Article 390

Underfloor Raceways

1. Scope. This article covers the use and installation requirements for underfloor raceways.
2. Definition.Underfloor Raceway. A raceway and associated components designed and intended for installation beneath or flush with the surface of a floor for the installation of cables and electrical conductors.
3. Use.
   1. Permitted. The installation of underfloor raceways shall be permitted beneath the surface of concrete or other flooring material or in office occupancies where laid flush with the concrete floor and covered with linoleum or equivalent floor covering.
   2. Not Permitted. Underfloor raceways shall not be installed
      1. where subject to corrosive vapors or (2) in any hazardous (classified) locations, except as permitted by 504.20 and in Class I, Division 2 locations as permitted in 501.10(B)(3). Unless made of a material approved for the condition or unless corrosion protection approved for the condition is provided, metal underfloor raceways, junction boxes, and fittings shall not be installed in concrete or in areas subject to severe corro‐ sive influences.
4. Covering. Raceway coverings shall comply with 390.4(A) through (D).
   1. Raceways Not over 100 mm (4 in.) Wide. Half-round and flat-top raceways not over 100 mm (4 in.) in width shall have not less than 20 mm (3∕4 in.) of concrete or wood above the raceway.Exception: As permitted in 390.4(C) and (D) for flat-top raceways.
   2. Raceways over 100 mm (4 in.) Wide But Not over 200 mm (8 in.) Wide. Flat-top raceways over 100 mm (4 in.) but not over 200 mm (8 in.) wide with a minimum of 25 mm (1 in.) spacing between raceways shall be covered with concrete to a depth of not less than 25 mm (1 in.). Raceways spaced less than 25 mm (1 in.) apart shall be covered with concrete to a depth of 38 mm (11∕2 in.).
   3. Trench-Type Raceways Flush with Concrete. Trench-type flush raceways with removable covers shall be permitted to be laid flush with the floor surface. Such approved raceways shall be designed so that the cover plates provide adequate mechani‐ cal protection and rigidity equivalent to junction box covers.
   4. Other Raceways Flush with Concrete. In office occupan‐ cies, approved metal flat-top raceways, if not over 100 mm (4 in.) in width, shall be permitted to be laid flush with the concrete floor surface, provided they are covered with substan‐ tial linoleum that is not less than 1.6 mm (1∕16 in.) thick or with equivalent floor covering. Where more than one and not more than three single raceways are each installed flush with the concrete, they shall be contiguous with each other and joined to form a rigid assembly.
5. Size of Conductors. No conductor larger than that for which the raceway is designed shall be installed in underfloor raceways.
6. Maximum Number of Conductors in Raceway. The combined cross-sectional area of all conductors or cables shall not exceed 40 percent of the interior cross-sectional area of the raceway.
7. Splices and Taps. Splices and taps shall be made only in junction boxes.For the purposes of this section, so-called loop wiring (continuous, unbroken conductor connecting the individual outlets) shall not be considered to be a splice or tap.Exception: Splices and taps shall be permitted in trench-type flush race‐ way having a removable cover that is accessible after installation. The conductors, including splices and taps, shall not fill more than 75 percent of the raceway area at that point.
8. Discontinued Outlets. When an outlet is abandoned, discontinued, or removed, the sections of circuit conductors supplying the outlet shall be removed from the raceway. No splices or reinsulated conductors, such as would be the case with abandoned outlets on loop wiring, shall be allowed in raceways.
9. Laid in Straight Lines. Underfloor raceways shall be laid so that a straight line from the center of one junction box to the center of the next junction box coincides with the centerline of the raceway system. Raceways shall be firmly held in place to prevent disturbing this alignment during construc‐ tion.
10. Markers at Ends. A suitable marker shall be installed at or near each end of each straight run of raceways to locate the last insert.
11. Dead Ends. Dead ends of raceways shall be closed.

1. Junction Boxes. Junction boxes shall be leveled to the floor grade and sealed to prevent the free entrance of water or concrete. Junction boxes used with metal raceways shall be metal and shall be electrically continuous with the raceways.
2. Inserts. Inserts shall be leveled and sealed to prevent the entrance of concrete. Inserts used with metal raceways shall be metal and shall be electrically continuous with the raceway. Inserts set in or on fiber raceways before the floor is laid shall be mechanically secured to the raceway. Inserts set in fiber raceways after the floor is laid shall be screwed into the race‐ way. When cutting through the raceway wall and setting inserts, chips and other dirt shall not be allowed to remain in the race‐ way, and tools shall be used that are designed so as to prevent the tool from entering the raceway and damaging conductors that may be in place.
3. Connections to Cabinets and Wall Outlets. Connec‐ tions from underfloor raceways to distribution centers and wall outlets shall be made by approved fittings or by any of the wiring methods in Chapter 3, where installed in accordance with the provisions of the respective articles.

390.17 Ampacity of Conductors. The ampacity adjustment factors, in 310.15(B)(3), shall apply to conductors installed in underfloor raceways.