How to Size Junction Boxes

Junction boxes are plastic or metal housings where electrical house wires which have been spliced together are tucked away. The National Electrical Code mandates all builders and contractors make wire splices inside junction boxes to eliminate risk of shock and electrical fires. Outdoor junction boxes are usually mounted onto an outside wall. Interior junction boxes are typically installed on studs or joists inside attic crawl spaces. There are many different sizes and styles of junction boxes. The size and shape of junction box you use will be somewhat dependent on the job you will be doing.

Things You'll Need

• Tape measure
• Pencil and paper

Instructions

1. Count the number of wires you will actually be running into your junction box.

2. Estimate the diameter of each wire that will be fed into the junction box and jot it down on a piece of paper.

3. Measure the height, width and depth of the junction box you intend to use with a tape measure and write these figures down.

4. Add up the diameters of the cables you intend to run into the junction box. For example, if you will be running a total of six cables into the box (five with 4-inch diameters and one with an 8-inch diameter) the total diameter size would equal 20 inches for the five 4-inch diameter cables and 8 inches for the largest cable, bringing the total diameter size to 28 inches.

5. Compare the total diameter of the wires you will be feeding into your junction box to both the input hole you will be feeding the wires through and the depth of the junction box you intend to use. If the total wire diameter is larger than either of these figures, you will need to select a larger junction box.

Tips & Warnings

• Jamming wires that really do not fit into a junction box can be hazardous because the action can strip sheathing off the wires or cables.

References

• EC&M: Sizing Junction Boxes
• Washington State Department of Transportation: NEMA Junction Box Sizing