Electrical Engineering

In Article " Conduit Fittings and Supports ", I explained the following points:

Types of Conduit Fittings and Supports,
Notes for Conduit Supporting.

Today, I will explain The Types of Electrical Boxes as follows.

You can review the following articles in the same course for more information:

Types of Electrical Conduits

1- Functions of the Electrical Boxes

In any installation, the electrical boxes are used to fulfill the following requirements:

The electrical boxes enclose the wire connection or cable splices.
The electrical boxes shield wood building framing members or other adjacent flammable objects from electrical sparks should the wiring be defective.
The electrical Boxes also protect people from accidental electrical shock and keep moisture and dust out of wire connections or splices.

2- Electrical Boxes Materials

Most electrical boxes are made from the following materials:

Sheet steel and have a painted or galvanized finish,
Cast iron,
Aluminum,
PVC, or
Clear polycarbonate.

3- Where an electrical box is required in the wiring system?

Section 300.15 of the NEC contains specific requirements for where a box or conduit body is required in the wiring system. As can be seen, there is a general statement with several exceptions.

1- Box or Conduit Body

A box or conduit body complying with Article 314 is required to be installed at each conductor splice connection point, outlet, switch point, junction point, or pull point for the connection of conduit, electrical metallic tubing, surface raceway or other raceways.

2- Box Only

A box is required to be installed at each conductor splice connection point, outlet, switch point, junction point or pull point for the connection of Type MC cable, mineral-insulated, metal-sheathed cable, or other cables. A box shall be installed at the connection point between any such cable system and a raceway system.

Note:

Boxes should be installed only when needed. Too many boxes would introduce so much splicing of conductors that the likelihood of problems from improperly spliced conductors would increase.

4- General Rules for Boxes

All boxes must be large enough to provide for sufficient free space for all conductors that will be enclosed within them to prevent overcrowding and possible physical damage when the devices or splices are installed or completed.

No box is permitted to have an internal depth of less than 1/2 inch. A box that is intended to enclose flush devices is required to have an internal depth of not less than 15/16 inch.

In completed installations each box is required to have a cover, faceplate or fixture canopy.

Metal covers used with nonmetallic boxes are required to be grounded.

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 high rise buildings, where heavy feeder conductors are used.

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.

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.

5- Mounting of Electrical Boxes

Any type of box must be set flush in a wall or ceiling of wood or other combustible material. In this way, material that will burn is kept away from the wires in case of arcs or sparks in the box. If the box is set in a wall or ceiling of material that will not burn, the box may be recessed up to 1/4 inch.

At least 6 inches of conductor must be left at an outlet or switch box to connect fixtures or devices. This measurement is taken from the point where the conductor emerges from the conduit or cable sheath. Where the box opening is less than eight inches in any dimension, each conductor shall extend 3 inches outside the opening.

6- Types of Electrical Boxes

There are many types of boxes used in a conduit installation. The application and site conditions determine the type of box used. The electrical boxes can be divided to:

The outlet boxes,
Device boxes,
Pull and junction boxes.

The following is a list of the more common boxes and a brief description of their intended use.

First: Outlet Boxes

The 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 course.

The type of enclosures used as outlet and device boxes for supporting, enclosing and securing wiring devices such as switches, receptacles or other equipment on the same yoke or strap are available in various sizes and shapes. These enclosures may be used in one, two, three or more gang types.

6.1 Rectangular outlet boxes

Rectangular outlet box

Rectangular outlet 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.

Rectangular outlet 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.

6.2 Octagonal Outlet Boxes

An octagon box often referred to as a “fixture box”, connects light fixtures. Octagon boxes have cover plates to protect conductors when the box is used as a junction box. Table 314-16(a) lists the sizes of boxes with or without devices, but only #14 through #6 wires may be used. Octagon boxes are available in different sizes to accommodate cable clamps, fitting, and devices (switches or receptacles).

These boxes are conventionally measured diagonally, where their actual dimensions are 4 in., but they have nominal 4-in. dimensions when measured between their flat sides. They can be fastened to ceiling joists or studs with nails or screws, or can be supported by telescoping metal braces that extend to fit between adjacent joists. These boxes are frequently used to support luminaires if there are studs or hickeys in the box, on the box covers, or on cross straps.

Notes for fixture boxes:

Boxes for supporting lighting fixtures and ceiling paddle fans are available in the round and octagonal types.

Generally, device boxes are not acceptable for securing or supporting lighting fixtures. One of the reasons is that device boxes have 6-32 tapped screw holes and round or octagonal boxes have 8-32 tapped screw holes to support the weight of the lighting fixture.

6.3 Round Ceiling Pan Boxes

Round ceiling pan boxes with diameters of 3.25 in. have applications similar to those of octagonal boxes because they can be mounted on walls or ceilings to support light fixtures. Round boxes with depths of 0.5 in. have volumes of 4 in.³, and those with depths of 0.75 in. have volumes of 5.5 in.³. A choice of internal cable clamps is offered with these boxes, and they can accept round covers that fit both round and octagonal boxes.

6.4 Square Outlet Boxes

The square box measures four inches by four inches. They are large enough to gang two switches or two duplex receptacles in one box or to protect cable splices.

It encloses switches, thermostats, timers and receptacles. They have more cubic-inch volume than octagon boxes and allow more conductors to be installed. They also have accessories such as plaster (mud) rings to mount fixtures or devices. It also functions as a junction box or pull box.

Where a square metal box is used, it may be used for the support of a fixture or device, depending on the type of raised cover used. These covers are often referred to as “plaster” or “mud” rings.

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.

6.5 The Utility Box

The utility box or outlet box mounts on surfaces. It is often referred to as a “handy box.” The utility box encloses switches, receptacles, thermostats, or push button units. The box is a minimum of 1.5 inches deep. Boxes must meet the minimum cubic inch capacity per number of conductors per Table 314-16(a) and Table 314-16(b) of Article 314 in the NEC. Connect conduit to any one of the knockout holes in the box. This type of box is used for exposed work.

6.6 The Conduit Device Box

A conduit device box installs in drywall. The sides can be removed to enlarge it by attaching another box. This is referred to as "ganging the boxes." A conduit device box installs in offices and homes for switches and receptacles.

6.7 The masonry box

The masonry box is special conduit device boxes. The masonry box installs in concrete block and brick walls. Available in various sizes, the masonry box does not enlarge.

6.8 The weatherproof box

The weatherproof box is special conduit device boxes and used outside and in damp or wet areas. The weatherproof box is made of heavy cast metal and has a threaded hub to connect to conduit.