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Box Fill Calculator

Verifies conductor, fitting, and device count fit within outlet or junction box per NEC 314.16.

Conductors

Devices, Clamps & Grounds

Results

Total Fill
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Box Volume
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Fill %
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Status
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Fill Breakdown

Based on NEC 314.16 and Table 314.16(B). Verify with your local AHJ.

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How to Use This Calculator

1. Select the box type and size. Common residential boxes are single gang 18 cu in. Junction boxes are typically 4" square or round.

2. Enter conductor counts by gauge. Count every wire entering the box. Pigtails made inside the box do not count unless they leave the box.

3. Add devices, clamps, and grounds. Each device (switch/outlet) counts as 2 volumes. All clamps together count as 1 volume. All grounds together count as 1 volume.

NEC 314.16 Volume Allowances

#14 AWG: 2.00 cu in per conductor

#12 AWG: 2.25 cu in per conductor

#10 AWG: 2.50 cu in per conductor

#8 AWG: 3.00 cu in per conductor

#6 AWG: 5.00 cu in per conductor

Device: 2x volume of largest conductor connected

Clamps: 1x volume of largest conductor (total, not each)

Grounds: 1x volume of largest ground (total, not each)

Fittings: 1x volume of largest conductor (total)

Frequently Asked Questions

How do you calculate box fill per NEC 314.16?
Each conductor counts as one volume allowance. Devices count as two volumes each. All clamps together count as one volume. All grounds together count as one volume. Use the volume allowance from NEC Table 314.16(B) based on the largest wire gauge.
What are the NEC box fill volumes by wire size?
#14 AWG = 2.00 cu in, #12 AWG = 2.25 cu in, #10 AWG = 2.50 cu in, #8 AWG = 3.00 cu in, #6 AWG = 5.00 cu in per Table 314.16(B).
Does the ground wire count for box fill?
Yes, but all grounding conductors together count as only one volume allowance, based on the largest ground wire in the box.

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