These are all different types of electrical boxes
Before we get into specific types of boxes, let's go over some things that are applicable to all types of electrical boxes.
All electrical connections must be contained inside an electrical box. The box shields the building material and other flammable materials in the event of electrical sparks.
All boxes must be accessible. Never cover a box with drywall, paneling or other wall coverings.
If an electrical junction box holds only spliced wires and no device, such as a switch, it should be covered with a blank cover plate.
An electrical box should be installed with the front edge flush with the finished surface of the wall or ceiling. If the space between the finished surface and the edge of the box is greater than 1/8', then a box extender should be installed.
Make sure your box is deep enough to avoid crowding the wires. It must be deep enough so a switch or receptacle can be installed easily without crimping or damaging the wires. Electrical codes determine how many wires of what size each size of box can accommodate based on the cubic-inch capacity of the box. For example, a #14 wire occupies 2 cubic inches and a #12 wire occupies 2.25 cubic inches. When counting wires, count the fixture or device as one wire. It's always safe to use a large box unless you don't have room in the wall or ceiling. There is a chart at Ask-The-Electrician.com. that shows the size of different boxes and the wires they will accommodate.
Electrical boxes come in different materials and different shapes. By familiarizing yourself with the different types of boxes, you'll be able to select the correct box for your home wiring project.
Indoor boxes are usually either plastic or metal.
Plastic electrical boxes are the most widely used boxes for indoor residential wiring. They're inexpensive and easy to install. However, since you cannot ground a plastic box, so some local codes do not allow them or they are only allowed for certain uses. Check with your local building department before using a plastic box.
Some plastic boxes have holes w/knockout tabs. These boxes do not have built-in clamps so the cable is not held in place by the box. You must use cable clamps and staple the cable within 8 inches of the box if you use this type of box.
Plastic boxes are easier to damage than metal boxes, so buy extra boxes just in case. Never install a cracked box.
Most are brittle; don't use them where they are not built into the wall. The exception is an outdoor box made of extra strong PVC.
Don't use with heavy light fixtures and fans.
Some plastic boxes include nails for anchoring the box to the framing material.
Metal electrical boxes are stronger and provide better ground connection than plastic boxes.
Metal boxes must be grounded to the circuit grounding system. Connect the circuit grounding wires to the box with a pigtailed green wire and wire nut, or with a grounding clip.
The cable entering a metal box must be clamped.
'Gangable' boxes can be dismantled and ganged together to make space for two or more devices.
These are sometimes called old-work or cut-in boxes.
Remodel electrical boxes are used when running cable to install new devices into an old wall.
Plastic remodel boxes have 'wings' and metal remodel boxes have expandable clips or bendable ears that hold them in the wall.
Outdoor boxes are usually molded plastic or cast aluminum.
These boxes are used with PVC conduit in outdoor wiring and exposed indoor wiring.
These are required for outdoor fixtures connected with metal conduit.
They have sealed seams and threaded openings to keep moisture out.
Rectangular (2'X3') Trade Name 'One-Gang':
These boxes are used for switches and receptacles.
One-gang boxes may have detachable sides that allow them to be ganged together to form two-gang boxes.
Square (4'X4') Trade Name 'Four-Square':
'Plaster Rings' are used as adapters to accommodate the following configurations: One-Gang, Two-Gang, Three-Inch or Four-Inch Round.
When a square box is used only for splicing cables, it is called an electrical junction box and a blank cover plate must be used.
Octagonal Trade Name 'Three-0':
These contain wire connections for ceiling fixtures.
Some octagonal electrical boxes have extendable braces that will fit any joist spacing and are nailed or screwed to the framing material.
While selecting the correct electrical box for your project will help to ensure the successful completion of your wiring project, always respect electricity and follow safety precautions. Never work on live circuits. Before work begins, the circuit should be identified and turned off at the panel, tagging it to let others know that work is being done on that circuit. Confirm that the power is off with a voltage tester. Electrical work should only be performed by a confident, experienced person or by a licensed electrical contractor.
Dave Rongey is a Licensed Electrical Contractor with over 35 years of experience. He is also an avid Do It Yourselfer.
More information on electrical boxes can be found in the Electrical Boxes section of his website: Ask-The-Electrician.com.