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Electrical Wiring Junction Boxes

Summary: Understanding electrical junction boxes and what they are used for. Home electrical wiring is the process of installing electrical wire to a location that will serve electrical devices or an appliance.

Electrical Junction Boxes, Fixture Boxes, Receptacle Boxes, Splice Boxes

One very important component is the box where the wire will be installed. The type and size of the home wiring electrical boxes will depend upon the circuit size, application and its location.

Box Fill - How many wires you can put in a box? The NEC Code requires wiring to be protected and enclosed when devices such as receptacles, fixtures and switches are being installed. Sometimes it is necessary to junction or splice wires in home wiring electrical boxes.

When splicing wires, an accessible electrical junction box box is necessary to enclose the spliced wires, thus the trade name is electrical enclosure.
This list shows various electrical boxes and displays the box name and describes the typical application.

The most common type of materials that home wiring electrical boxes are made of are: plastic, fiberglass, steel, aluminum and bakealight. The metal boxes come with knock-outs for using conduit such as EMT (electrical metallic tubing) or flex- steel or aluminum.

Why don't you see the exact same electrical box at your supplier? The availability of the types of boxes will depend upon which electrical manufacturer or purchasing channel your supplier has a contract with.

Boxes and Wire-Fill
How many wires can be installed into an electrical box will depend on how many Cubic Inches the box has and what size wires are being installed.

Below is a chart of a few common home wiring electrical boxes and the actual information which should be stamped or molded into each enclosure.

A good rule of thumb is to never over fill a box, leave yourself plenty of room, and don't cut your wire too short either - you can always trim off a little, but you can't stretch it. Keep in mind that the Ground Wires are not counted.

Box CU.IN.
#14 Wires
#12 Wires
#10 Wires
1-GANG NAIL PLASTIC 20.5 10 9 8
2-GANG CUT-IN FIBERGLASS 28.0 14 12 11
3-GANG CUT-IN FIBERGLASS 42.5 21 18 17
2-GANG NAIL PLASTIC 43.5 21 19 17

More about Electrical Box Fill Calculations

Electrical Code for Electrical Box Fill Capacity
National Electric Codes - Wire in Electrical Box NEC-Table370-16a


Common Home Electrical Wiring Boxes and How to Use Them

Electrical Box Part Name Electrical Applications
electrical wiring box - 1 gang plastic nail box 1-Gang Plastic Nail Box used for Type NM Cable wiring. Also known as a Rough-In Box. Outlets and Switches.
electrical wiring box - 1-gang cut in box 1-Gang Fiberglass Cut-In Box with a Back Bracket. Outlets and Switches. Type NM Cable wiring. Used with cable wiring in Remodel applications.
electrical wiring box - 1-gang metal cut in box 1-Gang Metal Cut-In Box with Adjusting Mounting Ears. Outlets and Switches, with knock-outs for steel or aluminum flex.
electrical wiring box - 1-gang handy box 1-Gang Handy Box. Outlets and Switches. Surface mount Microwave Receptacle. Garage or Shop area with Conduit.
electrical wiring box - 2-gang electrical nail box

2-Gang Plastic Nail Box.

Outlets and Switches.
electrical wiring box - 2-gang cut-in box 2-Gang Fiberglass Cut-In Box. Outlets and Switches.
electrical wiring box - 2-gang facenail box 2-Gang Plastic Face Nail Box. Outlets and Switches. Great for tight areas with limited room to nail.
electrical wiring box - 3-gang electrical box 3-Gang Plastic Side Nail Box. Outlets and Switches.
electrical wiring box - 3-gang face nail box 3-Gang Face Nail Box. Outlets and Switches.
electrical wiring box - 3-gang cut-in box 3-Gang Fiberglass Cut-In Box. Outlets and Switches.
electrical wiring box - 4-gang plastic nail box 4-Gang Plastic Nail Box. Switches and Lighting Controls.
electrical wiring box - 3 inch fiberglass nail box 3-Inch Fiberglass Nail Box. Lighting Fixtures and Junctions.
electrical wiring box - 3 inch fiberglass-cut-in-box 3-Inch Fiberglass Cut-In or Remodel Box. Lighting Fixtures and Smoke Detectors.
electrical wiring box - 3-inch-pancake-box 3-Inch Metal Pancake Box, also available in Fiberglass. Lighting Fixtures. Mounted on the surface of the framing to be flush with finished sheet rock.
electrical wiring box, 4-inch-plastic-nail-box 4-Inch Plastic Nail Box. Lighting Fixtures
electrical wiring box - 4-inch-ceiling-fan-box 4-Inch Ceiling Fan Box. Ceiling Fan Pancake Box, approved for 70 Lb Support.
electrical wiring box - 4-square-metal-box 4-Square Metal Box. Use with raised rings for 1 or 2 Gangs, 3-0 or 4-0 openings. Receptacles and Switches. Surface mount for use in Garage or Shop area with EMT or Flex Conduit.

More about Electrical Junction Boxes

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