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Electrical 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.

Home Wiring using Junction Boxes, Fixture Boxes, Receptacle 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.
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.

ELECTRICAL BOX EXAMPLES	Box CU. IN.	#14 Wires	#12 Wires	#10 Wires
3-INCH PANCAKE METAL	4.5	2	2	0
3-INCH CUT-IN FIBERGLASS	14.0	7	6	5
1-GANG CUT-IN FIBERGLASS	16.0	8	7	6
1-GANG CUT-IN FIBERGLASS	18.0	9	8	7
1-GANG NAIL PLASTIC	20.5	10	9	8
2-GANG CUT-IN FIBERGLASS	28.0	14	12	11
2-GLASS FACE NAIL PLASTIC	32.0	16	14	12
3-GANG CUT-IN FIBERGLASS	42.5	21	18	17
2-GANG NAIL PLASTIC	43.5	21	19	17

Additional Resources about Electrical Box Fill Calculations

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

Electrical Junction Boxes - top of page

Common Home Electrical Wiring Boxes and their Application

Electrical Box	Trade Name	Application
	1-Gang Plastic Nail Box used for Romex wiring. Also known as a Rough-In Box.	Outlets and Switches.
	1-Gang Fiberglass Cut-In Box with a Back Bracket.	Outlets and Switches. Romex wiring. Used with Romex wiring in Remodel applications.
	1-Gang Metal Cut-In Box with Adjusting Mounting Ears.	Outlets and Switches. With knock-outs for steel or aluminum flex.
	1-Gang Handy Box.	Outlets and Switches. Surface mount Microwave Receptacle. Garage or Shop area with Conduit.
	2-Gang Plastic Nail Box.	Outlets and Switches.
	2-Gang Fiberglass Cut-In Box.	Outlets and Switches.
	2-Gang Plastic Face Nail Box.	Outlets and Switches. Great for tight areas with limited room to nail.
	3-Gang Plastic Side Nail Box.	Outlets and Switches.
	3-Gang Face Nail Box.	Outlets and Switches.
	3-Gang Fiberglass Cut-In Box.	Outlets and Switches.
	4-Gang Plastic Nail Box.	Switches and Lighting Controls.
	3-Inch Fiberglass Nail Box.	Lighting Fixtures and Junctions.
	3-Inch Fiberglass Cut-In or Remodel Box.	Lighting Fixtures and Smoke Detectors.
	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.
	4-Inch Plastic Nail Box.	Lighting Fixtures
	4-Inch Ceiling Fan Box.	Ceiling Fan Pancake Box. Approved for 70 Lb Support.
	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.