Electrical Codes for Junction Boxes

Summary: Listing of electrical codes for junction boxes with examples of electrical junction boxes codes for home electrical wiring.
© By: Dave Rongey

Electrical Code Definitions for Home Electrical Junction Boxes

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NEC 314.16 Wiring Methods Junction Box Fill

The volume of electrical boxes shall be sufficient for the number of conductors, devices, and cable clamps contained within the box. Code Adoption Information: The codes shown are examples only and may not be current or accurate for your application or jurisdiction. Contact your local building authority for complete information.

Nonmetallic boxes are marked with their cubic inch capacity.
Use the following table to calculate the minimum box size:
Conductor Size................................14 AWG........12 AWG
Each separate insulated wire..............2 in-3............2.25 in-3
All ground wires (combined)................2 in-3............2.25 in-3
For each device (switch/receptacle).....4 in-3............4.4 in-3
All internal cable clamps (combined)....2 in-3...........2.25 in-3

Sample Box Fill Calculation:
Four 14/2 w/ground type NMB cables:
8 insulated wires...................16 cubic inches
All ground wires......................2 cubic inches
1 switch.................................4 cubic inches
1 receptacle...........................4 cubic inches
All cable clamps (combined)....2 cubic inches
Minimum Box Volume...........28 cubic inches

NEC 370-29 Do not conceal junction boxes in walls, ceilings, or non-accessible attics and under-floor areas.
Residential Electrical Code Examples for Junction Boxes

IRC Codes
Boxes: Junction, outlet, and lighting
1.Unused openings in boxes and conduit bodies shall be closed. 2003 IRC E3806.4
2.All boxes shall be accessible. NEC 314.29
3.Boxes shall be installed at each conductor splice point, outlet, switch point, junction point, or pull point. NEC 314
4.Non-metallic boxes of rectangular shape (2 1/4 inch x 4 inches) can be used in both walls and ceilings. No clamps necessary if wire is fastened within 8 inches of the box. NEC 314.17

Always contact your local building authority for complete and up to date code information.

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More about Electrical Codes for Boxes

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Codes for Wires in Electrical Box 1 »

Questions about Electrical Codes for Junction Boxes

Questions about Residential Electrical Wiring Codes for Boxes

Question from Jan, a Homeowner in Atascadero, California.

Can I install junction boxes in a crawl space?

I’m working in a 1890’s very small house with a single wall. The house needs to be rewired. The crawl and attic areas are accessible. I plan to run the circuit feeds to the receptacles and switches inside surface mount metal Wiremold V 700 boxes etc. I plan to run the feeding circuits in dry crawl space with 2×8 16 OC joist system. Can I use NM #12 wiring for feeders and make wirenut connections inside 4×4 metal J-boxes inside the crawl space? I will be installing the appropriate cable connectors into and out of those J-boxes.

This electrical wiring question came from Jan, a Homeowner in Atascadero , California.

Dave’s Answer:

Jan, the electrical code requires that the junction boxes are accessible, therefore your plan should be fine. Be sure to obtain a permit and have your work inspected.

Electrical Tips to Help You Wire it Right

The Safest Way to Test Electrical Devices and Identify Electric Wires!

The Non-Contact Electrical Tester
This is a testing tool that I have had in my personal electrical tool pouch for years, and is the first test tool I grab to help identify electrical wiring. It is a Non-contact tester that I use to easily Detect Voltage in Cables, Cords, Circuit Breakers, Lighting Fixtures, Switches, Outlets and Wires. Simply insert the end of the tester into an outlet, lamp socket, or hold the end of the tester against the wire you wish to test. Very handy and easy to use.

The Quickest Way to Check for Faulty Electrical Wiring!

The Plug-In Outlet Tester
This is the first tool I grab to troubleshoot a problem with outlet circuit wiring. This popular tester is also used by most inspectors to test for power and check the polarity of circuit wiring.
It detects probable improper wiring conditions in standard 110-125 VAC outlets Provides 6 probable wiring conditions that are quick and easy to read for ultimate efficiency Lights indicate if wiring is correct and indicator light chart is included Tests standard 3-wire outlets UL Listed Light indicates if wiring is incorrect Very handy and easy to use.

Strip Off Wire Insulation without Nicking and Damaging the Electric Wire!

The Wire Stripper and Wire Cutter
My absolute favorite wire stripping tool that I have had in my personal electrical tool pouch for years, and this is the tool I use to safely strip electrical wires.
This handy tool has multiple uses:
The wire gauges are shown on the side of the tool so you know which slot to use for stripping insulation.
The end of the tool can be used to grip and bend wire which is handy for attaching wire onto the screw terminals of switches and outlets..

The wire stripper will work on both solid and stranded wire. This tool is Very Handy and Easy to Use.

Residential Electrical Parts and Accessories

Light Switches

120volt Outlets

Circuit Breakers

Electrician Tools

Voltage Testers

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