NEC 314.16 · Box fill

How to calculate box fill with NEC 314.16

What counts, what doesn’t, and how many cubic inches you need — the counting rules in plain language with two worked examples and a free link to the official tables.

Need the actual tables? Look up 314.16 and Tables 314.16(A) and (B) in the free NFPA viewer →

What counts toward box fill (314.16(B)) — in our words

Item in the box	How many allowances	Sized by
Each conductor that enters from outside and terminates or is spliced	1 each	That conductor’s size
Conductor passing through unbroken	1	That conductor’s size
Unbroken conductor looped/coiled (long enough to be cut into two free ends)	2	That conductor’s size
Pigtails / jumpers that never leave the box	0	—
Internal cable clamps (one or many)	1 total	Largest conductor in the box
Luminaire studs or hickeys	1 for each type	Largest conductor in the box
Each device yoke/strap (switch, receptacle)	2 per yoke	Largest conductor connected to that device
Device wider than a single 2-in. gang	2 per gang it needs	Largest conductor connected to it
Equipment grounding conductors (all of them)	1 total	Largest EGC in the box
Up to four small fixture wires from a domed luminaire canopy (plus an EGC)	0	Allowed to be omitted under the listed conditions

Our own summary, not the code text (2023 NEC). Look up 314.16 and Tables 314.16(A) and (B) in the free NFPA viewer (nfpa.org/freeaccess) and verify against the edition adopted by your AHJ.

How to run a box-fill calc, step by step

Step	What you do	Watch out for
1	List every item in the box using the counting rules above	Count per yoke for devices, not per device — a duplex receptacle is one yoke
2	Find the volume allowance for each conductor size in Table 314.16(B)	Allowances are per conductor size, not per insulation type
3	Multiply each count by its allowance and add them up	Items sized by "largest conductor" use the largest one present, not an average
4	Get the box volume: marked volume on the box, or Table 314.16(A) for standard metal boxes	Plastic and non-standard boxes: use the marked volume — the table only covers standard metal boxes
5	Add any marked volume from plaster rings, extension rings, or raised covers	Only if the ring or cover is marked with its volume (or listed in the table)
6	Box volume must be at least the calculated total	If it isn’t, go deeper, go to a 4-in. square with a ring, or add an extension

Our own summary, not the code text (2023 NEC). Look up 314.16 and Tables 314.16(A) and (B) in the free NFPA viewer (nfpa.org/freeaccess) and verify against the edition adopted by your AHJ.

Worked example 1 — receptacle box, two 12/2 NM cables with ground, internal clamps

Item	Count	Allowance	Volume
Insulated 12 AWG conductors (2 per cable)	4	2.25 in³	9.00 in³
Internal cable clamps	1	2.25 in³	2.25 in³
Duplex receptacle (one yoke)	2	2.25 in³	4.50 in³
All EGCs together	1	2.25 in³	2.25 in³
Total required	8		18.00 in³

12 AWG = 2.25 in³ per Table 314.16(B). The box (plus any marked ring) must be marked at 18.0 in³ or more. Worked-example values only — not the table.

Worked example 2 — switch box, two 14/2 NM cables with ground, no internal clamps

Item	Count	Allowance	Volume
Insulated 14 AWG conductors	4	2.00 in³	8.00 in³
Single-pole switch (one yoke)	2	2.00 in³	4.00 in³
All EGCs together	1	2.00 in³	2.00 in³
Pigtail from ground splice to switch	0	—	—
Total required	7		14.00 in³

14 AWG = 2.00 in³ per Table 314.16(B). Many nonmetallic boxes have no internal clamps — count the clamp allowance only if clamps are inside the box. Worked-example values only.

Common field mistakes

Mistake	What to do instead
Counting each ground wire separately	All EGCs together count as one allowance, sized by the largest
Counting a device as one	Each yoke counts as two, sized by the largest conductor connected to that device
Counting pigtails	Wires that start and end inside the box don’t count
Using the Table 314.16(A) volume for a plastic box	Use the volume marked on the box — the table is for standard metal boxes
Forgetting the plaster ring or mud ring volume	A marked ring adds volume; an unmarked one adds nothing
Counting a clamp allowance for external connectors	Only clamps inside the box count — connectors outside the box don’t

Our own summary, not the code text (2023 NEC). Look up 314.16 and Tables 314.16(A) and (B) in the free NFPA viewer (nfpa.org/freeaccess) and verify against the edition adopted by your AHJ.

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Box fill is a counting problem

Look up 314.16 and its tables in the free NFPA viewer. NFPA offers free read-only access to current NEC editions. This page explains how to use the rules (2023 NEC); it does not reproduce the tables.

Two tables, one comparison. Table 314.16(A) gives the volume of standard metal boxes (and the max number of same-size conductors they hold). Table 314.16(B) gives a volume allowance per conductor, by size. Everything else in 314.16(B) is a counting rule that converts clamps, devices, fittings, and grounds into conductor allowances. Add them up and the box has to be at least that big.

The counting rules in one breath. Each wire coming in that terminates or splices counts one; a wire passing through counts one (two if it is looped long enough to cut into two free ends); pigtails that never leave the box count zero. All internal clamps together count one. Each type of luminaire stud or hickey counts one. Each device yoke counts two, sized by the biggest wire on that device. All equipment grounding conductors together count one, sized by the largest. Clamps and fittings are sized by the largest conductor in the box.

Where the volume comes from. Standard metal boxes can use the table; anything else — plastic boxes, most specialty boxes — uses the volume marked in the box. Plaster rings, extension rings, and raised covers add their marked volume. That’s why a 4-in. square box with a deep ring can often solve a fill problem without changing the rough-in.

Worked example. A receptacle box with two 12/2 NM cables and internal clamps: 4 insulated conductors + 1 for clamps + 2 for the receptacle + 1 for all grounds = 8 allowances. At 2.25 in³ each for 12 AWG, the box needs 18.0 in³. A switch box on 14 AWG with no internal clamps is 7 × 2.00 in³ = 14.0 in³.

Skip the math with the free box fill calculator. For raceway fill instead of box fill, see the conduit fill chart and the conduit fill calculator; for conductor sizing, the NEC 310.16 ampacity chart. Field PM keeps rough-in photos and inspection sign-offs with the job.

FAQ

How do you calculate box fill?+

Count every conductor that enters and terminates or splices, add one allowance for all internal clamps, one for each type of luminaire stud or hickey, two for each device yoke, and one for all equipment grounding conductors. Multiply each count by the Table 314.16(B) volume for its conductor size, add them up, and compare with the box volume. Look up the tables free at nfpa.org/freeaccess.

How many cubic inches per 12 AWG and 14 AWG wire?+

Table 314.16(B) allows 2.25 in³ for each 12 AWG and 2.00 in³ for each 14 AWG conductor. Look up other sizes in the free NFPA viewer.

Do ground wires count in box fill?+

Yes — but all of them together count as a single allowance, based on the largest equipment grounding conductor in the box.

Why does a receptacle count as two conductors?+

Each device yoke or strap takes up space where wires would otherwise go, so 314.16(B)(4) assigns it a double allowance based on the largest conductor connected to it. A device too wide for a single 2-in. gang counts double for each gang it needs.

Does a pigtail count toward box fill?+

No. A conductor that originates in the box and never leaves it — like a splice-to-device pigtail — is not counted.

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