Article 425

Fixed Resistance and Electrode Industrial Process Heating Equipment
The equipment shall be considered a continuous duty load.

Scope. This article covers fixed industrial process heat‐ ing employing electric resistance or electrode heating technol‐ ogy. For the purpose of this article, heating equipment shall include boilers, electrode boilers, duct heaters, strip heaters, immersion heaters, process air heaters, or other approved fixed electric equipment used for industrial process heating. This article shall not apply to heating and room air conditioning for personnel spaces covered by Article 424, fixed heating equip‐ ment for pipelines and vessels covered by Article 427, induc‐ tion and dielectric heating equipment covered by Article 665, and industrial furnaces incorporating silicon carbide, molybde‐ num, or graphite process heating elements.
Other Articles. Fixed industrial process heating equip‐ ment incorporating a hermetic refrigerant motor-compressor shall also comply with Article 440.
Branch Circuits.

Branch-Circuit Requirements. Individual branch circuits shall be permitted to supply any volt-ampere or wattage rating of fixed industrial process heating equipment for which they are rated.
Branch-Circuit Sizing. Fixed industrial process heating equipment and motors shall be considered continuous loads.

425.6 Listed Equipment. Fixed industrial process heating equipment shall be listed.

Part II. Installation

General.
1. Location. Fixed industrial process heating equipment shall be located with respect to building construction and other equipment so as to permit access to the equipment. 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.
2. Working Space. Working space about electrical enclosures for fixed industrial process heating equipment that require examination, adjustment, servicing, or maintenance while energized shall be accessible, and the work space for personnel shall comply with 110.26 and 110.34, based upon the utilization voltage to ground.Exception: With special permission, in industrial establishments only, where conditions of maintenance and supervision ensure that only qualified persons will service the installation, working space less than that required in 110.26 or 110.34 shall be permitted.
3. Above Grade Level, Floor, or Work Platform. Where the enclosure is located above grade, the floor, or a work platform, all of the following shall apply:
1. The enclosure shall be accessible.
2. The width of the working space shall be the width of the enclosure or a minimum of 762 mm (30 in.), whichever is greater.
3. The depth of the workspace shall comply with 110.26(A) or 110.34 based upon the voltage to ground.
4. All doors or hinged panels shall open to at least 90 degrees.
Approval. All fixed industrial process heating equip‐ ment shall be installed in an approved manner.
Special Permission. Fixed industrial process heating equipment and systems installed by methods other than covered by this article shall be permitted only by special permission.
Supply Conductors. Fixed industrial process 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.
Locations.
1. Exposed to Physical Damage. Where subject to physical damage, fixed industrial process heating equipment shall be protected in an approved manner.
2. Damp or Wet Locations. Fixed industrial process heating equipment installed in damp or wet locations shall be listed for such locations 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: See 110.11 for equipment exposed to dete‐ riorating agents.
Spacing from Combustible Materials. Fixed industrial process 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.
Infrared Lamp Industrial Heating Equipment. In industrial occupancies, infrared industrial process heating equipment lampholders shall be permitted to be operated in series on circuits of over 150 volts to ground, provided the volt‐ age rating of the lampholders is not less than the circuit volt‐ age.

Each section, panel, or strip carrying a number of infrared lampholders, including the terminal wiring of such section, panel, or strip, shall be considered as infrared industrial heat‐ ing equipment. The terminal connection block of each assem‐ bly shall be considered an individual outlet.

Part III. Control and Protection of Fixed Industrial Process Heating Equipment

425.19 Disconnecting Means. Means shall be provided to simultaneously disconnect the heater, motor controller(s), and supplementary overcurrent protective device(s) of all fixed industrial process heating equipment from all ungrounded conductors. 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 simulta‐ neously disconnect all ungrounded conductors that it controls. The disconnecting means specified in 425.19(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

Heating Equipment with Supplementary Overcurrent Protection. The disconnecting means for fixed industrial process heating equipment with supplementary overcurrent protection shall be within sight from the supplementary over‐ current protective device(s), on the supply side of these devi‐ ces, if fuses, and, in addition, shall comply with either 425.19(A)(1) or (A)(2).
1. Heater Containing No Motor Rated over 1∕8 Horsepower. The disconnecting means specified in 425.19 or unit switches complying with 425.19(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. The disconnecting means is in sight from the motor controller(s) and the heater and complies with Part IX of Article 430.
  2. 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.
Heating Equipment Without Supplementary Overcurrent Protection.
1. Without Motor or with Motor Not over 1∕8 Horsepower.For fixed industrial process heating equipment without amotor 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 fixed industrial process heating equipment with a motor rated over 1∕8 hp, a disconnecting means shall be located within sight from the motor controller or shall be permitted to comply with the requirements in 425.19(A)(2).
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. The branch circuit switch or circuit breaker, where readily accessible for servicing of the fixed heater, shall be permitted as the other disconnecting means.

425.21 Switch and Circuit Breaker to Be Indicating. Switches and circuit breakers used as disconnecting means shall be of the indicating type.

425.22

Branch-Circuit Devices. Fixed industrial process heating equipment other than such motor-operated equipment as required by Articles 430 and 440 to have additional overcurrent protection shall be permitted to be protected against overcur‐ rent where supplied by one of the branch circuits in Arti‐ cle 210.
Resistance Elements. Resistance-type heating elements in fixed industrial process heating equipment shall be protected at not more than 60 amperes. Equipment rated more than 48 amperes and employing such elements shall have the heating elements subdivided, and each subdivided load shall not exceed 48 amperes. Where a subdivided load is less than 48 amperes, the rating of the supplementary overcurrent protec‐ tive device shall comply with 425.3(B). A boiler employing resistance-type immersion heating elements contained in an ASME-rated and stamped vessel shall be permitted to comply with 425.72(A).
Overcurrent Protective Devices. The supplementary over‐ current protective devices for the subdivided loads specified in 425.22(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 No. 1: See 240.10. Where cartridge fuses are used to provide this overcurrent protection, a single disconnect‐ ing means shall be permitted to be used for the several subdivi‐ ded loads.Informational Note No. 2: For supplementary overcurrent protection, see 240.10.Informational Note No. 3: disconnecting means for cartridge fuses in circuits of any voltage, see 240.40.
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 425.22(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.
Conductors for Subdivided Loads. Field-wired conduc‐ tors between the heater and the supplementary overcurrent protective devices for fixed industrial process heating equip‐ ment shall be sized at not less than 125 percent of the load served. The supplementary overcurrent protective devices specified in 425.22(C) shall protect these conductors in accord‐ ance with 240.4. Where the heaters are 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:

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

Part IV. Marking of Heating Equipment

Nameplate.
1. Marking Required. Fixed industrial process heating equipment shall be provided with a nameplate giving the iden‐ tifying name and the normal rating in volts and watts or in volts and amperes.Fixed industrial process heating equipment intended for use on alternating current only, direct current only, or both shall be marked to so indicate. The marking of equipment consist‐ ing 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.
Marking of Heating Elements. All heating elements that are replaceable in the field and are part of industrial proc‐ ess heating equipment shall be legibly marked with the ratings in volts or watts or in volts or amperes.

425.45 Concealed Fixed Industrial Heating Equipment — Inspection and Tests. Concealed fixed industrial heating equipment installations shall be made with due care to prevent damage to the heating equipment and shall be inspected and approved before heating equipment is covered or concealed.

Part V. Fixed Industrial Process Duct Heaters

General. Part V 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.
Identification. Heaters installed in an air duct shall be identified as suitable for the installation.
Airflow. Means shall be provided to ensure uniform airflow over the face of the heater in accordance with the manufacturer’s instructions.Informational Note: Some heaters installed within 1.2 m (4 ft) of the outlet of an air-moving device, elbows, baffle plates, or other obstructions in ductwork use 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.
Elevated Inlet Temperature. Duct heaters intended for use with elevated inlet air temperature shall be identified as suitable for use at the elevated temperatures.

Fan Circuit Interlock. Means shall be provided to ensure that the fan circuit, where present, is energized when any heater circuit is energized. However, time- or temperature- controlled delay in energizing the fan motor shall be permit‐ ted.
Limit Controls. Each duct heater shall be provided with an approved, integral, automatic-reset temperature limit‐ ing 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 discon‐ nects a sufficient number of conductors to interrupt current flow. This device shall be manually resettable or replaceable.
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 425.19(A).

Part VI. Fixed Industrial Process Resistance-Type Boilers

Scope. The provisions in Part VI 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 VII of this article.
Identification. Resistance-type boilers shall be identi‐ fied as suitable for the installation.
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 425.3(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 425.3(B).
3. Supplementary Overcurrent Protective Devices. The supplementary overcurrent protective devices for the subdivi‐ ded loads as required by 425.72(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.
4. 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 subdivi‐ ded circuits. See 240.40.
5. 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, 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- or pressure-actuated device controls the cyclic operation of the equipment.
6. Conductors for Subdivided Loads. Field-wired conductors between the heater and the supplementary overcurrent protec‐ tive devices shall be sized at not less than 125 percent of the load served. The supplementary overcurrent protective devices specified in 425.72(C) shall protect these conductors in accord‐ ance with 240.4. Where the heaters are 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:
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.
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.
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 VII. Fixed Industrial Process Electrode-Type Boilers

Scope. The provisions in Part VII of this article shall apply to boilers for operation at 600 volts, nominal, or less, inwhich heat is generated by the passage of current between elec‐ trodes through the liquid being heated.
Identification. Electrode-type boilers shall be identi‐ fied as suitable for the installation.
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).Exception: The provisions of this section shall not apply to conductors that form an integral part of an approved boiler. Where an electrode boiler is rated 50 kW or more, the conductors supplying the boiler elec‐ trode(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 condi‐ tions are met:
1. The electrode boiler is marked with a minimum conductor size.
2. The conductors are not smaller than the marked minimum size.
3. A temperature- or pressure-actuated device controls the cyclic oper‐ ation of the equipment.
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.
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.
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 electrodes shall be isolated and electrically insulated from ground.
Markings. All electrode-type boilers shall be marked to show the following:

The manufacturer’s name.
The normal rating in volts, amperes, and kilowatts.
The electrical supply required specifying frequency, number of phases, and number of wires.
The marking “Electrode-Type Process Heating Boiler.”
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.

426.1 ARTICLE 426 — FIXED OUTDOOR ELECTRIC DEICING AND SNOW-MELTING EQUIPMENT

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