Electrical Boxes

NEC 2002, Article 314, “Outlet, Device, Pull, and Junction Boxes; Conduit Bodies; Fittings; and Manholes,” requires that wire connections or cable splices be contained inside approved metal or plastic boxes. This provision is intended to shield wood building framing members or other adjacent flammable objects from electrical sparks should the wiring be defective. Boxes also protect people from accidental electrical shock and keep moisture and dust out of wire connections or splices.

The outlet, device, pull, and junction boxes that must be sized, selected, and used in accordance with the latest NEC are classified by application. Boxes with supports are required in vertical raceways where the weight of the cable would place excessive strain on the conductor terminals. These conduit-support boxes are common in highrise buildings, where heavy feeder conductors are used. Most electrical boxes are made from sheet steel and have a painted or galvanized finish, but some are made of cast iron, aluminum, PVC, or clear polycarbonate

JUNCTION AND PULL BOXES

Junction and pull boxes provide access points for pulling and feeding conductors into a raceway system. Their use is mandatory in conduit runs where the number of bends between outlets exceeds the maximum number permitted by the NEC. Among the many standard commercial products available are the following :

  • Metal surface-mounted pull boxes finished with gray enamel paint are available with or without knockouts. Widths and heights are from 4 to 24 in., and depths are from 4 to 8 in. These boxes include covers attached with screws.
  • Galvanized metal surface-mounted pull boxes are available with or without knockouts. Widths and heights are from 4 to 36 in., and depths are from 4 to 12 in. These boxes also include covers attached with screw.
  • Metal junction pull boxes with hinged covers, finished in gray enamel, are also available with or without knockouts Widths are from 4.5 to 24 in., heights are from 5 to 12 in., and depths are from 3 to 6 in
  • Moisture-proof, gasketed boxeswith metal screw-on covers, finished in gray enamel, do not have knockouts. Widths and heights are from 6 to 12 in., and depths are from 4 to 6 in.
  • Double-door metal transformer cabinets, finished in gray enamel, do not have knockouts. Widths are 36 in., heights are 30 to 36 in., and depths are 10 in.

OUTLET BOXES

The NEC 2002, Article 314, “Outlet, Device, Pull, and Junction Boxes; Conduit Bodies; Fittings and Manholes,” requires that outlet boxes with the correct fittings be installed at the following locations:

  • Each conductor splice point
  • Each outlet, switch point, or junction point
  • Each pull point for the connection of conduit and other raceways
  • Transitions from conduit to open cables

A wide variety of standard metal and nonmetallic electrical or outlet boxes are available commercially for installing switches and receptacles, connecting lighting fixtures, and protecting splices. Only the most commonly used boxes and their features are discussed and illustrated in this chapter. These are the rectangular and square metal and plastic boxes for wall switches and duplex receptacles and the octagonal and round electrical boxes for making wire connections to ceiling or wall luminaires. All of these electrical boxes are available in different depths. The boxes used must be deep enough to permit switches and receptacles and their wiring to be inserted and removed easily without crimping or damaging the wires. This applies as well to boxes used to suspend luminaires.

Because so many different manufacturers offer these electrical boxes, the reader is advised to consult the latest manufacturers’catalogs for more details than can be given here. In addition to depth, boxes are available with different numbers of knockouts (KOs) or pryouts (POs) to accommodate different arrangements for switch levers, control knobs, receptacle sockets, or other devices.

Product lines are changed in response to customer demand, and manufacturers drop products lines that are no longer sold in large enough quantities to be profitable. The boxes listed below are readily available in hardware and electrical supply stores today.

Rectangular (3-in. x 2-in.) metal and plastic boxes are intended for installing a single switch or a duplex receptacle. Also called device boxes, they are normally mounted vertically in walls by fastening them to wall studs in new construction. A 3-in.x 2-in. rectangular metal switch box with a detachable side is shown in below Figure. This box has a depth of 2 1/2 in. and a volume of 12.5 in.3. Boxes with depths of 2.75 in. have minimum volumes of 14 in.3, and those with depths of 3 1/2 in. have minimum volumes of 18 in.

These 3-in. x 2-in. boxes can be ganged together with matching boxes to provide a single box with twice the volume. After removing the opposing removable sides of each box, they are fastened together with screws to form one rigid unit. Ganged boxes can accept two switches or two duplex receptacles with their associated wiring. Rectangular switch boxes are available with the following features:

  • Gangable or nongangable
  • Depths of 1 1/2 to 3 1/2 in., 2.56, 2.75, and 3.50 in.
  • Nonmetallic (NM) and armored cable clamps
  • Beveled back corners on some configurations
  • Screw-driven flexible expansion clamps for installing the boxes in drywall cutouts

Metal 4-in. x 2.125-in. utility “handy” boxes are used for mounting switches and receptacles. The box shown in Fig. 6-8 has a depth of 1.875 in. and a volume of 13 in . It has six knockouts (KOs), two each on the sides and back. Handy boxes are deepdrawn from a single sheet of steel and galvanized for protection. Some have attached mounting straps. There are also handy boxes with square corners and nine KOs. They are 1.5 in. deep and have volumes of 11.5 in.3.

Plastic 2 1/4-in. x 4 1/4-in. rectangular boxesfor mounting switches or duplex receptacles can only be used with nonmetallic (NM) cable. An example of a box with four NM cable clamps is shown in below Figure. This plastic box, nominally sized 3 in. x 2 in., has a volume of 5.5 in.3. It is intended for use where circuits are being extended in houses with finished walls. A cutout is made in the gypsum wallboard, and the wire or wires to be connected are fished out and clamped within the box. The box is then inserted in the cutout and the swing-out clamps on the top and bottom of the box are extended by turning the screws in the linkage to clamp the box securely to the back side of the wallboard.

These so-called retrofit boxes are popular after-market items that can be installed in finished rooms to add new switches or receptacles. Their use avoids removing and later replacing drywall sections to expose the wall studs needed for anchoring the boxes.

However, there are 3-in. x 2-in. plastic boxes with preattached nails in bosses directed at an angle for fastening the boxes to studs in new construction. The NEC requires that all wall switches installed in any of these plastic boxes have grounding screws.

Square metal boxes measuring 4 in. x 4 in. and 4 11/16 x 4 11/16 in. are large enough to gang two switches or two duplex receptacles in one box or to protect cable splices. A 4-in. x 4-in. box with a depth of 1.5 in. is shown in below Figure. This box has a minimum volume of 21.0 in.3. The box shown has 17 conduit KOs.

Square boxes are also normally mounted vertically in walls by nailing them to studs during original construction. A wide selection of cover plates is available for these boxes, with different cutouts to admit the toggles of strap-mounted switches or to expose both outlets of strap-mounted dual receptacles. Single switches or duplex receptacles can be installed in these boxes if an adapter cover plate with a centrally located cutout is used.

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