About Box Fill Calculator (NEC 314.16)
The box fill calculator applies NEC 314.16(B) to find the minimum volume an outlet, device, or junction box must have for the conductors and hardware inside it. Each insulated conductor entering or leaving the box counts one volume allowance from Table 314.16(B) — 2.00 in3 for 14 AWG up to 5.00 in3 for 6 AWG — and additional allowances cover internal cable clamps, support fittings such as studs and hickeys, device yokes at two allowances each, and equipment grounding conductors.
Enter the conductor counts per AWG size, tick the clamp box if the box has internal clamps, and add the yoke and grounding conductor details. The tool returns the total required volume in cubic inches and cubic centimetres with a line-by-line breakdown per rule, then checks the total against the volumes of common metallic and nonmetallic boxes — octagon, 4-inch square, 4-11/16 square, and device boxes — listing every box that passes with its utilization percentage.
How It Works
- Enter the number of insulated circuit conductors of each size (14 through 6 AWG) that enter, leave, or pass through the box; each counts one Table 314.16(B) allowance.
- Check "internal cable clamps" to add a single allowance of the largest conductor in the box, and enter any support fittings (studs/hickeys), each of which adds one largest-conductor allowance.
- Enter the number of device yokes/straps and the largest conductor size connected to a yoke; each yoke adds two allowances of that size (a device wider than a single gang counts as two yokes).
- Enter the total equipment grounding conductors and the largest EGC size: all EGCs together count one allowance, plus a quarter allowance for each EGC beyond four (NEC 2023 314.16(B)(5)).
- The tool totals the volume in in3 and cm3, shows the per-rule breakdown, and lists the standard boxes that meet the requirement with their utilization percentage.
Worked Example
Two 12/2 with ground NM cables enter a single-gang device box with internal clamps and feed one duplex receptacle. The four insulated 12 AWG circuit conductors count 4 x 2.25 = 9.00 in3. The two bare grounds together count one 12 AWG allowance, 2.25 in3, and the internal clamps add another single 12 AWG allowance, 2.25 in3. The receptacle yoke adds two allowances of the 12 AWG connected to it, 4.50 in3. The required volume is 9.00 + 2.25 + 2.25 + 4.50 = 18.00 in3, so an 18.0 in3 nonmetallic box is exactly at the limit (100% utilization) and a 20.0 in3 box passes with margin at 90%.
Formulas
- Total required box volume
V = V_conductors + V_clamps + V_fittings + V_devices + V_EGC- Conductor fill 314.16(B)(1)
V_conductors = sum( n_size * allowance_size )- Clamp and fitting fill 314.16(B)(2)-(3)
V_clamps = 1 * allowance_largest; V_fittings = n_fittings * allowance_largest- Device fill 314.16(B)(4)
V_devices = n_yokes * 2 * allowance_yoke- EGC fill 314.16(B)(5)
V_EGC = (1 + 0.25 * max(0, n_EGC - 4)) * allowance_largestEGC
Standards & References
- NEC 314.16(A) -- box volume (marked or Table 314.16(A))
- NEC 314.16(B) / Table 314.16(B) -- volume allowances per conductor
- NEC 2023 314.16(B)(5) -- EGC quarter allowance beyond four grounds
Frequently Asked Questions
How do conductors count toward box fill under NEC 314.16(B)(1)?
Each insulated conductor that originates outside the box and terminates or is spliced inside counts one volume allowance for its size. A conductor that passes through without splice also counts once, and it counts twice only when it is looped inside with at least twice the 300.14 minimum free length. Pigtails that both start and end inside the box do not count at all, and neither do small equipment-bonding jumpers or up to four fixture wires per 314.16(B)(1) exceptions.
How much volume does a receptacle or switch take up?
Each device yoke or strap counts as two volume allowances of the largest conductor connected to that yoke, per 314.16(B)(4). A standard duplex receptacle fed with 12 AWG therefore adds 2 x 2.25 = 4.50 in3. A device physically wider than a single 2-inch gang — some GFCIs, timers, or fan controls — counts as two yokes, doubling the deduction.
How are equipment grounding conductors counted?
All equipment grounding conductors in the box together count as a single allowance based on the largest EGC present, per 314.16(B)(5). Since the 2023 NEC, each additional EGC beyond four adds a further quarter allowance, so eight 12 AWG grounds count (1 + 4 x 0.25) = 2 allowances, 4.50 in3. Isolated grounding conductors run with the circuit get their own separate single allowance under the same rule.
When do cable clamps count toward box fill?
Only clamps that are inside the box count. One or more internal clamps together add a single allowance of the largest conductor in the box per 314.16(B)(2) — you never count each clamp separately. External connectors such as NM cable connectors that clamp outside the box wall, or clamps integral to a raised cover, add nothing.
How do I know the volume of my box?
Nonmetallic boxes are marked with their volume in cubic inches by the manufacturer (typical single-gang boxes are 18.0, 20.0, or 22.5 in3). Standard metallic boxes use Table 314.16(A): a 4 x 1-1/2 in octagon is 15.5 in3, a 4 x 2-1/8 square is 30.3 in3, and a 4-11/16 x 2-1/8 square is 42.0 in3. Marked plaster rings, extension rings, and domed covers add their marked volume to the total available.
What happens if my box is overfilled?
An overfilled box violates NEC 314.16 and is a real safety issue: crowded conductors are damaged during device installation, splices are stressed, and heat cannot dissipate, all of which lead to insulation failure and arcing faults. The fix is a deeper box, a box extension ring, a larger square box with a mud ring, or splitting the circuit into a second box. Inspectors routinely count box fill, so size it before rough-in.