Electrical Boxe Sizing Guide

The National Electrical Code restricts the number of conductors that are allowed in a single electrical box. How many conductors are allowed is determined by the size of the box and the size of the conductors. Each conductor that terminates or is spliced within the box is counted once, and each conductor that passes through the box without splice or termination is counted once.

: 

The size of the box is determined by its volume, the volume of the box is found by using this formula. Height x Width x Length = The total cubic inches or the volume of the box. Some boxes will have its volume listed on the box while others will not. New nonmetallic boxes should have its volume listed on the box, while most metal boxes will not.

The minimum volume required per conductor is as follows:
18g - 1.50 cubic inches per conductor.
16g - 1.75 cubic inches per conductor.
14g - 2.00 cubic inches per conductor.
12g - 2.25 cubic inches per conductor.
10g - 2.50 cubic inches per conductor.
8g - 3.00 cubic inches per conductor.
6g - 5.00 cubic inches per conductor.

Note: Remember these are single conductors, a 12/2 romex has 3 single conductors in it.

Standard Metal Electrical Boxes.
Box Size Volume Maximum Number of Conductors
Round/octagonal Volume 18g 16g 14g 12g 10g 8g 6g
4 x 1 1/4 12.5 8 7 6 5 5 5 2
4 x 1 ½ 15.5 10 8 7 6 6 5 3
4 x 2 1/8 21.5 14 12 10 9 8 7 4

Square Boxes Volume 18g 16g 14g 12g 10g 8g 6g
4 x 1 ¼ 18.0 12 10 9 8 7 6 3
4 x 1 ½ 21.0 14 12 10 9 8 7 4
4 x 2 1/8 30.3 20 17 15 13 12 10 6
4 11/16 x 1 ¼ 25.5 17 14 12 11 10 8 5
4 11/16 x 1 ½ 29.5 19 16 14 13 11 9 5
4 11/16 x 2 1/8 42.0 28 24 21 18 16 14 8

Device Boxes Volume 18g 16g 14g 12g 10g 8g 6g
3 x 2 x 1 ½ 7.5 5 4 3 3 3 2 1
3 x 2 x 2 10.0 6 5 5 4 4 3 2
3 x 2 x 2 ¼ 10.5 7 6 5 4 4 3 2
3 x 2 x 2 ½ 12.5 8 7 6 5 5 4 2
3 x 2 x 2 ¾ 14.0 9 8 7 6 5 4 2
3 x 2 x 3 ½ 18.0 12 10 9 8 7 6 3
4 x 2 1/8 x 1 ½ 10.3 6 5 5 4 4 3 2
4 x 2 1/8 x 1 7/8 13.0 8 7 6 5 5 4 2
4 x 2 1/8 x 2 1/8 14.5 9 8 7 6 5 4 2

Notes to Remember when installing your Electrical Box:

1) Remember that you need to properly ground a metal box with a green ground screw.

2) Use the proper size romex connector to bring the cables in the box.

3) In damp or wet locations a weather proof box that is rated as such will be needed.

4) Properly fasten the box flush with the finished surface.

5) If the box is going to be used for a heavy lighting fixture, make sure the box is rated for the weight of the fixture.

6) If the box is for a ceiling fan, it will need to be rated for a ceiling fan.
:
BACK