Junction Box Sizing Calculator

A junction box sizing calculator is a tool used to determine the appropriate size of a junction box based on the number and size of conductors, devices, and fittings it needs to accommodate.

How do you size a junction box?

1. Determine the number and gauge of all conductors entering the box.
2. Include space for switches, receptacles, or other devices.
3. Add volume for cable clamps or connectors.
4. Sum up the required volume based on conductor count and size.
5. Choose a box with a volume equal to or greater than the calculated total.

Let’s size a junction box for the following:

• 4 #14 AWG conductors
• 2 #12 AWG conductors
• 1 grounding wire
• 1 single-gang device
• 2 cable clamps
• Volume for conductors:
  • #14 AWG: 4 * 2.0 cu.in. = 8.0 cu.in.
  • #12 AWG: 2 * 2.25 cu.in. = 4.5 cu.in.
  • Ground wire: 1 * 2.25 cu.in. = 2.25 cu.in.
• Device allowance: 1 * 4.0 cu.in. = 4.0 cu.in.
• Clamp allowance: 2 * 1.0 cu.in. = 2.0 cu.in.

Total required volume: 8.0 + 4.5 + 2.25 + 4.0 + 2.0 = 20.75 cu.in.

Choose a box with at least 20.75 cu.in. capacity, such as a 4″ square box with a volume of 21 cu.in.

Junction Box Sizing Chart

Junction Box Sizing Formula

The Junction Box Sizing Formula is based on the National Electrical Code (NEC) requirements.

Total Box Volume = (Conductor Volume + Device Volume + Clamp Volume) * 1.25

Where:

• Conductor Volume = Sum of individual conductor allowances
• Device Volume = Sum of device allowances
• Clamp Volume = Sum of clamp/connector allowances
• 1.25 is a 25% safety factor

To calculate the required box size for:

• 6 #14 AWG conductors
• 1 #12 AWG ground wire
• 1 single-gang device
• 2 cable clamps
• Conductor Volume: (6 2.0) + (1 2.25) = 14.25 cu.in.
• Device Volume: 1 * 4.0 = 4.0 cu.in.
• Clamp Volume: 2 * 1.0 = 2.0 cu.in.

Total Box Volume = (14.25 + 4.0 + 2.0) * 1.25 = 25.31 cu.in.

Choose a box with a volume of at least 25.31 cu.in.