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Article 424

Fixed Electric Space-Heating Equipment

Part I. General

  1. Scope. This article covers fixed electric equipment used for space heating. For the purpose of this article, heating equipment shall include heating cable, unit heaters, boilers, central systems, or other approved fixed electric space-heating equipment. This article shall not apply to process heating and room air conditioning.
  2. Other Articles. Fixed electric space-heating equipment incorporating a hermetic refrigerant motor-compressor shall also comply with Article 440.
  3. Branch Circuits.
  1. Branch-Circuit Requirements. Individual branch circuits shall be permitted to supply any volt-ampere or wattage rating of fixed electric space-heating equipment for which they are rated.Branch circuits supplying two or more outlets for fixed elec‐ tric space-heating equipment shall be rated not over 30 amperes. In other than a dwelling unit, fixed infrared heat‐ ing equipment shall be permitted to be supplied from branch circuits rated not over 50 amperes.
  2. Branch-Circuit Sizing. Fixed electric space-heating equip‐ ment and motors shall be considered continuous load.

424.6 Listed Equipment. Electric baseboard heaters, heating cables, duct heaters, and radiant heating systems shall be listed and labeled.

Part II. Installation

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  1. General. Permanently installed electric baseboard heat‐ ers equipped with factory-installed receptacle outlets, or outlets provided as a separate listed assembly, shall be permitted in lieu of a receptacle outlet(s) that is required by 210.50(B). Such receptacle outlets shall not be connected to the heater circuits.Informational Note: Listed baseboard heaters include instruc‐ tions that may not permit their installation below receptacle outlets.
  2. Special Permission. Fixed electric space-heating equipment and systems installed by methods other than covered by this article shall be permitted only by special permission.
  3. Supply Conductors. Fixed electric space-heating equipment requiring supply conductors with over 60°C insula‐ tion shall be clearly and permanently marked. This marking shall be plainly visible after installation and shall be permitted to be adjacent to the field connection box.
  4. Locations.
    1. Exposed to Physical Damage. Where subject to physical damage, fixed electric space-heating equipment shall be protected in an approved manner.
    2. Damp or Wet Locations. Heaters and related equipment installed in damp or wet locations shall be listed for such loca‐ tions and shall be constructed and installed so that water or other liquids cannot enter or accumulate in or on wired sections, electrical components, or ductwork.
    Informational Note No. 1: See 110.11 for equipment exposed to deteriorating agents.Informational Note No. 2: See 680.27(C) for pool deck areas.
  5. Spacing from Combustible Materials. Fixed electric space-heating equipment shall be installed to provide the required spacing between the equipment and adjacent combustible material, unless it is listed to be installed in direct contact with combustible material.

Part III. Control and Protection of Fixed Electric Space- Heating Equipment

  1. Disconnecting Means. Means shall be provided to simultaneously disconnect the heater, motor controller(s), and supplementary overcurrent protective device(s) of all fixed electric space-heating equipment from all ungrounded conduc‐ tors. Where heating equipment is supplied by more than one source, feeder, or branch circuit, the disconnecting means shall be grouped and identified as having multiple disconnecting means. Each disconnecting means shall simultaneously discon‐ nect all ungrounded conductors that it controls. The discon‐ necting means specified in 424.19(A) and (B) shall have an ampere rating not less than 125 percent of the total load of the motors and the heaters and shall be lockable in accordance with 110.25.
    1. Heating Equipment with Supplementary Overcurrent Protection. The disconnecting means for fixed electric space- heating equipment with supplementary overcurrent protection shall be within sight from the supplementary overcurrent protective device(s), on the supply side of these devices, iffuses, and, in addition, shall comply with either 424.19(A)(1) or (A)(2).(1) Heater Containing No Motor Rated over 1∕8 Horsepower. The disconnecting means specified in 424.19 or unit switches complying with 424.19(C) shall be permitted to serve as the required disconnecting means for both the motor controller(s) and heater under either of the following conditions:
      1. The disconnecting means provided is also within sight from the motor controller(s) and the heater.
      2. The disconnecting means is lockable in accordance with 110.25.
      (2) Heater Containing a Motor(s) Rated over 1∕8 Horsepower. The above disconnecting means shall be permitted to serve as the required disconnecting means for both the motor control‐ ler(s) and heater under either of the following conditions:
      1. Where the disconnecting means is in sight from the motor controller(s) and the heater and complies with Part IX of Article 430.
      2. Where a motor(s) of more than 1∕8 hp and the heater are provided with a single unit switch that complies with 422.34(A), (B), (C), or (D), the disconnecting means shall be permitted to be out of sight from the motor controller.
    2. Heating Equipment Without Supplementary Overcurrent Protection.
      1. Without Motor or with Motor Not over 1∕8 Horsepower. For fixed electric space-heating equipment without a motor rated over 1∕8 hp, the branch-circuit switch or circuit breaker shall be permitted to serve as the disconnecting means where the switch or circuit breaker is within sight from the heater or is lockable in accordance with 110.25.
      2. Over 1∕8 Horsepower. For motor-driven electric space- heating equipment with a motor rated over 1∕8 hp, a disconnect‐ ing means shall be located within sight from the motor controller or shall be permitted to comply with the require‐ ments in 424.19(A)(2).
    3. Unit Switch(es) as Disconnecting Means. A unit switch(es) with a marked “off” position that is part of a fixed heater and disconnects all ungrounded conductors shall be permitted as the disconnecting means required by this article where other means for disconnection are provided in the types of occupancies in 424.19(C)(1) through (C)(4).
      1. Multifamily Dwellings. In multifamily dwellings, the other disconnecting means shall be within the dwelling unit, or on the same floor as the dwelling unit in which the fixed heater is installed, and shall also be permitted to control lamps and appliances.
      2. Two-Family Dwellings. In two-family dwellings, the other disconnecting means shall be permitted either inside or outside of the dwelling unit in which the fixed heater is instal‐ led. In this case, an individual switch or circuit breaker for the dwelling unit shall be permitted and shall also be permitted to control lamps and appliances.
      3. One-Family Dwellings. In one-family dwellings, the service disconnecting means shall be permitted to be the other discon‐ necting means.
      4. Other Occupancies. In other occupancies, the branch- circuit switch or circuit breaker, where readily accessible for

    servicing of the fixed heater, shall be permitted as the other disconnecting means.
  2. Thermostatically Controlled Switching Devices.
    1. Serving as Both Controllers and Disconnecting Means. Thermostatically controlled switching devices and combination thermostats and manually controlled switches shall be permit‐ ted to serve as both controllers and disconnecting means, provided they meet all of the following conditions:
      1. Provided with a marked “off” position
      2. Directly open all ungrounded conductors when manually placed in the “off” position
      3. Designed so that the circuit cannot be energized automat‐ ically after the device has been manually placed in the “off” position
      4. Located as specified in 424.19
    2. Thermostats That Do Not Directly Interrupt All Ungroun‐ ded Conductors. Thermostats that do not directly interrupt all ungrounded conductors and thermostats that operate remote- control circuits shall not be required to meet the requirements of 424.20(A). These devices shall not be permitted as the disconnecting means.
  3. Switch and Circuit Breaker to Be Indicating. Switches and circuit breakers used as disconnecting means shall be of the indicating type.
  4. Overcurrent Protection.
  1. Branch-Circuit Devices. Electric space-heating equip‐ ment, other than such motor-operated equipment as required by Articles 430 and 440 to have additional overcurrent protec‐ tion, shall be permitted to be protected against overcurrent where supplied by one of the branch circuits in Article 210.
  2. Resistance Elements. Resistance-type heating elements in electric space-heating equipment shall be protected at not more than 60 amperes. Equipment rated more than 48 amperes and employing such elements shall have the heat‐ ing elements subdivided, and each subdivided load shall not exceed 48 amperes. Where a subdivided load is less than48 amperes, the rating of the supplementary overcurrent protective device shall comply with 424.3(B). A boiler employ‐ ing resistance-type immersion heating elements contained in an ASME-rated and stamped vessel shall be permitted to comply with 424.72(A).
  3. Overcurrent Protective Devices. The supplementary over‐ current protective devices for the subdivided loads specified in 424.22(B) shall be (1) factory-installed within or on the heater enclosure or supplied for use with the heater as a separate assembly by the heater manufacturer; (2) accessible, but shall not be required to be readily accessible; and (3) suitable for branch-circuit protection.Informational Note: See 240.10.Where cartridge fuses are used to provide this overcurrent protection, a single disconnecting means shall be permitted to be used for the several subdivided loads.Informational Note No. 1: For supplementary overcurrent protection, see 240.10.Informational Note No. 2: For disconnecting means for cartridge fuses in circuits of any voltage, see 240.40.
  4. Branch-Circuit Conductors. The conductors supplying the supplementary overcurrent protective devices shall be considered branch-circuit conductors.Where the heaters are rated 50 kW or more, the conductors supplying the supplementary overcurrent protective devices specified in 424.22(C) shall be permitted to be sized at not less than 100 percent of the nameplate rating of the heater, provi‐ ded all of the following conditions are met:
    1. The heater is marked with a minimum conductor size.
    2. The conductors are not smaller than the marked mini‐ mum size.
    3. A temperature-actuated device controls the cyclic opera‐ tion of the equipment.
  5. Conductors for Subdivided Loads. Field-wired conduc‐ tors 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 424.22(C) shall protect these conductors in accordance with 240.4.

Where the heaters are rated 50 kW or more, the ampacity of field-wired conductors between the heater and the supplemen‐ tary 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:

  1. The heater is marked with a minimum conductor size.
  2. The conductors are not smaller than the marked mini‐ mum size.
  3. A temperature-activated device controls the cyclic opera‐ tion of the equipment.

Part IV. Marking of Heating Equipment

  1. Nameplate.
    1. Marking Required. Each unit of fixed electric space- heating equipment shall be provided with a nameplate giving the identifying name and the normal rating in volts and watts or in volts and amperes.Electric space-heating equipment intended for use on alter‐ nating current only, direct current only, or both shall be marked to so indicate. The marking of equipment consisting of motors over 1∕8 hp and other loads shall specify the rating of the motor in volts, amperes, and frequency, and the heating load in volts and watts or in volts and amperes.
    2. Location. This nameplate shall be located so as to be visi‐ ble or easily accessible after installation.
  2. Marking of Heating Elements. All heating elements that are replaceable in the field and are part of an electric heater shall be legibly marked with the ratings in volts and watts or in volts and amperes.

Part V. Electric Space-Heating Cables

424.34 Heating Cable Construction. Factory-assembled nonheating leads of heating cables, if any, shall be at least

2.1 m (7 ft) in length.

  1. Marking of Heating Cables. Each unit shall be marked with the identifying name or identification symbol, catalog number, and ratings in volts and watts or in volts and amperes.•
  2. Clearances of Wiring in Ceilings. Wiring located above heated ceilings shall be spaced not less than 50 mm (2 in.) above the heated ceiling. The ampacity of conductors shall be calculated on the basis of an assumed ambient temperature of 50°C (122°F), applying the correction factors shown in the 0–2000 volt ampacity tables of Article 310. If this wiring is loca‐ ted above thermal insulation having a minimum thickness of

50 mm (2 in.), the wiring shall not require correction for temperature.

  1. Area Restrictions.
    1. Extending Beyond the Room or Area. Heating cables shall be permitted to extend beyond the room or area in which they originate.
    2. Uses Not Permitted. Heating cables shall not be installed as follows:
      1. In closets, other than as noted in 424.38(C)
      2. Over the top of walls where the wall intersects the ceiling
      3. Over partitions that extend to the ceiling, unless they are isolated single runs of embedded cable
      4. Under or through walls
      5. Over cabinets whose clearance from the ceiling is less than the minimum horizontal dimension of the cabinet to the nearest cabinet edge that is open to the room or area
      6. In tub and shower walls
      7. Under cabinets or similar built-ins having no clearance to the floor
    3. In Closet Ceilings as Low-Temperature Heat Sources to Control Relative Humidity. The provisions of 424.38(B) shall not prevent the use of cable in closet ceilings as low- temperature heat sources to control relative humidity, provided they are used only in those portions of the ceiling that are unobstructed to the floor by shelves or other permanent lumin‐ aires.
  2. Clearance from Other Objects and Openings. Heating elements of cables installed in ceilings shall be separated at least 200 mm (8 in.) from the edge of outlet boxes and junc‐ tion boxes that are to be used for mounting surface luminaires. A clearance of not less than 50 mm (2 in.) shall be provided from recessed luminaires and their trims, ventilating openings, and other such openings in room surfaces. No heating cable shall be covered by any surface-mounted equipment.
  3. Splices. The length of heating cable shall only be altered using splices identified in the manufacturer’s instruc‐ tions.
  4. Ceiling Installation of Heating Cables on Dry Board, in Plaster, and on Concrete.
    1. In Walls. Cables shall not be installed in walls unless it is necessary for an isolated single run of cable to be installed down a vertical surface to reach a dropped ceiling.
    2. Adjacent Runs. Adjacent runs of heating cable shall be installed in accordance with the manufacturer’s instructions.
    3. Surfaces to Be Applied. Heating cables shall be applied only to gypsum board, plaster lath, or other fire-resistant mate‐ rial. With metal lath or other electrically conductive surfaces, a coat of plaster or other means employed in accordance with the heating cable manufacturer’s instructions shall be appliedto completely separate the metal lath or conductive surface from the cable.Informational Note: See also 424.41(F).
    4. Splices. All heating cables, the splice between the heating cable and nonheating leads, and 75-mm (3-in.) minimum of the nonheating lead at the splice shall be embedded in plaster or dry board in the same manner as the heating cable.
    5. Ceiling Surface. The entire ceiling surface shall have a finish of thermally noninsulating sand plaster that has a nomi‐ nal thickness of 13 mm (1∕2 in.), or other noninsulating material identified as suitable for this use and applied according to specified thickness and directions.
    6. Secured. Cables shall be secured by means of approved stapling, tape, plaster, nonmetallic spreaders, or other approved means either at intervals not exceeding 400 mm (16 in.) or at intervals not exceeding 1.8 m (6 ft) for cables identified for such use. Staples or metal fasteners that straddle the cable shall not be used with metal lath or other electrically conductive surfaces.
    7. Dry Board Installations. In dry board installations, the entire ceiling below the heating cable shall be covered with gypsum board not exceeding 13 mm (1∕2 in.) thickness. The void between the upper layer of gypsum board, plaster lath, or other fire-resistant material and the surface layer of gypsum board shall be completely filled with thermally conductive, nonshrinking plaster or other approved material or equivalent thermal conductivity.
    8. Free from Contact with Conductive Surfaces. Cables shall be kept free from contact with metal or other electrically conductive surfaces.
    9. Joists. In dry board applications, cable shall be installed parallel to the joist, leaving a clear space centered under the joist of 65 mm (21∕2 in.) (width) between centers of adjacent runs of cable. A surface layer of gypsum board shall be moun‐ ted so that the nails or other fasteners do not pierce the heat‐ ing cable.
    10. Crossing Joists. Cables shall cross joists only at the ends of the room unless the cable is required to cross joists elsewhere in order to satisfy the manufacturer’s instructions that the installer avoid placing the cable too close to ceiling penetra‐ tions and luminaires.
  5. Finished Ceilings. Finished ceilings shall not be covered with decorative panels or beams constructed of materi‐ als that have thermal insulating properties, such as wood, fiber, or plastic. Finished ceilings shall be permitted to be covered with paint, wallpaper, or other approved surface finishes.
  6. Installation of Nonheating Leads of Cables.
    1. Free Nonheating Leads. Free nonheating leads of cables shall be installed in accordance with approved wiring methods from the junction box to a location within the ceiling. Such installations shall be permitted to be single conductors in approved raceways, single or multiconductor Type UF, Type NMC, Type MI, or other approved conductors.
    2. Leads in Junction Box. Not less than 150 mm (6 in.) of free nonheating lead shall be within the junction box. The marking of the leads shall be visible in the junction box.
    3. Excess Leads. Excess leads of heating cables shall not be cut but shall be secured to the underside of the ceiling and embedded in plaster or other approved material, leaving only a length sufficient to reach the junction box with not less than 150 mm (6 in.) of free lead within the box.
  7. Installation of Cables in Concrete or Poured Masonry Floors.•
    1. Adjacent Runs. Adjacent runs of heating cable shall beinstalled in accordance with the manufacturer’s instructions.
    2. Secured in Place. Cables shall be secured in place by nonmetallic frames or spreaders or other approved means while the concrete or other finish is applied.•
    3. Leads Protected. Leads shall be protected where theyleave the floor by rigid metal conduit, intermediate metal conduit, rigid nonmetallic conduit, electrical metallic tubing, or by other approved means.
    4. Bushings or Approved Fittings. Bushings or approved fittings shall be used where the leads emerge within the floor slab.
    5. Ground-Fault Circuit-Interrupter Protection. Ground- fault circuit-interrupter protection for personnel shall be provi‐ ded for cables installed in electrically heated floors of bathrooms and kitchens and in hydromassage bathtub loca‐ tions.
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  8. Installation of Cables Under Floor Coverings.
    1. Identification. Heating cables for installation under floor covering shall be identified as suitable for installation under floor covering.
    2. Expansion Joints. Heating cables shall not be installed where they bridge expansion joints unless provided with expan‐ sion and contraction fittings applicable to the manufacture of the cable.
    3. Connection to Conductors. Heating cables shall be connected to branch-circuit and supply wiring by wiring meth‐ ods described in the installation instructions or as recognized in Chapter 3.
    4. Anchoring. Heating cables shall be positioned or secured in place under the floor covering, per the manufacturer’s instructions.
    5. Ground-Fault Circuit-Interrupter Protection. Ground- fault circuit-interrupter protection for personnel shall be provi‐ ded.
    6. Grounding Braid or Sheath. Grounding means, such as copper braid, metal sheath, or other approved means, shall be provided as part of the heated length.
  9. Inspection and Tests. Cable installations shall be made with due care to prevent damage to the cable assembly and shall be inspected and approved before cables are covered or concealed.imageN
  10. Label Provided by Manufacturer. The manufacturers of electric space-heating cables shall provide marking labels that indicate that the space-heating installation incorporates electric space-heating cables and instructions that the labels shall be affixed to the panelboards to identify which branch circuits supply the circuits to those space-heating installations. If the electric space-heating cable installations are visible and

distinguishable after installation, the labels shall not be required to be provided and affixed to the panelboards.

Part VI. Duct Heaters

  1. General. Part VI shall apply to any heater mounted in the airstream of a forced-air system where the air-moving unit is not provided as an integral part of the equipment.
  2. Identification. Heaters installed in an air duct shall be identified as suitable for the installation.
  3. Airflow. Means shall be provided to ensure uniform airflow over the face of the heater in accordance with the manufacturer’s instructions.Informational Note: Heaters installed within 1.2 m (4 ft) of the outlet of an air-moving device, heat pump, air conditioner, elbows, baffle plates, or other obstructions in ductwork may require turning vanes, pressure plates, or other devices on the inlet side of the duct heater to ensure an even distribution of air over the face of the heater.
  4. Elevated Inlet Temperature. Duct heaters intended for use with elevated inlet air temperature shall be identified as suitable for use at the elevated temperatures.
  5. Installation of Duct Heaters with Heat Pumps and Air Conditioners. Heat pumps and air conditioners having duct heaters closer than 1.2 m (4 ft) to the heat pump or air condi‐ tioner shall have both the duct heater and heat pump or air conditioner identified as suitable for such installation and so marked.
  6. Condensation. Duct heaters used with air conditioners or other air-cooling equipment that could result in condensa‐ tion of moisture shall be identified as suitable for use with air conditioners.
  7. 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.
  8. Limit Controls. Each duct heater shall be provided with an approved, integral, automatic-reset temperature- limiting control or controllers to de-energize the circuit or circuits.In addition, an integral independent supplementary control or controllers shall be provided in each duct heater that disconnects a sufficient number of conductors to interrupt current flow. This device shall be manually resettable or replaceable.
  9. Location of Disconnecting Means. Duct heater controller equipment shall be either accessible with the discon‐ necting means installed at or within sight from the controller or as permitted by 424.19(A).
  10. Installation. Duct heaters shall be installed in accord‐ ance with the manufacturer’s instructions in such a manner that operation does 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 110.26 .

Informational Note: For additional installation information, see NFPA 90A -2015, Standard for the Installation of Air-Conditioning and Ventilating Systems, and NFPA 90B-2015, Standard for the Instal‐ lation of Warm Air Heating and Air-Conditioning Systems.

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Part VII. Resistance-Type Boilers

  1. Scope. The provisions in Part VII of this article shall apply to boilers employing resistance-type heating elements. See Part VIII of this article for electrode-type boilers.
  2. Identification. Resistance-type boilers shall be identi‐ fied as suitable for the installation.
  3. Overcurrent Protection.
    1. Boiler Employing Resistance-Type Immersion Heating Elements in an ASME-Rated and Stamped Vessel. A boiler employing resistance-type 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 heat‐ ing 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 424.3(B).
    2. 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 heat‐ ing 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 424.3(B).
    3. Supplementary Overcurrent Protective Devices. The supplementary overcurrent protective devices for the subdivi‐ ded loads as required by 424.72(A) and (B) shall be as follows:
      1. Factory-installed within or on the boiler enclosure or provided as a separate assembly by the boiler manufac‐ turer
      2. Accessible, but need not be readily accessible
      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 240.40.
    4. Conductors Supplying Supplementary Overcurrent Protec‐ tive Devices. The conductors supplying these supplementary overcurrent protective devices shall be considered branch- circuit conductors.Where the heaters are rated 50 kW or more, the conductors supplying the overcurrent protective device specified in 424.72(C) 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:
      1. The heater is marked with a minimum conductor size.
      2. The conductors are not smaller than the marked mini‐ mum size.
      3. A temperature- or pressure-actuated device controls the cyclic operation of the equipment.
    5. Conductors for Subdivided Loads. Field-wired conduc‐ tors 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 424.72(C) shall protect these conductors in accordance with 240.4.
    Where the heaters are rated 50 kW or more, the ampacity of field-wired conductors between the heater and the supplemen‐ tary 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:
    1. The heater is marked with a minimum conductor size.
    2. The conductors are not smaller than the marked mini‐ mum size.
    3. A temperature-activated device controls the cyclic opera‐ tion of the equipment.
  4. Overtemperature Limit Control. Each boiler designed so that in normal operation there is no change in state of the heat transfer medium shall be equipped with a temperature- sensitive limiting means. It shall be installed to limit maximum liquid temperature and shall directly or indirectly disconnect all ungrounded conductors to the heating elements. Such means shall be in addition to a temperature-regulating system and other devices protecting the tank against excessive pres‐ sure.
  5. Overpressure Limit Control. Each boiler designed so that in normal operation there is a change in state of the heat transfer medium from liquid to vapor shall be equipped with a pressure-sensitive limiting means. It shall be installed to limit maximum pressure and shall directly or indirectly disconnect all ungrounded conductors to the heating elements. Such means shall be in addition to a pressure-regulating system and other devices protecting the tank against excessive pressure.

Part VIII. Electrode-Type Boilers

  1. Scope. The provisions in Part VIII of this article shall apply to boilers for operation at 600 volts, nominal, or less, in which heat is generated by the passage of current between elec‐ trodes through the liquid being heated.Informational Note: For over 600 volts, see Part V of Article 490.
  2. Identification. Electrode-type boilers shall be identi‐ fied as suitable for the installation.
  3. Branch-Circuit Requirements. The size of branch- circuit conductors and overcurrent protective devices shall be calculated on the basis of 125 percent of the total load (motors not included). A contactor, relay, or other device, approved for continuous operation at 100 percent of its rating, shall be permitted to supply its full-rated load. See 210.19(A), Excep‐ tion. The provisions of this section shall not apply to conduc‐ tors that form an integral part of an approved boiler.Where an electrode boiler is rated 50 kW or more, the conductors supplying the boiler electrode(s) shall be permitted to be sized at not less than 100 percent of the nameplate rating of the electrode boiler, provided all the following conditions are met:
    1. The electrode boiler is marked with a minimum conduc‐ tor size.
    2. The conductors are not smaller than the marked mini‐ mum size.
    3. A temperature- or pressure-actuated device controls the cyclic operation of the equipment.
  4. Overtemperature Limit Control. Each boiler, designed so that in normal operation there is no change in state of the heat transfer medium, shall be equipped with a temperature- sensitive limiting means. It shall be installed to limit maximum liquid temperature and shall directly or indirectly interrupt all current flow through the electrodes. Such means shall be in addition to the temperature-regulating system and other devi‐ ces protecting the tank against excessive pressure.
  5. Overpressure Limit Control. Each boiler, designed so that in normal operation there is a change in state of the heat transfer medium from liquid to vapor, shall be equipped with a pressure-sensitive limiting means. It shall be installed to limit maximum pressure and shall directly or indirectly interrupt all current flow through the electrodes. Such means shall be in addition to a pressure-regulating system and other devices protecting the tank against excessive pressure.
  6. Grounding. For those boilers designed such that fault currents do not pass through the pressure vessel, and the pres‐ sure vessel is electrically isolated from the electrodes, all exposed non–current-carrying metal parts, including the pres‐ sure vessel, supply, and return connecting piping, shall be grounded.For all other designs, the pressure vessel containing the elec‐ trodes shall be isolated and electrically insulated from ground.
  7. Markings. All electrode-type boilers shall be marked to show the following:
  1. The manufacturer’s name.
  2. The normal rating in volts, amperes, and kilowatts.
  3. The electrical supply required specifying frequency, number of phases, and number of wires.
  4. The marking “Electrode-Type Boiler.”
  5. A warning marking, “All Power Supplies Shall Be Discon‐ nected Before Servicing, Including Servicing the Pressure Vessel.” A field-applied warning marking or label shall comply with 110.21(B).

The nameplate shall be located so as to be visible after instal‐ lation.

Part IX. Electric Radiant Heating Panels and Heating Panel Sets

  1. Scope. The provisions of Part IX of this article shall apply to radiant heating panels and heating panel sets.
  2. Definitions.Heating Panel. A complete assembly provided with a junction box or a length of flexible conduit for connection to a branch circuit.Heating Panel Set. A rigid or nonrigid assembly provided with nonheating leads or a terminal junction assembly identified as being suitable for connection to a wiring system.
  3. Markings.
    1. Location. Markings shall be permanent and in a location that is visible prior to application of panel finish.
    2. Identified as Suitable. Each unit shall be identified as suit‐ able for the installation.
    3. Required Markings. Each unit shall be marked with the identifying name or identification symbol, catalog number, and rating in volts and watts or in volts and amperes.
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  4. Installation.
    1. General.
      1. Manufacturer’s Instructions. Heating panels and heating panel sets shall be installed in accordance with the manufactur‐ er’s instructions.
      2. Locations Not Permitted. The heating portion shall not be installed as follows:
        1. In or behind surfaces where subject to physical damage
        2. Run through or above walls, partitions, cupboards, or similar portions of structures that extend to the ceiling
        3. Run in or through thermal insulation, but shall be permitted to be in contact with the surface of thermal insulation
        1. Separation from Outlets for Luminaires. Edges of panels and panel sets shall be separated by not less than 200 mm (8 in.) from the edges of any outlet boxes and junction boxes that are to be used for mounting surface luminaires. A clear‐ ance of not less than 50 mm (2 in.) shall be provided from recessed luminaires and their trims, ventilating openings, and other such openings in room surfaces, unless the heating panels and panel sets are listed and marked for lesser clearan‐ ces, in which case they shall be permitted to be installed at the marked clearances. Sufficient area shall be provided to ensure that no heating panel or heating panel set is to be covered by any surface-mounted units.
        2. Surfaces Covering Heating Panels. After the heating panels or heating panel sets are installed and inspected, it shall be permitted to install a surface that has been identified by the manufacturer’s instructions as being suitable for the installa‐ tion. The surface shall be secured so that the nails or other fastenings do not pierce the heating panels or heating panel sets.
        3. Surface Coverings. Surfaces permitted by 424.93(A)(4) shall be permitted to be covered with paint, wallpaper, or other approved surfaces identified in the manufacturer’s instructions as being suitable.
    2. Heating Panel Sets.
      1. Mounting Location. Heating panel sets shall be permitted to be secured to the lower face of joists or mounted in between joists, headers, or nailing strips.
      2. Parallel to Joists or Nailing Strips. Heating panel sets shall be installed parallel to joists or nailing strips.
      3. Installation of Nails, Staples, or Other Fasteners. Nailing or stapling of heating panel sets shall be done only through the unheated portions provided for this purpose. Heating panel sets shall not be cut through or nailed through any point closer than 6 mm (1∕4 in.) to the element. Nails, staples, or other
        fasteners shall not be used where they penetrate current- carrying parts.
      4. Installed as Complete Unit. Heating panel sets shall be installed as complete units unless identified as suitable for field cutting in an approved manner.
  5. Clearances of Wiring in Ceilings. Wiring located above heated ceilings shall be spaced not less than 50 mm (2 in.) above the heated ceiling. The ampacity shall be calculated on the basis of an assumed ambient temperature of 50°C (122°F), applying the correction factors given in the 0–2000 volt ampac‐ ity tables of Article 310. If this wiring is located above thermal insulations having a minimum thickness of 50 mm (2 in.), the wiring shall not require correction for temperature.
  6. Location of Branch-Circuit and Feeder Wiring in Walls.
    1. Exterior Walls. Wiring methods shall comply with Arti‐ cle 300 and 310.15(A)(3).
    2. Interior Walls. The ampacity of any wiring behind heating panels or heating panel sets located in interior walls or parti‐ tions shall be calculated on the basis of an assumed ambient temperature of 40°C (104°F), applying correction factors given in the 0–2000 volt ampacity tables of Article 310.
  7. Connection to Branch-Circuit Conductors.
    1. General. Heating panels or heating panel sets assembled together in the field to form a heating installation in one room or area shall be connected in accordance with the manufactur‐ er’s instructions.
    2. Heating Panels. Heating panels shall be connected to branch-circuit wiring by an approved wiring method.
    3. Heating Panel Sets.
      1. Connection to Branch-Circuit Wiring. Heating panel sets shall be connected to branch-circuit wiring by a method identi‐ fied as being suitable for the purpose.
      2. Panel Sets with Terminal Junction Assembly. A heating panel set provided with terminal junction assembly shall be permitted to have the nonheating leads attached at the time of installation in accordance with the manufacturer’s instructions.
  8. Nonheating Leads. Excess nonheating leads of heating panels or heating panel sets shall be permitted to be cut to the required length as indicated in the manufacturer’s installation instructions. Nonheating leads that are an integral part of a heating panel and a heating panel set, either attached or provi‐ ded by the manufacturer as part of a terminal junction assem‐ bly, shall not be subjected to the ampacity requirements of 424.3(B) for branch circuits.
  9. Installation in Concrete or Poured Masonry.•
    1. Secured in Place and Identified as Suitable. Heatingpanels or heating panel sets shall be secured in place by means specified in the manufacturer’s instructions and identified as suitable for the installation.
    2. Expansion Joints. Heating panels or heating panel sets shall not be installed where they bridge expansion joints unless provision is made for expansion and contraction.
    3. Spacings. Spacings shall be maintained between heating panels or heating panel sets and metal embedded in the floor.Grounded metal-clad heating panels shall be permitted to be in contact with metal embedded in the floor.
    4. Protection of Leads. Leads shall be protected where they leave the floor by rigid metal conduit, intermediate metal conduit, rigid nonmetallic conduit, or electrical metallic tubing, or by other approved means.
    5. Bushings or Fittings Required. Bushings or approved fittings shall be used where the leads emerge within the floor slabs.
  10. Installation Under Floor Covering.
    1. Identification. Heating panels or heating panel sets for installation under floor covering shall be identified as suitable for installation under floor covering.•
    2. Installation. Listed heating panels or panel sets, if instal‐led under floor covering, shall be installed on floor surfaces that are smooth and flat in accordance with the manufacturer’s instructions and shall also comply with 424.99(B)(1) through (C)(5).
      1. Expansion Joints. Heating panels or heating panel sets shall not be installed where they bridge expansion joints unless protected from expansion and contraction.
      2. Connection to Conductors. Heating panels and heating panel sets shall be connected to branch-circuit and supply wiring by wiring methods recognized in Chapter 3.
      3. Anchoring. Heating panels and heating panel sets shall be firmly anchored to the floor using an adhesive or anchoring system identified for this use.
      4. Coverings. After heating panels or heating panel sets are installed and inspected, they shall be permitted to be covered by a floor covering that has been identified by the manufac‐ turer as being suitable for the installation.
      5. GFCI Protection. Branch circuits supplying the heating panel or heating panel sets shall have ground-fault circuit- interrupter protection for personnel.imageN•
      6. Grounding Braid or Sheath. Excluding nonheating leads,
    grounding means, such as copper braid, metal sheath, or other approved means, shall be provided with or as an integral part of the heating panel or heating panel set.
    imageNPart X. Low-Voltage Fixed Electric Space-Heating Equipment
  11. Scope. Low-voltage fixed electric space-heating equipment shall consist of an isolating power supply, low- voltage heaters, and associated equipment that are all identi‐ fied for use in dry locations.
  12. Energy Source.
    1. Power Unit. The power unit shall be an isolating type with a rated output not exceeding 25 amperes, 30 volts (42.4 volts peak) ac, or 60 volts dc under all load conditions.
    2. Alternate Energy Sources. Listed low-voltage fixed electric space-heating equipment shall be permitted to be supplied directly from an alternate energy source such as solar photovol‐ taic (PV) or wind power. When supplied from such a source, the source and any power conversion equipment between the source and the heating equipment and its supply shall be listed and comply with the applicable section of the NEC for the

    source used. The output of the source shall meet the limits of 424.101(A).
  13. Listed Equipment. Low-voltage fixed electric space- heating equipment shall be listed as a complete system.
  14. Installation.
    1. General. Equipment shall be installed per the manufac‐ turer’s installation instructions.
    2. Ground. Secondary circuits shall not be grounded.
    3. Ground-Fault Protection. Ground-fault protection shall not be required.
  15. Branch Circuit.
  16. Equipment shall be permitted to be supplied from branch circuits rated not over 30 amperes.

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