390.1 Scope. This article covers the use and installation requirements for underfloor raceways.

390.2 Use.

(A) 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.

(B) 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 judged suitable for the condition or unless corrosion protection approved for the condition is provided, ferrous or nonferrous metal underfloor raceways, junction boxes, and fittings shall not be installed in concrete or in areas subject to severe corrosive influences.

390.3 Covering. Raceway coverings shall comply with

390.3(A) through (D).
(A) 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 (VA in.) of concrete or wood above the raceway.

Exception: As permitted in 390.3(C) and (D) for flat-top raceways.

(B) 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 ( 1 xh in.).

(C) 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 mechanical protection and rigidity equivalent to junction box covers.

(D)Other Raceways Flush with Concrete. In office oc- cupancies, 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 substantial linoleum that is not less than

1.6 mm ('/i6 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.

390.4 Size of Conductors. No conductor larger than that for which the raceway is designed shall be installed in underfloor raceways.

390.5 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.

390.6 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