Boxes: How Many Conductors Can You Run In Them?

The answer to this question is not straightforward, but it’s covered in Sec. 314.16 of the National Electrical Code. Here are some highlights:

• These requirements do not apply to the terminal housings supplied with motors or generators.
• You must perform box volume calculations and box fill calculations. In no case can the calculated box volume be less than the calculated box fill.
• The volume consists of the total of the assembled sections including domed covers and extension rings (if marked with their volume) [314.16(A)].
• If you’re using standard boxes not marked with their volume, you can use Table 314.16(A) to determine the volume [314.16(A)(1]]. If a box isn’t marked with its volume and isn’t covered by this table, you can’t use that box [314.16(A)(2)].
• Table 314.16(B) provides the volume allowance required for each conductor.
• You don’t have to allow for small fittings (e.g., locknuts and bushings) when calculating the box fill.
• You count a conductor only once, when it originates outside the box and terminates in, or is spliced in, the box [314.16(B)].