Junction Box Sizing Calculator
Size a pull or junction box to NEC 314.28. Pick the pull type (straight, angle or U), the largest raceway and the other raceways in each row — get the minimum box dimensions, the required spacing between raceways of a run, the governing minimum and a suggested box, with by-raceway and by-pull-type comparison tables and a downloadable PDF.
| Raceway | Straight (8×) | Angle/U (6×) | Governing |
|---|---|---|---|
| Pick a raceway to compare | |||
| Pull type | Rule | Governing min | Between |
|---|---|---|---|
| Pick a raceway to compare | |||
How to use the junction box sizing calculator
Four quick steps turn a pull type and raceway into NEC 314.28 minimum box dimensions — the required wall lengths, the spacing between raceways of a run, the governing minimum and a suggested box.
Pick the pull type
Straight, angle or U — how the conductors travel through the box.
Set the largest raceway
The trade size of the biggest conduit entering the box.
Add the others per wall
For angle or U pulls, the sum of the other raceways in each row.
Read results & save a PDF
Get the minimum dimensions and a suggested box — then download a report.
The rules behind the box size
NEC 314.28 sets the minimum dimensions from the raceways — eight times the largest for a straight pull, six times plus the others for an angle or U pull.
▶ Worked example
A straight pull with a 2" largest raceway.
The box length must be at least 8 × 2 = 16 in, wall to wall.
Straight pull, 2″ largest → box ≥ 16″ long
As an angle pull with the same 2″ largest and one other 1″ raceway on the wall, that wall must be 6 × 2 + 1 = 13 in, the other wall 6 × 2 = 12 in, and the two raceways of a run at least 12 in apart.
Pull box sizing quick reference
The NEC 314.28 rules, the straight-pull length for each raceway, and the six-times dimension for angle and U pulls.
| Pull type | Minimum dimension |
|---|---|
| Straight | 8 × largest raceway (length) |
| Angle | 6 × largest + sum of others, per wall |
| U pull | 6 × largest + sum of others, same wall |
| Between raceways of a run | 6 × largest raceway |
| Largest raceway | 8 × rule | Minimum box length |
|---|---|---|
| 1/2″ | 8 × 0.5 | 4 in |
| 3/4″ | 8 × 0.75 | 6 in |
| 1″ | 8 × 1 | 8 in |
| 1-1/4″ | 8 × 1.25 | 10 in |
| 1-1/2″ | 8 × 1.5 | 12 in |
| 2″ | 8 × 2 | 16 in |
| 3″ | 8 × 3 | 24 in |
| 4″ | 8 × 4 | 32 in |
| Largest raceway | 6 × rule | Base dimension & min spacing |
|---|---|---|
| 1/2″ | 6 × 0.5 | 3 in |
| 3/4″ | 6 × 0.75 | 4.5 in |
| 1″ | 6 × 1 | 6 in |
| 1-1/4″ | 6 × 1.25 | 7.5 in |
| 1-1/2″ | 6 × 1.5 | 9 in |
| 2″ | 6 × 2 | 12 in |
| 3″ | 6 × 3 | 18 in |
| 4″ | 6 × 4 | 24 in |
For angle and U pulls, add the sum of the trade sizes of the other raceways in the same row to the six-times base. The between-raceways spacing uses the base alone (6 × largest).
Everything the calculator returns
One calculation gives the full box picture — the required walls, the spacing, and a suggested box.
Box sizing at a glance
The NEC 314.28 multipliers, and the default result.
Built for the way you work
Whether you are pulling the wire, inspecting the install, or laying out the job, the box sizing starts from the same inputs.
Sizing a pull box for large feeders and need the minimum dimensions and a box that passes before you order it.
- Use the largest raceway
- Mind the between-run spacing
- Round up to a stock box
Verifying a box meets 314.28 or pricing one on a takeoff, and want the minimums for each pull type fast.
- Confirm the right clause
- Compare straight vs angle
- Flag undersized boxes
Specifying pull boxes on a drawing and want the dimensions and spacing for the raceways in each wall.
- Size each wall by its row
- Set the run spacing
- Note the suggested box
Size a pull box right
Seven things that keep a 314.28 box legal and practical.
Straight is 8×, angle is 6×
A straight pull needs 8 times the largest raceway; angle and U pulls use 6 times plus the other raceways in the row.
Only the largest drives it
The multiplier always uses the single largest raceway. The others are added only as a plain sum on angle and U walls.
Don’t forget the spacing
On angle and U pulls the two raceways of a run must be at least 6 times the largest apart, not just the walls sized.
Check the conductor size
314.28 is for 4 AWG and larger. Smaller conductors are sized by box fill under 314.16 instead.
Round up to a real box
The rule gives a minimum. Pick the next stock box at or above it — a larger box is always allowed.
Depth is about fittings
314.28 does not set a depth. Make sure the locknuts, bushings and conductor bend fit and the cover closes freely.
Leave room for splices
If the box holds splices or taps, size it for those too — the pull dimensions alone may not leave enough room.
Frequently asked questions
Common questions about sizing pull and junction boxes to NEC 314.28, pull types and spacing.
A pull or junction box for conductors 4 AWG and larger is sized by NEC 314.28 from the raceways entering it, not from the conductor count. You pick the pull type — straight, angle or U — identify the largest raceway, and apply the matching multiplier.
For a straight pull, the box length must be at least eight times the largest raceway. For an angle or U pull, each wall with raceways must be at least six times the largest plus the sum of the other raceways in that row, and the raceways of a single run must be at least six times the largest apart.
For example, a straight pull with a 2 inch largest raceway needs a box at least 16 inches long. This calculator applies the right rule and gives the minimum dimensions and a suggested box.
They describe how the conductors travel through the box, and each has its own NEC rule. In a straight pull the conductors enter one wall and leave the opposite wall, running straight through.
In an angle pull they turn a corner — entering one wall and leaving an adjacent, perpendicular wall. In a U pull they enter and leave through the same wall, doubling back. Angle and U pulls need more room because the conductors have to bend inside the box.
That is why a straight pull uses the eight-times rule while angle and U pulls use the more demanding six-times-plus-others rule. This calculator lets you pick the type and applies the correct one.
For a straight pull, NEC 314.28(A)(1) sets the minimum box length at eight times the trade size of the largest raceway entering the box. Only the single largest raceway matters for this dimension; the others in the row do not add to it.
The width only needs to accommodate the raceways and their locknuts and bushings, and the depth likewise just has to fit the fittings.
For example, with a 2 inch largest raceway the box must be at least 8 times 2, which is 16 inches, long. This calculator computes the length for your largest raceway and suggests a box size.
For an angle or U pull, NEC 314.28(A)(2) sets each wall that has raceways at six times the trade size of the largest raceway, plus the sum of the trade sizes of the other raceways in the same row on that wall.
In addition, the distance between the two raceways of a single conductor run must be at least six times the largest raceway, so the conductors have room to make the bend.
For example, with a 2 inch largest raceway and one other 1 inch raceway on the wall, that wall must be at least 6 times 2 plus 1, which is 13 inches, and the raceways of a run must be at least 12 inches apart. This calculator works out each wall and the spacing.
When a conductor enters through one raceway and leaves through another, NEC 314.28(A)(2) requires the distance between those two raceways, measured between their nearest edges, to be at least six times the trade size of the larger raceway. This gives the large conductors room to bend without being damaged.
It applies to angle and U pulls, where the conductors change direction; a straight pull has no such requirement because the conductors run straight through.
For example, with a 2 inch largest raceway the two raceways of a run must be at least 12 inches apart. This calculator reports this minimum spacing alongside the box dimensions.
No. NEC 314.28 applies to pull and junction boxes and conduit bodies that contain conductors of 4 AWG or larger. For boxes with smaller conductors — 6 AWG and below — the box fill rules of NEC 314.16 govern instead, based on counting the conductors, devices and fittings.
So the size of the conductors decides which rule you use: the dimensional 314.28 method for the big conductors, the volume-based 314.16 method for the smaller ones.
This calculator covers the 314.28 dimensional method. For smaller conductors sized by box fill, use a box fill calculator instead.
NEC 314.28 sets the length and width of a pull or junction box from the raceways, but it does not set a specific depth. The depth only needs to be enough to install the raceways with their locknuts and bushings and to close the cover without cramping the conductors.
In practice the depth is driven by the fittings and the conductor bend, not by a multiplier. A box that meets the length and width rules is usually deep enough, but check that the largest fittings and the conductor bending radius fit.
This calculator gives the minimum length and width from 314.28; confirm the depth fits your conduits, fittings and the conductor bend.
It can. NEC 314.28 sizes the box from the raceways for the pull, but a box that also contains splices or taps must additionally have enough room for them — the splices, their insulation and the conductors cannot be crammed in.
For boxes with large conductors the code requires the splice to fit within the space, and good practice leaves working room. The dimensional pull rules are a minimum, not necessarily enough on their own when splicing.
This calculator gives the 314.28 pull dimensions; if the box holds splices, make sure the chosen box also has room for them and meets any fill or free-space requirements.
Disclaimer: US units, NEC 314.28 pull and junction boxes with conductors 4 AWG and larger. Straight pull: length ≥ 8 × the largest raceway trade size. Angle / U pull: each wall with raceways ≥ 6 × the largest + the sum of the other raceways in that row, and the raceways of a run ≥ 6 × the largest apart. It does not size boxes for conductors 6 AWG and smaller (NEC 314.16 box fill), set box depth, or account for splices, taps, grounding bars or working space. A reference aid only — confirm the box, pull dimensions and conductor sizes against the current NEC and your authority having jurisdiction.