Box Fill Chart

Volume allowance per conductor from 14 to 6 AWG, and how many conductors of each size four common box types will hold.

Data verified 2026-09-29 · based on NEC 2023 (NFPA 70-2023)

Conductor Volume Allowance and Box Capacity (NEC 314.16)

The volume allowance is per conductor, and it is not the conductor's physical volume — it is an allowance for the conductor, its insulation and the space needed to work it. So the count is lower than the geometry alone would suggest.

Several things count as conductors for fill even though they carry no current. Each conductor entering and leaving counts once; each device (switch, receptacle) counts as two conductors of the largest size connected to it; each grounding conductor counts as one; and cable clamps count as one conductor total. A box that looks half empty by wire count can still be over-filled once the device allowances are added.

The conductor count column in this table is a simplification — divide the box volume by the allowance, then subtract the device, clamp and grounding allowances before comparing against the actual conductor count.

What Counts Toward Box Fill

The rule that catches people out is that things which are not current-carrying conductors still consume box volume. Working through them:

Conductors — each one entering and leaving counts once, however long it is in the box. A conductor that passes through without being spliced counts once, not twice.

Devices — each switch or receptacle counts as two conductors of the largest size connected to it. That matters because 12 AWG wire on a receptacle adds 4.5 in³, not 2.25.

Grounding conductors — all of them together count as one conductor, based on the largest size present. All grounds in a box count as one, which is generous and is why a grounded box rarely fails on grounds alone.

Cable clamps — all clamps together count as one conductor of the largest size.

Pigtails — a short conductor spliced to a longer run counts as an additional conductor, because it occupies space.

The practical sequence is: total the allowances, subtract from the box volume, and compare what remains against the conductors you actually need to fit. Using the box's listed volume as the conductor count, as the table above does, overstates capacity by exactly those allowances.

Frequently Asked Questions

How many wires can be in an electrical box?

The box volume must be at least the sum of the volume allowances — 2.00 in³ per 14 AWG conductor, 2.25 per 12 AWG, 2.50 per 10 AWG — plus allowances for devices, clamps and grounds. A 21.0 in³ box holds nine 12 AWG conductors, fewer once a receptacle and clamp are counted.

Does a grounding wire count as a conductor in box fill?

Yes, but all grounding conductors in a box together count as only one conductor, sized to the largest one present. So adding a second grounding conductor does not change the box fill calculation.

How much box fill does a receptacle take?

Each device counts as two conductors of the largest size connected to it. A receptacle on 12 AWG wire therefore consumes 4.5 in³ — twice the 2.25 in³ allowance of a single conductor. That is the allowance most often forgotten when a box is close to its limit.

What is the minimum box size for a 12 AWG circuit?

It depends on the conductor count and the devices. A single receptacle on 12 AWG with one cable needs roughly 2 conductors plus a device plus a clamp plus a ground — around 13.5 in³ — which a 3 × 2 × 2-1/2 device box at 14.0 in³ just covers.

Do cable clamps count toward box fill?

Yes, but all clamps together count as one conductor of the largest size. Their combined allowance is often small enough to ignore when the box has plenty of volume, and decisive when it does not.

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Value Sources

Each data column on this page is tied to the source it came from. The numbers in square brackets correspond to the table headers above.

Data Sources

Cross-checked against:

• Volume allowances checked against NEC Table 314.16(B)
• The conductor-count columns recomputed from the box volume and allowance at build time
• Four values held as known-value checks including a recomputed count

Derived values — the following values on this page are calculated, not taken directly from the standard:

The conductor counts are upper bounds that ignore device, clamp and grounding allowances. A real box fill calculation must subtract those before comparing against the conductor count actually present. Box volumes are nominal and vary slightly by manufacturer.

Cite This Page

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