Box Fill Explained (Rule 12-3034)
Overfilled boxes cause inspection failures and overheating risk. Rule 12-3034 defines how much volume conductors, devices, and fittings consume. Try the free box fill calculator.
Volume allowances
Each conductor size has a fixed volume deduction. All current-carrying conductors, equipment grounding conductors, devices, and internal clamps count toward fill unless the code exempts them.
When to upsize
If calculated fill exceeds box volume, use a larger box or reduce splices. Plan box fill during rough-in—not at trim when options are limited.