Box Fill Calculator - NEC 314.16

Cable mode: Enter cables entering the box (e.g., "two 14/2 NM-B"). Conductors are auto-counted — each cable's hot and neutral count as individual conductors. Equipment grounds are counted per 314.16(B)(5): 1–4 EGCs = 1 conductor volume; each EGC beyond 4 adds ¼ conductor volume.

Common Scenarios

Cables Entering Box

Device Yokes (switches/receptacles)

Internal cable clamps?

Pigtail reminder: Pigtails and jumpers that originate and terminate inside the box are NOT counted for box fill. Only conductors that enter through a cable or raceway count. This is the #1 box fill mistake.

By clicking Apply, you acknowledge that results are for reference only and must be verified by a qualified electrician against the current National Electrical Code and local amendments.

Conductors 314.16(B)(1)

Count each conductor that enters the box. Pigtails and jumpers that originate and terminate inside the box are not counted.

14 AWG conductors

12 AWG conductors

10 AWG conductors

8 AWG conductors

6 AWG conductors

Devices 314.16(B)(4)

Each device yoke counts as 2x the volume of the largest conductor connected to it.

Number of device yokes

Largest conductor to device

Clamps 314.16(B)(2)

All internal clamps count as 1x the volume of the largest conductor in the box.

Internal cable clamps present?

Equipment Grounding Conductors 314.16(B)(5)

1–4 EGCs = 1 conductor volume (based on largest). Each EGC beyond 4 adds ¼ conductor volume. Isolated ground counted separately.

Largest EGC size

Number of EGCs entering box

Additional EGC from device (isolated ground)?

By clicking Apply, you acknowledge that results are for reference only and must be verified by a qualified electrician against the current National Electrical Code and local amendments.

Reverse lookup: Select a box and see the maximum number of conductors that fit per NEC 314.16(A).

Select Box

Conductor Size

By clicking Apply, you acknowledge that results are for reference only and must be verified by a qualified electrician against the current National Electrical Code and local amendments.

Box Fill Calculator NEC 314.16

Cables Entering Box

Conductors 314.16(B)(1)

Devices 314.16(B)(4)

Clamps 314.16(B)(2)

Equipment Grounding Conductors 314.16(B)(5)

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