Box Fill Calculator
Calculate the maximum number of conductors allowed in an electrical box per NEC guidelines.
Published by:
isCalculator Editorial Team
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How to use this tool?
How to Use the Box Fill Calculator
- Select Box Type – Choose from Standard, Square, Round, or Octagonal (for reference only; volume is based on your input).
- Enter Box Size – Input the box volume in cubic inches (e.g., 18). This is required.
- Add Conductors – Enter the count for each wire gauge (#14 to #6). The calculator uses NEC 314.16 volumes per conductor (e.g., #14 = 2 cu. in., #12 = 2.25 cu. in., etc.).
- Add Additional Items – Enter counts for internal clamps, support fittings, devices (yokes), and equipment grounding conductors. Each counts as 1 cu. in., except devices count as 2 cu. in.
- Click "Calculate" – The result shows if the box is within limit (with remaining capacity) or overfilled. Details break down each volume.
Note: All values must be non-negative. Box size must be greater than zero. Use imperial units (cubic inches) only.
Previous Results
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Frequently Asked Questions
Box fill refers to the total volume occupied by conductors, clamps, devices, and fittings inside an electrical box, which must not exceed the box's capacity per NEC.
Per NEC 314.16, each conductor has a specific volume allowance: #14 = 2 cu. in., #12 = 2.25, #10 = 2.5, #8 = 3, #6 = 5.
Each current-carrying conductor, neutral, and ground count. Equipment grounding conductors count as one (largest). Pigtails do not count.