NEC 314.16(B) Conductor Volume Allowance Table
Use this table as a field lookup for common conductor volume allowances used in box-fill checks. The required cubic inches come from conductor size, conductor count, device/yoke allowances, grounding conductor allowance, and internal fitting allowances.
| Conductor Size | Volume Allowance | Field Use |
|---|---|---|
| 18 AWG | 1.50 cu in | Used where 18 AWG conductors are allowed and present in the box-fill count. |
| 16 AWG | 1.75 cu in | Use this allowance when 16 AWG conductors are part of the box-fill calculation. |
| 14 AWG | 2.00 cu in | Common on 15A branch circuits where #14 conductors are installed. |
| 12 AWG | 2.25 cu in | Common on 20A branch circuits and receptacle layouts. |
| 10 AWG | 2.50 cu in | Use when larger branch-circuit conductors enter, splice, terminate, or pass through the box. |
| 8 AWG | 3.00 cu in | Often relevant when a larger device box or junction box is used with upsized conductors. |
| 6 AWG | 5.00 cu in | Large conductor volumes can quickly exceed small box capacity and usually need a more deliberate box selection. |
What Counts Toward Box Fill Under NEC 314.16(B)
Box fill is not just a wire count. Devices, grounding conductors, internal clamps, and fixture fittings can add volume allowances that are easy to miss during rough-in checks.
| Box-Fill Item | Counting Treatment | Field Note |
|---|---|---|
| Insulated conductor entering and terminating or spliced | One conductor volume | Count each conductor by its conductor size allowance. |
| Conductor passing through without splice or termination | One conductor volume | A pass-through conductor still occupies box volume. |
| Equipment grounding conductors | One combined allowance | Use the largest equipment grounding conductor in the box. For grounding conductor sizing context, review NEC 250.122 Equipment Grounding Conductor Sizing. |
| Device yoke or strap | Two conductor volumes | Use the largest conductor connected to the device yoke. This is a common miss on receptacle and switch boxes. |
| Internal cable clamps | One conductor volume | Applies where clamps are internal to the box and occupy box volume. |
| Fixture studs or hickeys | One conductor volume | Count when present inside the box. |
| Pigtails that originate and terminate in the same box | Usually not a separate allowance | Do not treat every short internal pigtail the same as a conductor entering the box. |
Field Boundary
A Box-Fill Pass Is Not the Whole Installation Check
Box fill confirms cubic-inch capacity for the conductors, devices, grounding conductors, and internal fittings being counted. It does not by itself confirm conductor ampacity, terminal temperature limits, raceway fill, box support, device listing, conductor protection, manufacturer instructions, or AHJ acceptance.
How to Calculate Box Fill
- 1Identify the conductor sizes present in the box and use the matching conductor volume allowance.
- 2Count conductors that enter and terminate, splice, or pass through the box.
- 3Add device yoke or strap allowances using the largest conductor connected to each device yoke.
- 4Add the combined equipment grounding conductor allowance and any internal clamp or fixture-fitting allowance that applies.
- 5Compare the required cubic inches against the marked box volume, extension ring volume, or listed box capacity information.
NEC 314.16(A), 314.16(B), and 314.16(C) Field Scope
Searches for NEC 314.16 often reference the subsections directly. Use this scope table to keep the field question pointed at the right part of the box-fill rule.
| Section | Primary Field Use | Boundary |
|---|---|---|
| 314.16(A) | Box volume and capacity basis | Confirm the box, extension ring, or listed assembly capacity instead of relying only on trade size. |
| 314.16(B) | Box-fill calculation allowances | This is where conductor, grounding conductor, device, clamp, and fitting allowances are commonly checked. |
| 314.16(C) | Boxes enclosing devices or utilization equipment | Use when the box contains devices or equipment that change the box-volume check. |
Box Fill Is Not Conduit Fill
Box fill checks the cubic-inch volume inside an outlet, device, or junction box. Raceway fill is a separate check under NEC Chapter 9, and should be reviewed using the NEC Chapter 9 Conduit Fill Tables reference when conductors are installed in raceways.
For raceways 24 inches or less, the separate nipple fill boundary belongs in the NEC Chapter 9 Nipple Rule reference rather than the box-fill calculation.
Common Box Fill Misses
- Using the box trade size instead of the marked cubic-inch capacity.
- Forgetting that a device yoke can add two conductor volume allowances.
- Counting every equipment grounding conductor separately instead of applying the combined grounding conductor allowance.
- Missing internal clamps, fixture studs, hickeys, or extension-ring volume conditions.
- Ignoring a larger conductor size mixed into the same box.
- Treating a box-fill pass as approval for conductor ampacity, box support, raceway fill, bending space, or device listing.
Field Example
A device box has two 12 AWG cables entering the box and one receptacle yoke. A basic field count may include four insulated conductors, one device yoke counted as two conductor allowances, and the equipment grounding conductors counted together as one allowance.
Example Count
Using 12 AWG volume allowances, the screening count is seven conductor allowances. Multiply the applicable 12 AWG volume allowance by seven and compare that total to the box cubic-inch marking. Internal clamps, additional devices, larger conductors, and fixture fittings can change the count.
Calculator Application
TradeHub Calculator Application
TradeHub treats NEC 314.16 as a box-volume screening reference. The calculator helps organize conductor count, conductor size, device yoke allowance, grounding conductor allowance, internal clamps, and available box cubic inches before the result is used in the field.
wire size basis → Box Fill
NEC 314.16 check → Raceway Fill
separate Chapter 9 check → Field Verification
box marking and conditions
- Use the Box Fill Calculator when conductor count, device count, grounding conductors, clamps, and box cubic inches need to be compared.
- Use the Wire Size Calculator separately when conductor size or overcurrent protection is still being selected.
- Use the Conduit Fill Calculator for the separate raceway-fill check after conductors leave the box.
Related TradeHub Calculators
Related References
Related NEC Field References
Source Scope
Source Alignment and Use Scope
This page is a field reference based on NEC 314.16 box-fill concepts and related TradeHub source alignment records. It supports screening and planning only; it does not reproduce proprietary code text, approve final box selection, replace box markings or listing instructions, override adopted local code, or determine AHJ acceptance. Review the TradeHub Code Citation & Source Log for source alignment records and the TradeHub Methodology page for how field references are scoped.
FAQ
Box Fill FAQ
What does NEC 314.16 cover?
NEC 314.16 covers box volume and box-fill calculation requirements for outlet boxes, device boxes, junction boxes, and similar enclosures where conductor volume allowances must be compared with available box capacity.
Do grounding conductors count separately for box fill?
Equipment grounding conductors are generally counted together as a single conductor volume allowance based on the largest equipment grounding conductor in the box. Internal grounding pigtails that originate and terminate in the same box are not counted the same way.
How much does a device yoke count for box fill?
A device yoke or strap is commonly counted as two conductor volume allowances based on the largest conductor connected to that device yoke.
Is box fill the same as conduit fill?
No. Box fill checks cubic-inch capacity inside a box. Conduit fill is a separate raceway-fill check under NEC Chapter 9 and should be reviewed separately when conductors are installed in raceways.