Box Fill Calculator

Size electrical boxes to NEC 314.16 by counting conductors, devices, clamps, fittings and grounds.

Compare the required cubic inches with a standard metal box or a marked volume and see how much room is left.

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Results will appear here once you enter your values.

The 314.16(B) counting rules

Box fill uses volume allowances. Each allowance takes the per-conductor volume from Table 314.16(B): 18 AWG is 1.50 in³, 16 AWG 1.75, 14 AWG 2.00, 12 AWG 2.25, 10 AWG 2.50, 8 AWG 3.00 and 6 AWG 5.00. Add up the allowances:

  • Conductors from outside the box: one allowance each. A wire that passes through unbroken counts once. An unspliced loop at least twice the 300.14 free length counts twice.
  • Internal cable clamps: one allowance for all clamps together, based on the largest conductor in the box.
  • Support fittings such as luminaire studs and hickeys: one allowance each, based on the largest conductor.
  • Device yokes: two allowances per yoke, based on the largest conductor connected to that device.
  • Equipment grounding conductors: one allowance for all of them, based on the largest EGC or bonding jumper.
  • Pigtails and jumpers that never leave the box, plus small fittings such as locknuts and bushings, do not count.

Worked example: receptacle fed by two 12/2 NM cables

Two 12/2 cables with grounds enter a box with internal clamps and feed one duplex receptacle.

  • Four 12 AWG circuit conductors: 4 x 2.25 = 9.00 in³
  • Internal clamps: 1 x 2.25 = 2.25 in³
  • Receptacle yoke: 2 x 2.25 = 4.50 in³
  • Both grounds together: 1 x 2.25 = 2.25 in³

The total is 18.00 in³. A 3 x 2 x 3-1/2 in. device box (18.0 in³) just passes. A 3 x 2 x 2-1/2 in. box (12.5 in³) is 5.5 in³ short. A 4 x 1-1/2 in. square box (21.0 in³) leaves 3 in³ spare. You can add a marked plaster ring or extension ring to the box volume.

Grounding conductors: NEC 2023 versus older editions

In the 2020 and 2023 NEC, 314.16(B)(5) counts a single volume allowance for all equipment grounding conductors and bonding jumpers in the box, sized by the largest one. The 2017 and earlier editions added a second allowance when an isolated-ground set of EGCs was present under 250.146(D). The calculator includes that field for jurisdictions still on an older edition. Leave it at None for 2020 or 2023.

Other common errors are counting pigtails, forgetting that a switch or receptacle counts as two conductors, and using the Table 314.16(A) volume for a nonmetallic box. Plastic boxes must use the volume molded into the box. Boxes containing 4 AWG or larger conductors are sized under 314.28 instead of by volume.

Wider devices and multi-gang boxes

A device or piece of equipment wider than a single 2 in. device box counts two allowances for each gang it needs.

For a multi-gang box, add the volume of each gang section, or use the total marked on the box.

Then count every yoke separately.

Leftover volume is not wasted: devices with back wiring and GFCI or smart switches take up far more space than the code minimum assumes.

Frequently Asked Questions

Common questions about the Box Fill Calculator

Ten, with nothing else in the box: 21.0 cubic inches divided by 2.00 cubic inches per 14 AWG conductor. Subtract allowances for any clamps, devices and grounds.

Sources & References

NFPA 70, National Electrical Code (2023 edition)

The code document itself; free read-only access is available after registering with NFPA.
→Source: National Fire Protection Association

NFPA 70 free access edition

Read-only online view of the 2023 NEC, including Chapter 9 and Article 314.
→Source: National Fire Protection Association

Mike Holt Enterprises code training articles

Widely used NEC training source with worked raceway fill and box fill examples.
→Source: Mike Holt Enterprises

Steel City / ABB Installation Products box volume data

Manufacturer catalog listing marked cubic-inch capacities for metal and nonmetallic outlet boxes.
→Source: ABB Installation Products

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