), and (b)(3) above, a single general-purpose enclosure shall be acceptable for the assembly. Where such an assembly includes any of the equipment described in (b)(2) above, the maximum obtainable surface temperature of any component of the assembly shall be clearly and permanently indicated on the outside of the enclosure. Alternatively, approved equipment shall be permitted to be marked to indicate the temperature range for which it is suitable, using the identification numbers of Table 500-3(d). (5) Fuses. Where general-purpose enclosures are permitted in (b)(1), (b)(2), (b)(3), and (b)(4) above, fuses for overcurrent protection of instrument circuits not subject to overloading in normal use shall be permitted to be mounted in general-purpose enclosures if each such fuse is preceded by a switch complying with (b)(1) above. (6) Connections. To facilitate replacements, process control instruments shall be permitted to be connected through flexible cord, attachment plug, and receptacle, provided (1) a switch complying with (b)(1) above is provided so that the attachment plug is not depended on to interrupt current; and (2) the current does not exceed 3 amperes at 120 volts, nominal; and (3) the power-supply cord does not exceed 3 feet (914 mm), is of a type approved for extra-hard usage or for hard usage if protected by location, and is supplied through an attachment plug and receptacle of the locking and grounding type; and (4) only necessary receptacles are provided; and (5) the receptacle carries a label warning against unplugging under load. 501-4. Wiring Methods. Wiring methods shall comply with (a) and (b) below. (a) Class I, Division 1. In Class I, Division 1 locations, threaded rigid metal conduit, threaded steel intermediate metal conduit, or Type MI cable with termination fittings approved for the location shall be the wiring method employed. All boxes, fittings, and joints shall be threaded for connection to conduit or cable terminations and shall be explosionproof. Threaded joints shall be made up with at least five threads fully engaged. Type MI cable shall be installed and supported in a manner to avoid tensile stress at the termination fittings. Where necessary to employ flexible connections, as at motor terminals, flexible fittings approved for Class I locations shall be used. Exception: As provided in Section 501-11. (b) Class I, Division 2. In Class I, Division 2 locations, threaded rigid metal conduit, threaded steel intermediate metal conduit, enclosed gasketed busways, enclosed gasketed wireways, or Type PLTC cable in accordance with the provisions of Article 725, Type MI, MC, MV, TC, or