Article 470

Resistors and Reactors

Part I. 1000 Volts, Nominal, and Under

  1. Scope. This article covers the installation of separate resistors and reactors on electrical circuits.Exception: Resistors and reactors that are component parts of other apparatus.This article also covers the installation of resistors and reac‐ tors in hazardous (classified) locations as modified by Arti‐ cles 501 through 504.
  2. Location. Resistors and reactors shall not be placed where exposed to physical damage.
  3. Space Separation. A thermal barrier shall be required if the space between the resistors and reactors and any combusti‐ ble material is less than 305 mm (12 in.).
  4. Conductor Insulation. Insulated conductors used for connections between resistance elements and controllers shall be suitable for an operating temperature of not less than 90°C (194°F).

Exception: Other conductor insulations shall be permitted for motor starting service.


Part II. Over 1000 Volts, Nominal

  1. General.
    1. Protected Against Physical Damage. Resistors and reactors shall be protected against physical damage.
    2. Isolated by Enclosure or Elevation. Resistors and reactors shall be isolated by enclosure or elevation to protect personnel from accidental contact with energized parts.
    3. Combustible Materials. Resistors and reactors shall not be installed in close enough proximity to combustible materials to constitute a fire hazard and shall have a clearance of not less than 305 mm (12 in.) from combustible materials.
    4. Clearances. Clearances from resistors and reactors to grounded surfaces shall be adequate for the voltage involved.
    5. Temperature Rise from Induced Circulating Currents. Metallic enclosures of reactors and adjacent metal parts shall be installed so that the temperature rise from induced circulat‐ ing currents is not hazardous to personnel or does not consti‐ tute a fire hazard.
  2. Grounding. Resistor and reactor cases or enclosures shall be connected to the equipment grounding conductor.Exception: Resistor or reactor cases or enclosures supported on a struc‐ ture designed to operate at other than ground potential shall not be connected to the equipment grounding conductor.
  3. Oil-Filled Reactors. Installation of oil-filled reactors, in addition to the above requirements, shall comply with appli‐ cable requirements of Article 450.