Electrician Prep

Box Fill Calculator

NEC 314.16

Calculate electrical box fill per NEC 314.16. Add conductors, clamps, devices, and grounds to verify box capacity.

About this calculator

Box fill rules in NEC 314.16 limit how many conductors, devices, clamps, and grounds you can pack into an electrical box. The rule exists to prevent insulation damage from overcrowding and to leave enough room for safe terminations. Each item counts for a number of "conductor volumes" based on the largest conductor in the box, and the sum has to fit inside the box volume listed in NEC 314.16(A) for standard boxes or stamped on the box for nonstandard ones.

Formula

Total fill in cubic inches equals the sum of conductor allowances, plus device allowances (two times the largest conductor), plus a single allowance for all internal cable clamps, plus a single allowance for all equipment grounding conductors. Compare the total to the listed box volume.

Fill = (conductor count × volume per conductor) + (devices × 2 × volume per conductor) + (1 × volume per conductor for clamps) + (1 × volume per conductor for grounds)

Reference: NEC 314.16(B)

How to use

  1. Enter the box type and trade size, or the cubic-inch volume stamped on the box.
  2. Add the count and AWG of each conductor entering the box (current-carrying and equipment grounding conductors).
  3. Indicate whether the box has internal cable clamps.
  4. Add any devices - switches, receptacles, etc. - which each count as two conductor volumes.
  5. Read the total fill against the box volume to see if you are compliant.

Worked example

Setup

A 4" × 1-1/2" square box (21 in³) with two 14/2 NM cables and one 12/2 NM cable, internal clamps, one duplex receptacle, no pigtails.

Calculation

Largest conductor is #12, which is 2.25 in³ per NEC Table 314.16(B). Conductors: 2 × #14 + 2 × #14 + 2 × #12 = 4 × 2.00 + 2 × 2.25 = 12.5 in³. Clamps: 1 × 2.25 = 2.25 in³. Grounds (counted as one conductor of the largest size): 1 × 2.25 = 2.25 in³. Device: 2 × 2.25 = 4.5 in³. Total = 21.5 in³.

Answer

21.5 in³ exceeds the 21 in³ box volume by half a cubic inch. Use a 4-11/16" square box (about 30 in³) or a deeper 4" square instead.

Frequently asked questions

Do equipment grounding conductors count?
Yes, but all grounding conductors entering the box count as a single conductor of the largest size present. Add 0.25 in³ for each isolated equipment grounding conductor beyond the first if you have separate isolated grounds.
Do wire nuts and pigtails add to the fill?
Wire nuts do not count. A pigtail that originates and terminates inside the box (used to connect multiple conductors to a single device terminal) does not count separately - the conductors entering the box already account for it.
What about plaster rings and extension rings?
A plaster ring or extension ring with its volume marked by the manufacturer adds that marked volume to the total box volume. Unmarked rings cannot be counted.