4 × #12 AWG conductors, one device yoke, internal clamps and one grounding conductor need 18.0 in³ — the smallest listed box that holds them is a 4" square x 1-1/4" at 18.0 in³.
Calculator
Minimum Box Volume
18.0in³
Conductors
9.0 in³
Devices
4.5 in³
Clamps
2.25 in³
Grounds
2.25 in³
Fittings
0.0 in³
Suitable Boxes
4" square x 1-1/4" — 18.0 in³
3" x 2" x 3-1/2" device — 18.0 in³
FD 2-3/8" deep, single cover/gang — 18.0 in³
FS 1-3/4" deep, multiple cover/gang — 18.0 in³
4" square x 1-1/2" — 21.0 in³
Per NEC 2023 314.16(B), NEC 2023 Table 314.16(A), NEC 2023 Table 314.16(B)(1)
How it’s calculated
V = conductors + 2 × yokes + clamps + grounds + fittings, each in the volume for its conductor size
conductors
one volume for each conductor entering the box, at its own size: #14 AWG 2.0 in³, #12 AWG 2.25 in³, #10 AWG 2.5 in³, #8 AWG 3.0 in³, #6 AWG 5.0 in³
yokes
two volumes per yoke, at the largest conductor connected to that yoke
clamps
one volume for all internal clamps together, at the largest conductor in the box
grounds
one volume for up to four, at the largest of them, plus a quarter for each beyond four
fittings
one volume for each type of luminaire stud or hickey, at the largest conductor
The box then has to be at least that volume. The listed boxes come from the table of standard metal boxes; a box with its volume marked by the manufacturer is read from the marking instead.
A conductor that begins and ends inside the box is not counted, and wire connectors get no allowance.
Assumptions
Conductors 14 AWG through 6 AWG, the sizes this form counts. Where 4 AWG or larger conductors enter, the box also has to meet the separate rules for pull and junction boxes.
The box's own volume is the table's figure for a standard metal box, or the volume the manufacturer marked on it. Nonmetallic boxes, and metal boxes the table does not list, carry that marking.
A terminal block has no field of its own: it counts one allowance at the largest conductor terminated to it, so add it as one more conductor of that size.
Each yoke is counted at one conductor size: where two devices in a box are wired in different sizes, use the larger.
Isolated grounding conductors are counted with the other equipment grounding conductors; they get no allowance of their own.
Volumes are rounded to 0.01 in³ and the total to 0.1 in³.
Worked example
A junction box with two circuits and two receptacles
Six #12 AWG and three #10 AWG conductors, two device yokes wired in #12 AWG, internal clamps, and three grounding conductors of which the largest is #10 AWG.
Conductors, each at the volume for its own size:6 × 2.25 + 3 × 2.5= 21.0 in³
Two yokes, each twice the volume of the largest conductor connected to it:2 × 2 × 2.25= 9.0 in³
All the internal clamps together, once, at the largest conductor in the box (#10 AWG):1 × 2.5= 2.5 in³
Three grounding conductors — four or fewer share one allowance, at the largest of them:1 × 2.5= 2.5 in³
Total21.0 + 9.0 + 2.5 + 2.5= 35.0 in³
Answer: 35.0 in³, so a 4-11/16" square x 2-1/8" at 42.0 in³ is the smallest listed box that takes it.
A yoke is two volumes, not one. A tool that counts a receptacle as a single conductor is one whole allowance short per device.
Check: whether the other tool doubles the device.
Counting each ground
Giving every grounding conductor its own allowance overstates a box with several cables in it. Four share one allowance, and the fifth onwards adds a quarter each.
Check: how the other tool treats the second and later grounds.
Counting pigtails and wire nuts
Neither is counted. A tool that counts every conductor end inside the box, rather than the conductors entering it, comes out high.
Check: whether the other tool asks for conductors entering the box or wires in it.
The wrong conductor size for the extras
Clamps, fittings and yokes are counted at the largest conductor they apply to, not at the smallest or at an average. In a box holding both 12 AWG and 10 AWG that is a quarter of a cubic inch each time.
Check: which size the other tool used for the clamp and device allowances.
Box volume from a catalog
A manufacturer's box is often larger than the table's standard figure, and its marked volume governs. A tool built on the table will sometimes call for a bigger box than the one in your hand needs.
Check: the volume marked on the box itself.
Questions
What size box do I need for 4 #12 AWG conductors and a receptacle?
18.0 in³ with internal clamps and one grounding conductor, so a 4" square x 1-1/4" (18.0 in³) is the smallest box in the table that holds it. Two yokes in the same box would need 22.5 in³.
Does every grounding conductor count?
Not one each. Up to four grounding conductors share a single allowance, taken at the largest of them, and every one beyond four adds a quarter of that allowance. One ground here is 2.25 in³ and five is 2.81 in³.
Why does a switch or receptacle count twice?
A yoke or strap with a device on it is counted at twice the volume of the largest conductor connected to that yoke, because the device itself takes up room. A device too wide for one gang counts twice for each gang it needs to mount.
Do wire connectors and pigtails count?
No. Wire connectors get no allowance, and a conductor that both begins and ends inside the box — a pigtail or a jumper — is not counted at all. Only conductors that enter the box and either end there or pass through it are. An unbroken conductor left in a loop or coil at least twice the length the code asks you to leave at a box counts twice.
What about cable clamps?
All the clamps inside one box count once together, at the volume of the largest conductor in the box. A cable connector whose clamping mechanism sits outside the box gets no allowance.
Does this cover a big pull box?
Only the volume count, and only for conductors up to 6 AWG. Where 4 AWG or larger conductors enter a box, it also has to meet the separate rules for pull and junction boxes, which size it from the raceways entering it rather than from a count.
Written and maintained by Yusuf Can Samiloglu: MEP/HVAC design background, mechanical-engineering degree, not a licensed electrician.
How it’s checked: the calculation is the method of the Electric Master app, ported to the web and tested case by case against the app’s source code. Every table value was checked against the NEC 2023 values in at least two independent published sources.
The App Store version of Electric Master (2.2.0) predates the NEC corrections this page follows; until its next update, its result can differ from this page for some inputs. Version 2.2.0 has an on/off switch for grounds and counts them as one allowance at 12 AWG whatever their number or size, counts every yoke at the largest conductor in the box, adds a deep-device allowance of its own and does not stop at two fitting types, so with more than four grounds, or a ground larger than 12 AWG, it asks for less room than this page.
: The total volume is rounded up, so the figure shown is never below the true total.
: First version, NEC 2023. Conductors by size, device yokes counted by gang with their own conductor size, internal clamps, fitting types, and grounding conductors by count at the largest ground’s size; the boxes that hold the total come from Table 314.16(A).