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Junction Box Fill Calculator: NEC Article 314.16 Box Volume

Count Conductors and Calculate Required Box Size per NEC Table 314.16(B)

Free NEC junction box fill calculator per Article 314.16. Enter the number of conductors by AWG size, cable clamps, support fittings, devices (switches/receptacles), and equipment grounding conductors to calculate the total volume required and minimum box size. Uses conductor volume allowances from NEC Table 314.16(B).

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Box fill violations are one of the most common NEC inspection failures. Overfilled boxes cause wire damage, increase fire risk from heat buildup, and make devices difficult to install properly. This calculator counts every conductor and fitting per NEC rules and recommends the smallest compliant box.

Pro Tip: Count your wires before you buy your boxes. A single 3-way switch with two 14/2 cables and a 14/3 traveler fills a standard single-gang box to its NEC limit. Add a ground pigtail and you are over. It costs nothing to upsize the box during rough-in, but it costs a lot to change it after drywall.

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Junction Box Fill Calculator
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How It Works

  1. Enter Conductor Count by Size

    Count all conductors entering the box by AWG size (#14, #12, #10, #8, #6). Each conductor (hot, neutral, traveler) counts individually. Do not count pigtails that originate inside the box.

  2. Count Cable Clamps

    Count the number of cable clamps or connectors inside the box. All internal clamps together count as one conductor volume, regardless of how many clamps there are.

  3. Count Devices

    Count switches and receptacles. Each single yoke device (switch, receptacle) counts as two conductor volumes of the largest wire connected to it. A combination device on one yoke still counts as two.

  4. Count Equipment Grounds

    All equipment grounding conductors together count as one conductor volume of the largest ground wire present. If an equipment bonding jumper is present, add one more volume of the largest EGC.

  5. Review Required Volume

    The calculator sums all conductor volume allowances and shows the minimum box volume in cubic inches. It recommends standard box sizes that meet or exceed the requirement.

Built For

  • Electricians selecting box sizes during rough-in to pass inspection on the first try
  • Electrical inspectors verifying box fill compliance during inspection
  • Apprentice electricians learning NEC box fill counting rules for journeyman exams
  • Estimators determining correct box sizes for material takeoffs
  • Electrical engineers specifying box sizes on construction drawings
  • Maintenance electricians verifying existing box capacity before adding circuits
  • Residential electricians sizing boxes for 3-way and 4-way switch configurations

Features & Capabilities

NEC Table 314.16(B) Volumes

Conductor volume allowances per NEC: #14 AWG = 2.0 cu. in., #12 = 2.25 cu. in., #10 = 2.5 cu. in., #8 = 3.0 cu. in., #6 = 5.0 cu. in.

Complete Counting Rules

Implements all NEC 314.16(B) counting rules: conductors, cable clamps (1 volume total), devices (2 volumes each), EGC (1 volume for all), equipment bonding jumpers.

Standard Box Recommendations

Shows standard box sizes (single-gang, double-gang, 4" square, 4-11/16" square, etc.) with volumes. Recommends the smallest box that meets the calculated requirement.

Multiple Wire Sizes

Handles mixed wire sizes in the same box. Uses the largest conductor size for clamp and device volume calculations per NEC rules.

Visual Volume Breakdown

Shows the volume contribution of each component (conductors, clamps, devices, grounds) so you can see what is using box capacity.

PDF Export

Export box fill calculations as a branded PDF for inspection documentation or submittal packages.

Frequently Asked Questions

No. Wire connectors (wire nuts, Wagos, push-in connectors) are not counted separately in box fill calculations. The conductors connected by them are already counted individually.
A pigtail that originates and terminates inside the box does not count. A pigtail that connects to a device counts as part of the device allowance (the device gets 2 conductor volumes). Only conductors that enter the box from outside are counted individually.
A 14/3 cable contains 3 insulated conductors plus 1 equipment ground. Count each insulated conductor (black, red, white) individually at the #14 volume (2.0 cu. in. each = 6.0 cu. in.). The ground wire is counted with all other grounds as 1 volume total.
Count each conductor at its own AWG volume. For clamps and devices, use the volume of the largest conductor in the box. For example, if the box has #14 and #12 wires, clamps count at #12 volume (2.25 cu. in.) and each device counts as 2 × 2.25 = 4.5 cu. in.
NEC 314.27(A) requires boxes used for switch or receptacle mounting to have a flat mounting surface. Round boxes should not be used for devices. Use single-gang, double-gang, or 4" square boxes with appropriate plaster rings.
Disclaimer: Box fill calculations are based on NEC Article 314.16 and Table 314.16(B). Local amendments may modify NEC requirements. Always verify with the authority having jurisdiction (AHJ). This calculator is a reference tool. Final box selection must comply with the edition of the NEC adopted in your jurisdiction.

Learn More

Electrical

NEC 314.16 Box Fill: How to Count Conductors and Pass Inspection

NEC 314.16 box fill rules explained. How to count conductors, clamps, devices, and grounds for junction box cubic inch calculations.

Read Guide →

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