(A) Insulation; conductor size.
(1) A conductor with insulation capable of withstanding heat will have a higher ampacity rating than a conductor of the same size with a lower insulator temperature rating. National Electric Code Table 310-13 lists the various types of insulation available.
(2) The NEC § 210-19(c) specifies the minimum conductor size permitted in house wiring as 14 A WG.
(B) Cable support. NEC § 336-5 states that “type NM cable must be fastened by staples or straps so designed and installed as not to injure the cable”. The NEC requires a staple or strap be placed every 12 inches from every box or fitting, and four and one- half feet apart on the run between boxes, including vertical and horizontal runs.
(C) Fastening table to box. NEC § 370-7(c) states that “where non-metallic boxes are used, non-metallic-sheathed cable shall extend into the box no less than one-fourth inch through a non-metallic sheathed cable knockout opening”. When used with a single gang box and where the cable is fastened within eight inches of a box measured along the sheath, the cable extends into the box so no less than one-fourth inch, so that securing the cable to the box shall not be required.
(D) Cable protection. When drilling through studs or joists, the NEC requires non-metallic cable be at least one and one-half inches from the outer edge of the wooden member. Where the hole is drilled closer than the one and one-half inches limit, a one-sixteenth- inch steel plate is fastened to the wooden member in front of the cable to protect it. (See NEC 373.) NEC § 300-14 requires at least six inches of free conductor must be left at each outlet and switch point.
(E) Boxes.
(1) Number of conductors in a box. NEC Table 370-6(a) (Metal Boxes) shall apply where no fittings or devices, such as fixture studs, cable clamps, hickeys, switches or receptacles are contained in the box and where no grounding conductors are part of the wiring within the box. Where one or more fixture studs, cable clamps or hickeys are contained in the box, the number of conductors shall be reduced by one less than shown in NEC Table 370-6(a), and an additional deduction of one conductor shall be made for each strap containing one or more devices. A further deduction of one conductor shall be made for one or more grounding conductors entering the box.
(a) A conductor running through the box shall be counted as one conductor, and each conductor originating outside of the box and terminating inside the box shall be counted as one conductor. Conductors, no part of which leave the box, shall not be counted.
(b) The volume of a wiring enclosure (box) shall be the total volume of the assembled sections and, where used, the space provided by plaster rings, domed covers extension rings and the like, that are marked with their volume in cubic inches. (See NEC 370-6 (a)(1).)
(2) Marking boxes. The National Electric Code requires that boxes other than those described in NEC Table 370-6(a) must be “durably and legibly marked”.
(3) Other boxes. NEC § 370-6(b). Boxes 100 cubic inches or less, other than those described in NEC Table 370-6(a), non-metallic boxes and conduit bodies having provisions for more than two conduit entries shall be durably and legibly marked by the manufacturer with their cubic inch capacity. The maximum number of conductors permitted shall be computed using the volume per conductor listed in NEC Table 370-6(b) and the reductions provided for in NEC § 370-6(a)(1).
(4) Installation.
(a) NEC § 300-15. The box shall be installed at each conductor splice connection point, outlet, switch point, junction point or pull point for the connection of metal-clad cable, mineral-insulated metal sheathed cable, type AC cable, non-metallic- sheathed cable, type AC cable, non-metallic-sheathed cable or other cables, at the connection point between the cable system and a raceway system and at each outlet and switch-point for concealed knob and tube wiring.
(b) NEC § 370-10. In non-combustible material boxes, or fitting, the box shall be so installed that the front edge of the box or fitting will not sit back of the finished surface more than one-fourth inch. In combustible material boxes and fittings, the box shall be flush with the finished surface.
(Prior Code, § 152.24)