thickness of 13 mm (!/2 in.), or other noninsulating material identified as suitable for this use and applied ac cording to specified thickness and directions.

(F) Secured. Cables shall be secured by means of ap proved 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.
(G) Dry Board Installations. In dry board installations, the entire ceiling below the heating cable shall be covered with gypsum board not exceeding 13 mm (V2 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.

(H) Free from Contact with Conductive Surfaces.

Cables shall be kept free from contact with metal or other electrically conductive surfaces.
(I) Joists. In dry board applications, cable shall be installed parallel to the joist, leaving a clear space centered under the joist of 65 mm (2'/2 in.) (width) between centers of adjacent runs of cable. A surface layer of gypsum board shall be mounted so that the nails or other fasteners do not pierce the heating cable.

(J) 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 penetrations and luminaires.

424.42 Finished Ceilings. Finished ceilings shall not be covered with decorative panels or beams constructed of materials 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.

424.43 Installation of Nonheating Leads of Cables.

(A) 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.

(B) 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.

(C) 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.

424.44 Installation of Cables in Concrete or Poured

Masonry Floors.

(A) Watts per Linear Meter (Foot). Constant wattage heating cables shall not exceed 54 watts/linear meter (I6V2 watts/linear foot) of cable.

(B) Spacing Between Adjacent Runs. The spacing be- tween adjacent runs