Box fill wrapper
Why you care (60 seconds)
Overfilled boxes cause heat buildup, damaged conductors, and inspection failures. Box fill is a quick calculation that prevents rework.
Where people lose time
- Forgetting to count internal clamps or devices.
- Miscounting conductor equivalents for grounds and yokes.
- Selecting a box size before counting conductors.
This is
The rule in plain language
Match the number and type of conductors and devices in a box to the volume the box can safely accommodate.
When it applies
Any time conductors, splices, or devices are installed in a box or fitting.
What you must do (checklist)
- Count conductors entering and leaving the box.
- Include allowances for equipment grounding conductors.
- Include devices, internal clamps, and fittings in the count.
- Verify box volume matches the total conductor equivalents.
Quick examples
- Adding a device can require a larger box even if conductors stay the same.
- An internal clamp can change the required volume.
This is not
Common misreads
- Assuming a standard box is always big enough for any device.
- Ignoring grounds because they share a splice.
What it doesn't cover
- Pull box sizing rules.
- Raceway fill calculations.
False friends
- Confusing the box volume on the label with usable space after fittings.
Exceptions & edge cases
- Listed boxes may have special allowances or limitations.
- Shallow boxes often require extension rings to meet fill needs.
Cross-references (NEC map)
- Primary: 314.16
Exam traps
- Missing yoke counts when a device has multiple conductors.
- Forgetting to include internal fittings in the total.
Field notes
- Keep a simple box fill calculator on your phone or field notebook.
- Stock extension rings to avoid delays during rough-in.
AHJ / Local amendments notes (placeholder)
- Add local amendments or interpretations here.
Revision notes
- Draft wrapper created for box fill fundamentals.