Enter the total number of conductors and volume allowances into the calculator to determine the box size. This calculator can also evaluate any of the variables given the others are known.

Electrical Box Size Formula

The following formula is used to calculate the size of an electrical box. 

BS = (N * D) + A

Variables:

  • BS is the box size in cubic inches
  • N is the total number of conductors
  • D is the volume allowance in cubic inches per conductor
  • A is the volume allowance in cubic inches for any device or fitting

To calculate the electrical box size, multiply the total number of conductors by the volume allowance per conductor. Then, add the volume allowance for any device or fitting. The result is the required box size in cubic inches.

What is an Electrical Box Size?

An electrical box size refers to the physical dimensions of the box that houses electrical connections, such as switches, outlets, and light fixtures. The size of the box is determined by the number and size of the wires, devices, and fittings that will be contained within it. It is crucial to choose the correct size to ensure safety and adherence to electrical codes. The box size is typically measured in cubic inches and can vary depending on the specific electrical application.

How to Calculate Electrical Box Size?

The following steps outline how to calculate the Electrical Box Size.

  1. First, determine the total number of conductors (N).
  2. Next, determine the volume allowance in cubic inches per conductor (D).
  3. Next, determine the volume allowance in cubic inches for any device or fitting (A).
  4. Next, gather the formula from above = BS = (N * D) + A.
  5. Finally, calculate the Electrical Box Size (BS).
  6. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

total number of conductors (N) = 8

volume allowance in cubic inches per conductor (D) = 4

volume allowance in cubic inches for any device or fitting (A) = 10