Electrical Engineering

In Article " Electrical Boxes - Part One ", I explained that there are many types of Electrical 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.

And I explained the first type: the outlet boxes in this Article.

Today, I will explain other types of Electrical Boxes as follows.

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

Types of Electrical Conduits

Conduit Fittings and Supports

Second: Device Boxes

Device boxes are usually rectangular outlet boxes and they are intended for installing a single switch or a duplex receptacle. They are normally mounted vertically in walls by fastening them to wall studs in new construction.

Device 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.

A 3-in. x 2-in. rectangular metal switch box with a detachable side is shown in above image. This box has a depth of 2.5 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 3.5 in. have minimum volumes of 18 in.³.

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.

Device switch boxes are available with the following features:

Gangable or nongangable,
Depths of 1.5 to 3.5 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.

Third: Junction and pull boxes

A pull box is a conduit box or condulet, made of sheet metal, cast metal, or a non-metallic material, that is used to pull wires through. These boxes provide a way to pull conductors long distances without placing excessive strains on the wire or insulation. A pull box allows long runs of conductors to be pulled in shorter intervals. It is used for straight pulls, or pulls that make single 90 degree turns.

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:

1- Metal surface-mounted pull boxes

They 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.

2- Galvanized metal surface-mounted pull boxes

They 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 screws.

3- Metal junction pull boxes with hinged covers

They finished in gray enamel, are also available with or without knockouts Widths are from 41_2 to 24 in., heights are from 5 to 12 in., and depths are from 3 to 6 in.

4- Moisture-proof, gasketed boxes with metal screw-on covers

They 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.

5- Double-door metal transformer cabinets

They finished in gray enamel, do not have knockouts. Widths are 36 in., heights are 30 to 36 in., and depths are 10 in.

Sizing of Junction and pull boxes according to NEC Section 314-28

The size of the box is based on the size and number of conductors, as well as the number of raceways and their diameter As per Section 314-28(a) which applies for 3/4 inch or larger raceways (conduit or cable) with #4 or larger conductors or cables pulled through the raceway.

1- For a Straight Pull

The width of a box for a straight pull is determined by the size of the largest conduit and by the space required by the locknuts and bushings. The length cannot be less than eight times the largest conduit. This rule applies to raceways 3/4 inch or larger with conductors #4 or larger.

For example, trade size 2 conduit containing four 4/0 AWG, Type THHW conductors requires (8 x 2 in. = 16 in. long pull box).

However, although 16 in. is the required minimum length, a longer pull box may be desired for maximum ease in handling this size conductor.

2- For Angle Pull, or U-Pull, or Splices

In an angle pull, or U-pull, a group of conduits enters a pull or junction box at one side and leaves at the top or bottom. When the minimum length is figured from the rules of 314-28(a)(2), this length sets the minimum width also. The inside length of the box must not be less than six times the largest raceway plus the sum of the diameters of all additional raceways entering the box. The wall, or row, with the most conduit entries must be used to size the junction box.

The diagonal distance between the centers of the raceways where they enter the box must be six times the raceway’s diameter.

Adjusting the previous example of trade size 2 conduit containing four 4/0 AWG, Type THHW conductors, provided the conductors were spliced within the enclosure, the required pull box dimension could be reduced to a 12-in.-long pull box (6 x 2 in. = 12 in.).

Another example for sizing of U Pull box is illustrated in the below image.

Special case:

If the raceway enclosing the same conductor, the raceway are required to have a minimum separation between them. The intent is to provide adequate space for the conductor to make the bend.

For example, the calculations for raceway having the same conductor are made in below image.