How to Size a Pull Box or Junction Box
Updated August 29, 2026 • Written by the field team at Arizona Electrical Solutions. All field guides →
Pull boxes get sized wrong for a simple reason: people reach for the box-fill rules they know from device boxes, and those rules don't apply. Once the conductors in a raceway are 4 AWG or larger, NEC 314.28 takes over — and it doesn't care about cubic inches at all. It cares about geometry, because the failure mode it's preventing isn't overcrowding; it's destroying insulation by bending big conductors too hard around a box that's too small.
The whole section boils down to two multipliers. A straight pull needs a box eight times as long as the largest raceway's trade size. An angle or U pull needs six times the largest raceway — plus the other raceway entries on the same wall — between each entry wall and the wall opposite. Learn those two and the rest is bookkeeping.
Underground work is where this bites hardest: feeder sweeps into switchgear rooms, duct-bank transitions, and site lighting junctions all live or die on 314.28, and a too-small box discovered at rough-in inspection means demo. It's a check our commercial electrical crews run before any box gets ordered, not after.
We do this work. Arizona Electrical Solutions self-performs commercial electrical on commercial projects across Arizona. Feeder pulls, duct-bank transitions, and gear rooms laid out so the boxes pass inspection before the wire ever ships.
Safety first. This work is for qualified, licensed electricians only. De-energize and lock out every circuit passing through a pull or junction box before opening it, and verify absence of voltage with a tested meter — pull boxes routinely carry multiple circuits from multiple sources. Wear PPE selected per NFPA 70E. Box installations are inspected work; the locally adopted NEC edition and local amendments govern — confirm with your AHJ.
What you'll need
- Current NEC codebook or code app (2023 numbering used here) — Article 314, especially 314.28
- The raceway schedule for every conduit entering the box: trade sizes, walls, and which conductors turn where
- A pull box or junction box of the calculated dimensions, NEMA-rated for the location
- Tape measure — dimensions are checked to the entry positions, not just the box shell
- Struts, anchors, or supports rated for the box and conductor weight
- Locknuts, bushings, and grounding bushings as required — insulated throats or bushings for 4 AWG and larger per 300.4(G)
- Bonding jumpers and lugs for concentric or eccentric knockouts
- Permanent marker or labels for circuit identification at the box
Code references
| NEC 314.28 | Pull and junction box sizing for conductors 4 AWG and larger — the section this whole guide walks. |
| NEC 314.28(A)(1) | Straight pulls: box length at least 8 × the trade size of the largest raceway. |
| NEC 314.28(A)(2) | Angle and U pulls: 6 × largest raceway plus other entries in the same row; 6 × spacing between entries enclosing the same conductor. |
| NEC 314.28(A)(3) | Smaller boxes permitted only where listed and permanently marked for the conductors installed. |
| NEC 314.16 | Cubic-inch box fill — governs boxes with conductors 6 AWG and smaller. |
| NEC 314.29 | Boxes and conduit bodies must remain accessible without removing part of the building. |
| NEC 300.4(G) | Insulated fittings where 4 AWG and larger conductors enter enclosures. |
| NEC 110.12 | Unused openings closed; workmanship — the things inspectors check right after the tape measure. |
Section numbers follow the 2023 NEC; the edition adopted by your jurisdiction governs.
Step by Step
Size a Pull Box or Junction Box — step by step
1. Confirm which rulebook applies: 314.16 or 314.28
Count conductor sizes first. Boxes containing only conductors 6 AWG and smaller are sized by cubic-inch fill under 314.16 — that's device-box territory, and our box fill calculator does the math. The moment any conductor in the box is 4 AWG or larger and the box is used as a pull or junction point, 314.28 governs, and the dimensions come from raceway trade sizes, not conductor counts.
Note what that means in practice: a pull box with two 3-inch conduits and just three 500 kcmil conductors has a huge minimum size, while the same box stuffed with forty 12 AWG wires under 314.16 could be far smaller. Big wire needs bending room; that's the entire logic of the section.
2. Straight pulls: eight times the largest raceway
314.28(A)(1): where conductors pass straight through, the box length must be at least eight times the trade size of the largest raceway. A 3-inch conduit in and a 3-inch conduit out means a box at least 24 inches long in the direction of the pull. A 4-inch run needs 32 inches.
Only the dimension in line with the pull is set by this rule — width and depth just have to accommodate the raceways and their locknuts and bushings. And the multiplier reads off the largest raceway even if it's only one of several: a wall with one 3-inch and three 1-inch entries still starts at 24 inches for the straight-through direction.
3. Angle and U pulls: six times, plus the neighbors
314.28(A)(2): where conductors enter one wall and leave through an adjacent wall (an angle pull), or enter and leave through the same wall (a U pull), the distance from each entry wall to the opposite wall must be at least six times the trade size of the largest raceway on that wall — plus the sum of the trade sizes of all the other raceway entries in the same row on the same wall.
Worked example: one 3-inch and two 2-inch conduits enter the left wall and turn down to the bottom. Left wall to right wall: 6 × 3 + 2 + 2 = 22 inches minimum. Then run the same math for the bottom wall — with the same three raceways landing there, it's also 22 inches. The box is at least 22 × 22 for those two dimensions; if the walls carried different raceway mixes, each wall's own math would set its own dimension.
4. Check the entry-to-entry distance on every angle and U pull
Same section, second sentence, and it's the one that catches people: the distance between raceway entries enclosing the same conductor must be at least six times the trade size of the larger raceway. For the 3-inch conduit turning a corner, its wall entry and its bottom exit must be at least 18 inches apart, measured between the nearest edges.
This check is what stops the tempting shortcut of putting both entries in the corner of an oversized box. The box can pass the wall-to-wall math and still fail here — and on U pulls in narrow boxes, this rule usually governs. Lay the entries out on paper before anything gets punched.
5. Multiple rows: size from the worst row
When a wall carries more than one row of raceway entries, 314.28(A)(2) says to calculate each row separately — six times the largest raceway in that row plus the other entries in that same row — and size the box from whichever row demands the most. Rows don't add to each other; the governing row wins.
Duct-bank stub-ups into a large junction box are the classic case: two rows of four 4-inch conduits calculate as 6 × 4 + 4 + 4 + 4 = 36 inches per row, not 72. Depth then has to be honest about the two rows physically fitting with bushings and spacers — the code minimum is a floor, and racking clearance often pushes past it.
6. Splices change nothing — and smaller boxes need a listing
A junction box where conductors are spliced uses exactly the same dimensional rules as a pull-through; 314.28(A)(2) applies its angle-pull math to any wall with a splice-fed entry. Don't shrink a box because 'nothing pulls through' — the bending radius a splice needs is the same radius a pull needs.
Going smaller than the calculation is only legal one way: 314.28(A)(3) allows a box listed for, and permanently marked with, the maximum number and size of conductors — a manufactured, tested product, not a field judgment call. If the marking isn't on the box, the full-size math stands.
7. Respect the conductor rules inside the box
314.28(B) requires conductors 4 AWG and larger to be protected where they enter — that pairs with 300.4(G)'s insulated fittings — and conductors in the box can't be doubled back in a way that defeats the bending space the dimensions bought. Rack long runs neatly along the walls; slack is good, spaghetti is not.
Every box has to stay accessible per 314.29 — no burying a junction box behind drywall or above a hard lid with no access panel. On exterior and site work, pick the enclosure rating for the real environment: NEMA 3R minimum outdoors, gasketed and shaded where Tucson sun will cook gaskets, and listed covers torqued down with all their hardware.
8. Order the box from the math, then verify at layout
Do the arithmetic for all three dimensions before the box is ordered: straight-pull length at 8×, each angle-pull wall at 6× plus neighbors, entry spacing at 6× the larger raceway, plus real-world racking depth. Write the governing rule next to each dimension on the shop drawing — the inspector will run the same numbers, and showing your work ends the conversation early.
At layout, mark every knockout location on the box before punching and re-measure the entry-to-entry distances as marked. Knockouts drift during fab; the 18 inches that existed on paper has a way of becoming 15 in the field, and hole placement is the one thing you can't fix after the punch.
Watch Out
Common mistakes
- Sizing a pull box by cubic-inch fill when it holds 4 AWG or larger conductors — 314.28's geometry rules govern, and they're almost always bigger.
- Forgetting to add the other raceway entries on the same wall to the 6× figure on angle pulls.
- Passing the wall-to-wall math but failing the 6× entry-to-entry spacing by clustering both entries near a corner.
- Adding rows together instead of sizing from the single worst row.
- Shrinking a junction box because it 'only has splices' — splices follow the same angle-pull dimensions.
- Ordering the box before laying out knockout positions, then discovering the entries can't achieve the required spacing.
- Burying a junction box behind finished surfaces with no access, in violation of 314.29.
- Using an indoor box outdoors, or a gasketed box baked in direct desert sun with no shade or drip protection until the gasket fails.
FAQ
Frequently asked questions
When does NEC 314.28 apply instead of regular box fill?
When the box is used as a pull or junction point for conductors 4 AWG or larger. Boxes holding only 6 AWG and smaller are sized by the cubic-inch fill rules of 314.16. The two sections measure completely different things — fill counts volume, 314.28 buys bending room.
How big must a pull box be for a straight pull with 3-inch conduit?
At least 24 inches long — eight times the 3-inch trade size, per NEC 314.28(A)(1). Width and depth only need to accommodate the raceways, bushings, and any other entries; the 8× applies to the dimension in line with the pull.
How do I size an angle pull with one 3-inch and two 2-inch conduits on the same wall?
Six times the largest raceway plus the sum of the other entries in the same row on that wall: 6 × 3 + 2 + 2 = 22 inches from that wall to the wall opposite. Repeat the calculation for each wall with entries, and also verify the 3-inch conduit's entry and exit are at least 18 inches apart — six times its trade size.
Do splice boxes really use the same dimensions as pull boxes?
Yes. NEC 314.28(A)(2) applies its angle-pull dimensions where splices are made, because a spliced 500 kcmil conductor needs the same bending room as one pulled through. The only path to a smaller box is 314.28(A)(3): a box listed and permanently marked for the specific conductors installed.
Does the 8× or 6× multiplier use the conduit's actual diameter or trade size?
Metric designator aside, the calculation uses the trade size of the raceway — a 3-inch conduit contributes 3 inches to the math regardless of its actual outside diameter. Largest raceway sets the multiplier; the other entries add their trade sizes.
What about boxes fed from multiple directions with different raceway mixes?
Run the numbers wall by wall. Each wall with entries generates its own minimum distance to the opposite wall — 6 × its largest raceway plus its same-row neighbors — and any straight-through pair generates an 8× length. The box must satisfy every one of those dimensions simultaneously; the governing case for each dimension wins.
Is there a depth requirement for pull boxes?
Not a multiplier like the length rules, but depth is still constrained: the raceway entries with their locknuts and bushings must physically fit, and rows need racking space. One relief valve exists — where a raceway enters the wall opposite a removable cover, 314.28(A)(2) permits the distance to that cover to follow the one-wire-per-terminal bending space of Table 312.6(A) instead of the full 6× math. Practical racking usually asks for more than the minimum anyway.
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