Box Fill Calculator

Box fill is one of the most routine NEC field calculations — and one of the most error-prone because 314.16(B) applies different counting rules to five distinct categories of items in the box. Conductors that enter or pass through count individually by AWG. Looped or coiled unbroken conductors with at least twice the required free-conductor length count as one allowance each. Internal cable clamps — regardless of how many are present — count as a single allowance based on the largest conductor in the box (small external fittings and locknuts do not count at all). Each device or equipment yoke counts as two allowances based on the largest conductor connected to that specific yoke. All equipment grounding conductors combined count as a single allowance based on the largest EGC. Getting any of these wrong produces an overfilled box that fails 314.16(A). This calculator tracks all five categories independently, applies the correct volume allowance from NEC Table 314.16(B) (1.50 in³ for 18 AWG through 5.00 in³ for 6 AWG), sums the total required volume, and compares it against the box's listed allowable fill. The Results tab shows a proportional Box Cross-Section visualization, a Fill Contribution Breakdown by category, and a Volume Comparison chart. The Trace tab walks through each category's calculation with NEC section citations. A single-size mode simplifies entry when all conductors are the same AWG. Special conditions include looped/coiled conductors, isolated grounding conductor sets (NEC 2017 treatment), and barrier-separated compartments. Five presets cover Receptacle, Switch with Clamp, Crowded, Undersized (intentional FAIL), and Isolated Ground (NEC 2017). The Learn tab covers the five counting rules, common mistakes, and a knowledge-check quiz. Supports NEC 2017, 2020, and 2026.

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A live look at the app you'll launch — every control on the left drives the visualizations on the right in real time.

• 01Apply the five NEC 314.16(B) counting categories — conductors, clamps, support fittings, device yokes, and equipment grounding conductors — and state the volume allowance rule for each
• 02Look up the volume allowance per conductor from NEC Table 314.16(B) for conductor sizes 18 AWG through 6 AWG and apply it correctly by category
• 03Explain why multiple internal cable clamps count as only one allowance and why external locknuts and fittings do not count at all
• 04Determine the device yoke volume contribution for a box containing two different yoke types connected to different AWG conductors
• 05Explain why all equipment grounding conductors — regardless of count — are counted as a single allowance sized by the largest EGC
• 06Identify the NEC 2017 treatment of isolated grounding conductors as an additional allowance separate from the standard EGC allowance