Article 424 in the National Electrical Code (NEC) addresses fixed electric space heating equipment, stating, “for the purpose of the article, heating equipment shall include heating cable, unit heaters, boilers, central systems, or other approved fixed electric space-heating equipment.” Since the scope clearly covers boilers, further investigation into Article 424 will determine if the NEC covers internal wiring of the boiler and any boiler-control panels or if Article 424 only covers field-installed wiring with internal wiring covered by UL 834, the Standard for Electric Heating, Water Supply, and Power Boilers. 


Article 424 addresses two specific types of boilers: resistance-type in Part VII and electrode-type in Part VIII. I discuss only resistance-type boilers in Part VII. 


According to the scope statement for resistance-type boilers in 424.70, “the provisions in Part VII 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.” Section 424.72(A) provides requirements for overcurrent protection for boilers employing resistance-type immersion heating elements in an American Society of Mechanical Engineers (ASME) rated and stamped boiler vessel. These units shall have the heating elements protected at not more than 150 amperes (A). (The ASME Boiler and Pressure Vessel Code provides rules for the design, fabrication and inspection of boilers and pressure vessels.) The boiler is stamped and rated to contain pressure that may occur where the water is boiling or turned to steam, so the heating elements can be rated at greater than the normal 48A maximum as may be found in nonrated vessels. Where the boiler is rated at more than 120A, the boiler must have the heating elements subdivided into loads not exceeding 120A. Where subdivided into 120A or lower loads, the rating of the overcurrent protective device shall comply with 424.3(B), requiring the loads to be calculated as continuous loads.


Where the boiler employs resistance-type heating elements rated more than 48A and not contained in an ASME-rated and stamped vessel, compliance with 424.72(B) is required. 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 60A. Such a boiler rated more than 48A shall have the heating elements subdivided into loads not exceeding 48A. Where a subdivided load is less than 48A, the rating of the overcurrent protective device shall comply with 424.3(B), requiring the loads to be calculated as continuous loads.


Section 424.72(C) states the supplementary overcurrent protective devices for the subdivided loads, as required by 424.72(A) and (B) (noted in the two paragraphs above), must be factory-installed within or on the boiler enclosure, or it must be provided as a separate assembly by the boiler manufacturer, accessible (but need not be readily accessible), and suitable for branch-circuit protection. Where cartridge fuses are used to provide this overcurrent protection, a single disconnecting means is permitted for the several subdivided circuits. In 424.72(D), the conductors supplying these supplementary overcurrent protective devices will be considered as branch-circuit conductors. In addition, where the heaters are rated 50 kilowatts (kW) or more, the conductors supplying the overcurrent protective device specified in 424.72(C) are permitted to be sized at not less than 100 percent of the nameplate rating of the heater, provided the heater is marked with a minimum conductor size, the conductors are not smaller than the marked minimum size, and a temperature- or pressure-actuated device controls the cyclic operation of the equipment.


Finally, 424.72(E) states that field-wired conductors between the heater and the supplementary overcurrent protective devices must be sized at not less than 125 percent of the load served. The supplementary overcurrent protective devices specified in 424.72(C) must protect these conductors in accordance with 240.4. However, where the heaters are rated 50 kW or more, the ampacity of field-wired conductors between the heater and the supplementary overcurrent protective devices are permitted to be not less than 100 percent of the load of their respective subdivided circuits, if the heater is marked with a minimum conductor size; the conductors are not smaller than the marked minimum size; and a 
temperature-activated device controls the cyclic operation of the equipment.


Compliance with NEC 424.72(C) is necessary for internally wired, factory-installed supplementary overcurrent devices or provided as a separate assembly by the boiler manufacturer. These devices must be accessible (although not required to be readily accessible), must be suitable for branch-circuit protection and must comply with the sizing requirements in 424.72(A) or (B). While the NEC must be followed as mentioned above, relating to overcurrent protection, boilers are required to comply with UL 834 as a listed assembly.