FIXED ELECTRIC SPACE HEATING EQUIPMENT 70-369

424-63. Fan Circuit Interlock. Means shall be provided to ensure that the fan circuit is energized when any heater circuit is energized. However, time- or temperature-controlled delay in energizing the fan motor shall be permitted. 424-64. Limit Controls. Each duct heater shall be provided with an approved, integral, automatic-reset temperature-limiting control or controllers to deenergize the circuit or circuits. In addition, an integral independent supplementary control or controllers shall be provided in each duct heater that will disconnect a sufficient number of conductors to interrupt current flow. This device shall be manually resettable or replaceable. 424-65. Location of Disconnecting Means. Duct heater controller equipment shall be accessible with the disconnecting means installed at or within sight from the controller. Exception: As permitted by Section 424-19(a). 424-66. Installation. Duct heaters shall be installed in accordance with the manufacturer's instructions in a manner so that operation will not create a hazard to persons or property. Furthermore, duct heaters shall be located with respect to building construction and other equipment so as to permit access to the heater. Sufficient clearance shall be maintained to permit replacement of controls and heating elements and for adjusting and cleaning of controls and other parts requiring such attention. See Section 110-16. (FPN): For additional installation information, see Installation of Air Conditioning and Ventilating Systems, NFPA 90A-1989 (ANSI) and Installation of Warm Air Heating and Air Conditioning Systems, NFPA 90B-1989 (ANSI). G. Resistance-type Boilers 424-70. Scope. The provisions in Part G of this article shall apply to boilers employing resistance-type heating elements. Electrode-type boilers shall not be considered as employing resistance-type heating elements. See Part H of this article. 424-71. Identification. Resistance-type boilers shall be identified as suitable for the installation. 424-72. Overcurrent Protection. (a) Boiler Employing Resistance-type Immersion Heating Elements in an ASME Rated and Stamped Vessel. A boiler employing resistancetype immersion heating elements contained in an ASME rated and stamped vessel shall have the heating elements protected at not more than 150 amperes. Such a boiler rated more than 120 amperes shall have the heating elements subdivided into loads not exceeding 120 amperes. Where a subdivided load is less than 120 amperes, the rating of the overcurrent protective device shall comply with Section 424-3(b). 70-370 NATIONAL ELECTRICAL CODE (b) Boiler Employing Resistance-type Heating Elements Rated More than 48 Amperes and Not Contained in an ASME Rated and Stamped Vessel. A boiler employing resistance-type heating elements not contained in an ASME rated and stamped vessel shall have the heating elements protected at not more than 60 amperes. Such a boiler rated more than 48 amperes shall have the heating elements subdivided into loads not exceeding 48 amperes. Where a subdivided load is less than 48 amperes, the rating of the overcurrent protective device shall comply with Section 424-3(b). (c) Supplementary Overcurrent Protective Devices. The supplementary overcurrent protective devices for the subdivided loads as required by Sections 424-72(a) and (b) shall be (1) factory-installed within or on the boiler enclosure or provided as a separate assembly by the boiler manufacturer; and (2) accessible, but need not be readily accessible; and (3) suitable for branch-circuit protection. Where cartridge fuses are used to provide this overcurrent protection, a single disconnecting means shall be permitted for the several subdivided circuits. See Section 240-40. (d) Conductors Supplying Supplementary Overcurrent Protective Devices. The conductors supplying these supplementary overcurrent protective devices shall be considered branch-circuit conductors. Exception: Where the heaters are rated 50 kW or more, the conductors supplying the overcurrent protective device specified in (c) above shall be permitted to be sized at not less than 100 percent of the nameplate rating of the heater, provided all of the following conditions are met: a. The heater is marked with a minimum conductor size; and b. The conductors are not smaller than the marked minimum size; and c. A temperature- or pressure-actuated device controls the cyclic operation of the equipment. (e) Conductors for Subdivided Loads. Field-wired conductors between the heater and the supplementary overcurrent protective devices shall be sized at not less than 125 percent of the load served. The supplementary overcurrent protective devices specified in (c) shall protect these conductors in accordance with Section 240-3. Exception: For heaters rated 50 kW or more, the ampacity of field-wired conductors between the heater and the supplementary overcurrent protective devices shall be permitted to be not less than 100 percent of the load of their respective subdivided circuits, provided all of the following conditions are met: a. The heater is marked with a minimum conductor size; and b. The conductors are not smaller than the marked minimum size; and c. A temperature-activated device controls the cyclic operation of the equipment. 424-73. Over-Temperature Limit Control. Each boiler designed, so that in normal operation there is no change in state of the heat transfer medium,