Field reference only | conduit fill review workflow
Use this box fill calculator to check cubic-inch capacity for insulated conductors, equipment grounding conductors, device yokes, internal clamps, and support fittings using NEC 314.16(B) screening logic.
Input Workflow
Enter box volume, conductors, EGCs, yokes, and internal hardware to compare required volume against available cubic-inch capacity
Use a standard volume helper or enter the stamped cubic-inch volume from the box.
Use the cubic-inch volume stamped on the box when available. Presets only help fill the field; if you edit a preset volume, the report labels it as a manual override from that preset.
Add only listed, stamped, or manufacturer-provided plaster ring / extension ring volume.
Insulated conductors entering, leaving, spliced, terminated, or passing through the box.
Do not count internal pigtails that start and end inside this box. Enter equipment grounding conductors in the grounding section. Count unbroken looped conductors over 12 inches as two conductors.
Enter grounding conductors separately from insulated conductors.
The default method counts the first four EGCs as one allowance based on the largest EGC, then adds 0.25 allowance for each EGC over four.
Isolated Equipment Grounding Conductors
Isolated equipment grounding conductors are counted as a separate grounding allowance, not blended into the standard EGC fraction method.
Device yokes count as two allowances per gang required for mounting, based on the largest conductor connected to that yoke.
Count each yoke or strap by the number of gangs required for mounting. A standard duplex receptacle on one strap is one gang. Wider devices can require two or more gang allowances.
Count built-in clamps and internal support fittings that occupy space inside the box.
Count built-in clamps or support fittings that occupy space inside the box. Do not count external cable connectors or clamps outside the box.
Count fixture studs, hickeys, and other internal support fitting types separately when present.
Common check: two 12/2 NM-B cables + one duplex yoke = 15.75 cu in required.
Enter box volume and accept the screening acknowledgment to reveal the result.
Box Fill Result
Common field options: listed extension ring, deeper box, larger box, or revised conductor/device layout.
Conductors larger than #6 AWG are outside this cubic-inch screening scope and should be reviewed under NEC 314.28 pull / junction box rules.
Required
Total Available
Fill Ratio
Largest Volume Contributor:
Box Volume Used:
Volume Source:
Insulated Conductors:
EGCs:
Devices / Yokes:
Internal Hardware:
EGC Allowance: Legacy 2017 method selected — verify local adopted NEC cycle.
Related Field Checks
Scope
This calculator screens box volume in cubic inches for insulated conductors by size, equipment grounding conductor allowance, device yoke or strap allowance, internal clamp allowance, support fitting allowance, required volume, remaining volume, and fill ratio.
Scope Boundaries
Source Scope
The calculation is scoped to NEC 314.16(B) and Table 314.16(B) conductor volume allowances for #18 through #6 AWG. The default EGC logic follows the 2020 / 2023 / 2026 method summarized in the EGC Box Fill Fraction Rules reference; the legacy 2017 method is available only as an advanced option.
Review the Code Citation & Source Log and the TradeHub Methodology page for how field references are scoped.
Calculator Application
Use this calculator after circuit load, conductor size, grounding conductor context, and listed box or ring volume are known. Box fill checks cubic-inch capacity for the box assembly; it does not replace conductor sizing, raceway fill, voltage drop, or larger pull-box review.
Related TradeHub Calculators
After the box-fill volume result is known, use conductor, raceway, derating, and voltage-drop tools only when those separate field conditions still need review. Box fill confirms cubic-inch capacity; it does not approve conductor sizing, raceway fill, or pull-box layout.
NEC 314.16 Box Fill
Cubic-inch box volume basis
EGC Box Fill Fraction Rules
Grounding conductor allowances
NEC 250.122 EGC Sizing
Size before counting allowance
NEC Chapter 9 Conduit Fill Tables
Raceway fill after box fill
NEC 314.28 Pull Box Sizing
Boundary outside #18 through #6
Code Citation & Source Log
Source alignment for this workflow
Method
Field Example
Two 12/2 NM-B cables enter the box: one line-in and one load-out. A standard duplex receptacle is installed on one yoke.
| 4 insulated #12 conductors × 2.25 | 9.00 cu in |
| 2 #12 EGCs count as one allowance | 2.25 cu in |
| 1 duplex receptacle yoke × 2 allowances × 2.25 | 4.50 cu in |
| Total required | 15.75 cu in |
An 18.0 cu in single-gang box has enough capacity for this screening example. A shallow 14.0 cu in box would show Capacity Exceeded because the required volume is 15.75 cu in.
Common Misses
FAQ
Internal pigtails that originate and terminate inside the same box are not counted as separate conductor allowances for this box-fill screening.
Equipment grounding conductors use the selected EGC method. The default method counts the first four EGCs as one allowance based on the largest EGC, then adds 0.25 allowance for each EGC over four.
Yes. Isolated equipment grounding conductors are entered separately and add their own allowance based on the largest isolated grounding conductor size.
Device yokes or straps count as two volume allowances per gang required for mounting, based on the largest conductor connected to that specific yoke or strap.
Enter the number of gangs required for mounting. A one-gang device counts as two allowances, a two-gang device counts as four allowances, and wider devices continue at two allowances per gang.
A standard duplex receptacle on one strap counts as one yoke and one gang unless the device requires a wider mounting space.
Yes. Internal clamps count once total based on the largest conductor present in the box. External cable connectors or clamps outside the box are not counted as internal clamp allowance.
Yes. Internal support fittings are counted by fitting type. If a box contains a fixture stud and a hickey, enter two support fitting types so the calculator adds two allowances based on the largest conductor present.
Yes. Enter the listed or stamped cubic-inch volume for the plaster ring, mud ring, or extension ring. The calculator adds that volume to the base box volume for the total available box volume.
No. This calculator is limited to #18 through #6 AWG box-fill screening. Conductors larger than #6 require NEC 314.28 pull or junction box review.
Wire connectors are not entered as a separate volume allowance in this screening calculator.
Professional Notice
This calculator is a field screening reference for NEC 314.16(B) box-fill volume. It does not replace adopted code requirements, manufacturer markings, engineered design, or local inspection authority review.
The is currently under review for code alignment and field validation.
Status: In Review