Table 5-1. - Minimum Cover Requirements for 0 to 600 Volts (Burial in Inches) - Continued
|In solid rock where covered by minimum of 2 inches concrete extending down to rock||2 inches, raceway only||2||2||2 inches, raceway only||2 inches, raceway only|
Note 1. For SI units; one inch = 25.4 millimeters.
Note 2. Raceways approved for burial only where concrete encased will require concrete envelope not less than 2 inches thick.
Note 3. Lesser depths are permitted where cables and conductors rise for terminations or splices or where access is otherwise required.
Note 4. Where one of the conduit types listed in columns 1 through 3 is combined with one of the circuit types in columns 4 and 5, the shallower depth of burial is permitted.
conductors. Ungrounded conductors will be distinguished by colors other than white, natural gray, or green, or by a combination of color plus distinguishing marking. Distinguishing markings also will be in a color other than white, natural gray, or green. and will consist of a stripe or stripes or a regularly spaced series of identical marks. Distin- guishing markings will not conflict in any manner with the surface markings required by the NEC® .
Underfloor raceway systems are used in office buildings for the installation of the wiring for telephone and signal systems and for convenience outlets for electrically operated office machinery. They provide a flexible system by which the location of outlets may be changed easily to accommodate the rearrangement of furniture and partitions. The NEC® allows their use when embedded in concrete or in the concrete fill of floors. Their installation is allowed only in locations that are free from corrosive or hazardous conditions. No wires larger than the maximum size approved for the particular raceway will be installed. The voltage of the system must not exceed 600 volts. The total cross-sectional area of all conductors in a duct must not be greater than 40 percent of the interior cross-sectional area of the duct.
An underfloor raceway system consists of ducts laid below the surface of the floor and interconnected by means of special cast-iron floor junction boxes. The ducts for underfloor raceway systems are made of either fiber or steel. Fiber ducts are made in two types-the open-bottom type and the completely enclosing type. Steel ducts are always of the completely enclosing type, usually having a rectangular cross section. In the underfloor raceway system, provision is made for outlets by means of specially designed floor-outlet fittings that are screwed into the walls of the ducts. When fiber ducts are used, the duct system is laid in the floor with or without openings or inserts for outlets. After the floor has been poured and finished as desired, the outlet fittings are installed into inserts or at any points along the ducts at which outlets are required. The method of installing outlet fittings is described in the next paragraph. When steel ducts are used, provision for the outlet fittings must be made at the time that the ducts are laid before the floor or floor fill is poured. The steel ducts are manufactured with threaded openings for outlet connections at regularly spaced intervals along the duct. During the installation of the raceway and the floor, these outlet openings are closed with specially constructed plugs whose height can be adjusted to suit the floor level. For telephone and similar circuits, much wider ducts can be obtained.
In general, underfloor raceways should be installed so that there is at least 3/4 inch of concrete or wood over the highest point of the ducts. However, in office-approved raceways, they may be laid flush with the concrete if covered with linoleum or equivalent floor covering. When two or three raceways are installed flush with the concrete, they must be contiguous with each other and joined to form a rigid assembly. Flat-top ducts over 4 inches wide but not over 8 inches, spaced less than 1 inch apart, must be covered with at least 1/2 inch of concrete. It is standard practice to allow 3/4-inch clearance between ducts run side by side. The center line of the ducts should form aContinue Reading