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 �	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 �	18.0	12	10	9	8	7	6	3
4 x 1 �	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 �	25.5	17	14	12	11	10	8	5
4 11/16 x 1 �	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 �	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 �	10.5	7	6	5	4	4	3	2
3 x 2 x 2 �	12.5	8	7	6	5	5	4	2
3 x 2 x 2 �	14.0	9	8	7	6	5	4	2
3 x 2 x 3 �	18.0	12	10	9	8	7	6	3
4 x 2 1/8 x 1 �	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.

Related Resources:

National Electrical Code Handbook

National Electrical Codebook

Stallcup's Illustrated Code Changes

The Complete Guide to Home Wiring (Black and Decker Complete Guides Series)

Wiring Simplified

Guide to Using the National Electrical Code

Residential Remodeling and Repair Professional Reference

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